EP4689611A1 - Gas analyzer - Google Patents

Abstract

Infrared gas analyzers and detector assemblies for infrared gas analyzers. In one example, an infrared gas analyzer includes a measurement module and a detector assembly coupled to the measurement module. The measurement module includes an infrared light source configured to emit an infrared light signal. The detector assembly includes an infrared sensing element and a sealed detector housing having an optical window transparent to the infrared light signal and an internal volume that is under vacuum. The infrared sensing element is disposed within the internal volume of the detector housing and configured to detect the infrared light signal via the optical window.

Description

GAS ANALYZER

STATEMENT OF GOVERNMENT INTEREST

[0001] This invention was made with United States Government assistance under Grant No. 1737184 awarded by the National Science Foundation. The United States Government has certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATION

[0002] This application claims priority to co-pending U.S. Provisional Application No. 63/498,429 filed on April 26, 2023 and titled “GAS ANALYZER,” which is hereby incorporated herein by reference in its entirety for all purposes.

BACKGROUND

[0003] Accurate observations of atmospheric composition and exchange of greenhouse gases between the ecosystems and the atmosphere are critical for constraining climate models and predicting future climate characteristics. Infrared gas analyzers, which may be implemented using either broad-band non dispersive or narrow-band tunable laser technologies, are widely used to obtain atmospheric measurements. Infrared gas analyzers use infrared (IR) detectors to measure the amount of infrared radiation emitted by an IR source that is absorbed by the gas when radiation passes through an optical sensing path. By comparing the infrared radiation before and after passing through the optical path the average concentration of the gas between the source and the detector can be inferred. However, research has found that IR gas analyzers deployed on moving platforms, such as buoys or ships, have errors in the gas concentration determinations due to the motion and orientation of the analyzer with respect to gravity. These errors limit the performance of the gas analyzer and have required the development of special correction methods.

SUMMARY

[0004] Aspects and embodiments are directed to gas analyzers having improved performance on moving platforms. According to certain examples, techniques are disclosed for eliminating or reducing carbon dioxide measurement error due to platform motion in open-path and closed- path infrared carbon dioxide (CO2) and/or water vapor (H2O) gas analyzers.

[0005] According to one example, a closed-path infrared gas analyzer comprises a housing and an infrared detector having an internal volume that is under vacuum. [0006] According to another example, an infrared gas analyzer comprises a measurement module including an infrared light source, and a detector assembly coupled to the measurement module. The detector assembly includes a sealed detector housing having an internal volume that is under vacuum, a sensing element, a temperature sensor, and a thermoelectric cooler, wherein the sensing element, the temperature sensor, and the thermoelectric cooler are disposed within the internal volume of the detector housing, and wherein the sensing element and the temperature sensor are coupled to the thermoelectric cooler.

[0007] Another example is directed to a detector assembly for an infrared gas analyzer. In one example, the detector assembly comprises a sealed detector housing having an internal volume that is under vacuum, the sealed detector housing including an optical window that is transparent to infrared radiation, an infrared sensing element disposed within the internal volume of the sealed detector housing and positioned to detect an infrared light signal via the optical window, a thermoelectric cooler disposed within the internal volume of the sealed detector housing and configured to cool the infrared sensing element, and a temperature sensor coupled to the thermoelectric cooler and configured to provide a temperature measurement, wherein the thermoelectric cooler is configured to regulate a temperature of the infrared sensing element to a selected temperature based on the temperature measurement.

[0008] According to another example, a closed-path infrared gas analyzer comprises a measurement module and a detector assembly coupled to the measurement module. The measurement module includes a housing having a first optical window transparent to infrared radiation, and an infrared light source configured to emit an infrared light signal, the infrared light source being disposed within the housing and positioned to direct the infrared light signal through the first optical window. The detector assembly includes a sealed detector housing having an internal volume that is under vacuum, the sealed detector housing including a second optical window transparent to the infrared radiation, an infrared sensing element disposed within the internal volume of the sealed detector housing and positioned to detect the infrared light signal via the second optical window, a thermoelectric cooler disposed within the internal volume of the sealed detector housing and configured to cool the infrared sensing element, and a temperature sensor coupled to the thermoelectric cooler and configured to provide a temperature measurement, wherein the thermoelectric cooler is configured to regulate a temperature of the infrared sensing element to a selected temperature based on the temperature measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Various aspects of at least one example are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide an illustration and a further understanding of the various aspects and are incorporated in and constitute a part of this disclosure. However, the figures are not intended as a definition of the limits of any particular example. The figures, together with the remainder of this disclosure, serve to explain principles and operations of the described and claimed aspects. In the figures, the same or similar components that are illustrated are represented by a like reference numeral. For purposes of clarity, every component may not be labeled in every figure. In the figures:

[0010] FIG. l is a block diagram of one example of an infrared gas analyzer system according to aspects of the present disclosure;

[0011] FIG. 2 is a block diagram of one example of an infrared detector that may be used in the system of FIG. 1, according to aspects of the present disclosure;

[0012] FIG. 3 is a graph illustrating an example of changes in detector spectral sensitivity as a function of the temperature of the sensing element, according to aspects of the present disclosure;

[0013] FIG. 4 is a graph illustrating an example of co-spectra of CO2 concentration and IRGA detector cooler voltage obtained from separate tests, showing moving vs. fixed platform result, according to aspects of the present disclosure;

[0014] FIG. 5A is a cross-sectional view of an example of a detector housing (vertical orientation) illustrating the temperature gradient between a cold sensing element and a hot housing and heat sink, according to certain aspects of the present disclosure;

[0015] FIG. 5B is a cross-sectional view of the detector housing of FIG. 5 A with the detector oriented 90 degrees from vertical, illustrating the temperature gradient between the cold sensing element and the hot housing and heat sink, according to aspects of the present disclosure;

[0016] FIG. 6A is a graph illustrating laboratory motion table test data showing standard EC 155 CO2 molar mixing ratio variation with time while platform pitch is modulated, according to aspects of the present disclosure;

[0017] FIG. 6B is a graph illustrating laboratory motion table test data showing standard EC 155 H2O molar mixing ratio variation with time while platform pitch is modulated, according to aspects of the present disclosure; [0018] FIG. 6C is a graph showing a scatter plot of the low-passed CO2 mixing ratio data versus the instantaneous tilt, according to aspects of the present disclosure;

[0019] FIG. 6D is a graph showing a scatter plot of the low-passed H2O mixing ratio data versus the instantaneous tilt, according to aspects of the present disclosure;

[0020] FIG. 7A is a graph illustrating laboratory motion table test data showing standard EC155 CO2 molar mixing ratio variation with time while platform roll is modulated, according to aspects of the present disclosure;

[0021] FIG. 7B is a graph illustrating laboratory motion table test data showing standard EC 155 H2O molar mixing ratio variation with time while platform roll is modulated, according to aspects of the present disclosure;

[0022] FIG. 7C is a graph showing a scatter plot of the low-passed CO2 mixing ratio data versus the instantaneous tilt, according to aspects of the present disclosure;

[0023] FIG. 7D is a graph showing a scatter plot of the low-passed H2O mixing ratio data versus the instantaneous tilt, according to aspects of the present disclosure;

[0024] FIG. 8A is a graph showing standard EC155 time series data similar to FIGS. 6A and 7A, but only showing CO2 IRGA data for pitch-only motion and with the addition of time variation in EC 155 detector temperature after subtracting its mean level, according to aspects of the present disclosure;

[0025] FIG. 8B is a graph showing standard EC155 time series data similar to FIGS. 6A and 7A, but only showing CO2 IRGA data for roll-only motion and with the addition of time variation in EC155 detector temperature after subtracting its mean level, according to aspects of the present disclosure;

[0026] FIG. 9A is a graph illustrating standard EC155 and example EC155P time series pitch test data showing an example of band-pass filtered variation in CO2 and detector temperature data after subtracting mean levels, according to aspects of the present disclosure;

[0027] FIG. 9B is a graph illustrating standard EC155 and example EC155P time series roll test data showing an example of band-pass filtered variation in CO2 and detector temperature data after subtracting mean levels, according to aspects of the present disclosure;

[0028] FIG. 10 is a graph illustrating autospectra of CO2 measurements during both static platform and mixed sea motion tests, according to aspects of the present disclosure

[0029] FIG. 11 A is a graph showing an example of motion induced error (MIE) on a moving buoy, illustrating pitch and roll motion CO2 test results, according to aspects of the present disclosure; [0030] FIG. 1 IB is a graph showing an example of MIE on the moving buoy, showing mixed sea motion CO2 results, according to aspects of the present disclosure;

[0031] FIG. 12A is a graph showing an example of MIE on the moving buoy, illustrating pitch and roll motion H2O test results, according to aspects of the present disclosure; and

[0032] FIG. 12B is a graph showing an example of MIE on the moving buoy, showing mixed sea motion H2O results, according to aspects of the present disclosure;

DETAILED DESCRIPTION

[0033] Infrared gas analyzers (IRGAs) are used to make atmospheric measurements to determine, among other things, concentrations of certain gases, such as CO2. Typically, such analyzers are installed on stationary meteorological towers over land; but an increasing number of systems are being deployed on mobile platforms and buoys to extend the spatial coverage and include measurements over water. For example, eddy covariance (EC) flux is one of few field measurement methods available to directly validate and refine the gas transfer models used in global estimation of CO2 exchange between the ocean and atmosphere. A CO2 sensor that can be used for these measurements is a high-rate non-dispersive IRGA. By necessity, airsea EC mass flux systems are usually employed in offshore field campaigns conducted aboard research vessels. However, a number of technological challenges are associated with the use of gas analyzers on moving platforms.

[0034] Accurate EC CO2 flux measurement at sea requires an IRGA CO2 mixing ratio measurement precision at or below 0.2 ppm. However, a long-standing technical problem affecting the accuracy of eddy correlation air-sea CO2 flux estimates has been motion contamination of the CO2 concentration measurement. Several research reports on gas transfer field experiments conducted using moving platforms have reported the IRGA motion contamination issues. These errors limit the performance of the system. For example, it has been found that the problem can lead to high frequency CO2 mixing ratio artifacts at 0.5-1.5 ppm levels. There has been speculation as to the cause of IRGA motion sensitivity in the literature, and various correction methods have been attempted, but the problem remains largely unsolved. For example, empirical correction methods may be used, but their universality is limited because the source of these sensor-related effects and their underlying mechanisms have not been understood. Accordingly, a number of non-trivial issues remain with respect to the development and use of gas analyzers on moving platforms.

[0035] Examples disclosed herein identify a cause of motion-induced error and provide an improve IRGA architecture that reduces such motion-induced error. As described further below, a large fraction of the motion sensitivity is associated with the detection approach common to most conventional closed-path and open-path IRGAs used for H20 and CO2 measurements. Examples provide a sensor architecture that addresses this issue and demonstrates significant improvement.

[0036] According to certain aspects, a dominant source of error is identified as orientationdependent temperature stabilization of a thermoelectrically cooled infrared detector used in the IRGA. As described in more detail below, differences in temperature between the cooled sensing element and surrounding detector housing cause density gradients that in conjunction with the gravitational field create natural free convection currents inside the detector enclosure. These circulation patterns affect the actual temperature of the infrared sensing element and are highly dependent on the orientation of the detector assembly. Accordingly, platform motion that changes the orientation of the detector assembly introduces gas concentration measurement errors as the gas analyzer is rotated with respect to the gravity field. Furthermore, changes in acceleration of the gas may cause similar effects, indicating that in addition to rotating motion, linear displacements may also introduce gas concentration errors.

[0037] To address these issues, examples disclosed herein provide an IRGA (e.g., a closed- path IRGA) with an improved infrared detector having an internal volume that is under vacuum. The use of an evacuated enclosure reduces acceleration and gravity related convection circulations and minimizes convection heat exchange. This allows more precise temperature control and eliminates the temperature dependency on orientation. As described further below, results in laboratory and deep-water tank tests of an IRGA according to certain embodiments show a factor of 4-10 reduction in CO2 error under typical at-sea buoy pitch and roll tilts in comparison to a conventional IRGA system. A similar noise reduction factor of 2-8 is observed in water vapor measurements. Examples demonstrate the ability to achieve improved flux measurements using embodiments of the IRGA disclosed herein on moving ocean observing and aircraft platforms.

[0038] Aspects and embodiments are directed to infrared gas analyzers for measuring atmospheric gases on moving platforms including the use of an evacuated detector to improve performance. According to one example, an infrared gas analyzer includes a measurement module and a detector assembly coupled to the measurement module. The measurement module includes an infrared light source configured to emit an infrared light signal. The detector assembly includes an infrared sensing element and a sealed detector housing having an optical window transparent to the infrared light signal and an internal volume that is under vacuum. The infrared sensing element is disposed within the internal volume of the detector housing and configured to detect the infrared light signal via the optical window.

[0039] These and other features and examples are described in more detail below.

[0040] Referring to FIG. 1, there is illustrated a block diagram of an example of an IRGA 100 according to certain examples. The IRGA 100 includes an electronics module 110 (also referred to as a measurement module) coupled to an infrared detector 120 (also referred to as a detector assembly). In some examples, the electronics module 110 includes a housing 112 that houses an infrared (IR) light source 114 along with various other components generally identified as electronics 116. In some examples, the electronics module 110 and the infrared detector 120 are housed together within a common enclosure 102. The IRGA 100 includes at least one I/O port 104 to allow gases to enter and exit the IRGA 100. In some examples, the electronics 116 includes a pump to draw the gas sample into the IRGA 100. The IRGA 100 may further include one or more power and/or data port(s) 106 coupled to the electronics 116 and configured to allow measurement data to be obtained from the IRGA (e.g., via one or more wired or wireless, analog or digital connections) and/or power and/or configuration data to be supplied to the IRGA 100.

[0041] In some examples, the IR light source 114 is a broadband infrared source. Light from the IR light source 114 is transmitted through a gas sample volume toward the detector 120. The detector 120 may include various components, some examples of which are illustrated in FIG. 2, housed in a sealed detector housing 122 having an optical window 124 to allow entry of infrared light from the IR light source 114. In some examples, the IRGA 100 uses the IR detector 120 to measure the amount of infrared radiation emitted by the IR source 114 that is absorbed by the gas sample when radiation passes through an optical sensing path. By comparing the infrared radiation before and after passing through the optical path the average concentration of the gas between the IR source 114 and the IR detector 120 can be inferred.

[0042] In some examples, the electronics 116 includes a rotating chopper wheel controller, and the light is transmitted through the rotating chopper wheel controller, that modulates the light at a high rate, alternating between signal and dark (null or reference) source levels. In other examples, other types of modulation may be applied to the light (source signal) from the IR light source 114. The IR detector 120, which may be positioned at the receiving end of the volume, is aligned to measure IR radiation changes for the repeating source and dark reference levels, with the detected IR levels changing with gas absorptance due to volumetric change in carbon dioxide and water vapor concentration inside the sample cell. The lowest (raw) level detector measurements within several separate IR bands may provide the demodulated light and dark reference data that can be used to calculate the mixing ratios for both gases.

[0043] Referring to FIG. 2, in some examples, the IR detector 120 includes an IR sensing element (SE) 202. The SE 202 may be coupled to a temperature sensor 204, such as a thermistor, for example. To increase the responsivity of the IR detector 120 and reduce the measurement noise, the IR sensing element 202 is cooled to sub-ambient temperature by a thermo-electric cooling (TEC) device 206. In some examples, the sensing element 202 and the thermistor 204 are mounted on the TEC 206. For environmental protection, the SE 202 and the TEC assembly 206 are enclosed in the sealed detector housing 122. In some examples, the sealed detector housing 122 is filled with dry, IR non-absorbing inert gas, such as nitrogen or xenon. In other examples, as described further below, the sealed detector housing 122 is maintained under vacuum conditions. The TEC 206 converts electrical energy to thermal and transfers heat from one side of the TEC to the other side and creates a temperature difference between the detector SE 202 on the cold side and a heat sink (502; FIGS. 5A and 5B) on the hot side. The heat sink 502 dissipates the thermal energy and keeps the hot side of the TEC close to ambient temperature. The temperature sensor (e.g., thermistor) 204 is mounted next to the SE 202 and used to measure and control the temperature of the SE to a constant value, usually -40 degrees Celsius. This is important to the performance of the IRGA 100 because the spectral response of the IR detector 120 depends on the temperature of the SE 202 and any changes could lead to errors in the gas concentration measurements.

[0044] FIG. 3 is a graph illustrating an example of changes in detector spectral sensitivity as a function of the temperature of the sensing element 202.

[0045] Referring again to FIGS. 1 and 2, in some examples, the coupling 108 between the electronics module 110 and the detector 120 may include one or more mechanical supports, such as struts, to hold the detector 120 in position with respect to the electronics module 110 such that the IR light source 114 is in alignment with the sensing element 202. In certain examples, the detector 120 is configured to receive the light from the IR light source 114 via the optical window 124. Accordingly, the sensing element 202 may be positioned centered under the optical window 124. Further, although not shown in FIG. 1, the housing 112 of the electronics module 110 may include another optical window positioned to allow the light from the IR light source 114 to travel though the optical window toward the detector 120. The coupling 108 may further include any electrical coupling paths needed to connect electrical components of the electronics module 110 and the detector 120. [0046] According to certain aspects, and in contrast to some other proposals, it has been determined that motion-induced error in gas concentration measurements resides primarily in the detector 120. Examples and experimental results described below demonstrate a determined cause of motion sensitivity in the IRGA 100, and improvements obtained using an example of the IRGA 100 configured to address the cause of motion sensitivity.

[0047] According to certain aspects, field trial data obtained from an EC system using a closed- path IRGA for EC CO2 mass flux measurements was analyzed to evaluate motion sensitivity. The field trial data was obtained from an EC system that included a dried sample line and a closed-path LI-7200 sensor (available from LI-COR biosciences, Inc.). The field trial data revealed that discus buoy motions, often much larger and more erratic than encountered on large research vessels, led to greater IRGA CO2 mixing ratio errors than previously reported, and errors that were uncorrectable using some existing empirical post-processing methods. Further investigation of the sensor itself after deployment, as well as an available open-path LI- 7500 unit (available from LLCOR Biosciences, Inc.), led to the observation that much of the CO2 motion contamination signal was correlated with a detector temperature control voltage associated with the receive end of the sensor.

[0048] FIG. 4 is a graph illustrating frequency co-spectra of CO2 molar density and receiver (Rx) control voltage measurements derived from fixed and moving 20 Hz measurement time series collected in similar winds and air-sea CO2 environments. Trace 402 (Cco2 Rx^Pier'^Flxed) represents data obtained from a test in which the buoy was on a pier at the UNH Coastal Marine Laboratory with no motion. Trace 404 (Cco2 Rx^Buoy) represents data obtained from a test in which the buoy was moored 6 miles offshore from the pier under wind-wave forcing. The data sets corresponding to traces 402 and 404 were collected in the coastal Gulf of Maine using an off-the-shelf closed-path IRGA (LI-7200). Wind speed was 7 ms-1 for both cases and the cospectra are derived using 10 minute data segments. Also shown in FIG. 4 are the autospectra of buoy roll (trace 406; SRoii^Buoy) and pitch (trace 408; Spitch^Buoy) motions for the moving case. In these examples, the sample line into the LI-7200 sensor was dried to limit water vapor contamination. The average buoy pitch and roll tilting angles were 8.5 degrees for these 7 ms-1 wind speed conditions, and the tilt spectra in FIG. 1 show the nominal buoy motion frequency band lies between 0.2 and 0.7 Hz. The CO2-Rx voltage covariance under motion within this band is apparent and at least 7 times larger than outside the motion pass band, whereas the fixed platform measurements under similar winds show no obvious correlation with detector control signals. [0049] As described above, aspects and examples provide an IRGA having improved performance on moving platforms. As noted above, evaluation of the measurement data presented in FIG. 4 has led to a determination that a key factor driving IRGA motion-induced noise is not at the transmitter (e.g., IR light source 114) end of the device, but rather resides with the detector 120. As described above, according to certain aspects, a dominant source of error is identified as orientation-dependent temperature stabilization of the thermoelectrically cooled sensing element 202 of the IRGA 100. Due to the differences in temperatures of the sensing element 202 and the detector housing 122, gas parcels in proximity to the cooled sensing element 202 have higher density compared to gas parcels close to the detector housing 122. When the detector housing 122 is filled with gas, these density gradients, in conjunction with the gravitational field, create natural free convection currents inside the detector housing 122, as illustrated in FIGS. 5A and 5B.

[0050] FIG. 5A is a cross-sectional view of an example of the detector housing 122, shown in a vertical orientation, illustrating the temperature gradient between the cold sensing element 202 and the hot detector housing 122 and a hot heat sink 502. Arrows indicate the free convection circulation of cold gas descending near the sensing element 202 and warmer gas rising near the bottom flange 504 (coupled to the heat sink 502) of the detector housing 122. The circulations are symmetrical with respect to the vertical axis 506.

[0051] FIG. 5B is a cross-sectional view of the detector housing 122 with the detector 120 oriented 90 degrees from vertical, illustrating the temperature gradient between the cold sensing element 202 and the hot detector housing 122 and heat sink 502.

[0052] The circulation patterns that occur within the detector housing 122 are highly dependent on the orientation of the detector 120, and affect the actual temperature of the temperature sensor 204, and thus the actual temperature of the sensing element 202. For example, referring to FIG. 5B, in this case the space above the sensing element 202 has stable stratification and limited free convection while the space below the sensing element 202 has unstable density stratification and enhanced free convection. As a result, the upper and the lower portions of the sensing element 202 have different temperatures. Consequently, depending on the location of the thermistor 204, the sensing element 202 may be controlled to different temperature, causing errors in the gas concentration measurement. Thus, platform motion that changes the orientation of the detector 120 introduces gas concentration measurement errors as the motion alters the circulation currents and thus the actual temperature of the sensing element 202.

[0053] Accordingly, to address this issue, in some examples, the detector 120 can be constructed with the inner volume of the detector housing 122 being evacuated rather than filled with a gas. In some examples, the inner volume of the detector housing 122 may be evacuated to less than 2E9 Torr. This prevents any acceleration and gravity related convection circulations and minimizes convection heat exchange allowing more precise temperature control of the sensing element 202 and eliminating or reducing the temperature dependency on orientation of the detector 120.

[0054] According to certain aspects, an open-path or closed-path IRGA 100 can be constructed using an evacuated detector housing 122. Other mechanical and/or electrical components and configurations of the IRGA 100 may remain the same. To demonstrate the temperature and orientation dependence of motion sensitivity, and improvements obtained using an IRGA with an evacuated detector 120, various experiments were performed and the results are presented below. In particular, experimental data is described below that demonstrates and compares the performance of an example closed-path IRGA with an infrared detector having an evacuated detector housing to standard models of commercially available IRGAs measuring CO2 and H2O. Tilt experiments with side-by-side mounted IRGAs were conducted on a controlled laboratory platform with independent pitch and roll axes. Over the ± 30 degree range of angular position, the orientation-correlated errors were reduced, in the example IRGA, by a factor of 4 to 10 on CO2 and a factor of 2 to 8 on H2O compared to the standard IRGA. Additional testing was performed duplicating realistic buoy motion in a deep-water tank with typical at-sea combined pitch and roll motion, as described below. In these tests, improvements in the measurement errors were similar to the laboratory experiments.

[0055] To demonstrate improved performance of an IRGA including an evacuated detector according to certain examples, a motion-induced error MIE) metric is formulated to accurately quantify the level of motion-induced CO2 or H20 error. MIE is defined using the ratio of signal variation when under motion to measurements collected with no motion, i.e., the static case, as shown in Equation (1) below. According to certain examples, the calculation is performed in the spectral domain across the motion frequency pass band (e.g., frequencies between /Hi and fLo) encountered in given conditions. This accounts for the inherent noise of each individual sensor when the platform is fixed. An MIE level approaching one means no motion impact on a given measurement.

[0056] In Equation (1), S(f) is either the CO2 or H2O concentration spectral density for a given motion test segment. MIE is calculated for each of many 60-120 s measurement test segments, li each having a specified time duration (e.g., in a range of 60 - 120 seconds), where the platform tilt standard deviation for each segment is computed according to Equation (2) below.

As described further below, MIE calculations performed using experimental test data demonstrate that an example IRGA according to certain embodiments has significantly improved performance relative to a standard IRGA of similar configuration.

[0057] Accordingly, aspects and embodiments provide an IRGA in which improved measurement accuracy can be obtained for moving platform based measurements. As described above, in certain examples, the IRGA includes a detector assembly that has a thermoelectrically cooled sensing element disposed, along with a temperature sensor, in a sealed, evacuated housing. By implementing the cooled sensing element under vacuum, orientation-dependent (e.g., motion-induced) temperature-based errors can be reduced or eliminated, thereby providing an IRGA with improved performance on moving platforms.

Experimental Results

[0058] Various comparative measurements were acquired using a standard, commercially available IRGA and a modified IRGA having an evacuated detector chamber, according to certain examples disclosed herein (referred to herein as the “example IRGA” or EC155P). The standard IRGA was an EC 155 unit available from Campbell Scientific Inc.(CSI), as is referred to herein as the EC 155 or standard IRGA.

[0059] The EC 155 is a closed-path IRGA system having a sensor configured to provide high rate and high precision CO2 and H20 mixing ratio measurements, where the desired data sampling rates are 5-20 Hz and the specified CO2 and H20 measurement precision is 0.15 j mol mol-1 and 0.006 mmol mol-1 respectively. The EC 155 also provides high precision temperature and pressure measurements inside the closed-path sample cell to adjust for ambient impacts on the flow when converting from the raw molar concentration measurements to mixing ratio. While the commercially available EC 155 field sample air collector employs a vortex intake device, this intake was bypassed in tests producing the measurement data presented herein to allow sole focus on motion-induced noise associated with the analyzer itself. EC155 control, transducer sampling, and data conversions were performed using a CSI EC 100 electronics unit, and a CSI CR6 data acquisition system was used to collect the measurements for all experiments. [0060] In operation of the EC 155 analyzer, light from a broadband infrared source is transmitted through a gas sample volume via a rotating chopper wheel controller that modulates the source signal at a high rate, alternating between signal and dark (null or reference) source levels. As in some examples described above, an IR detector at the receiving end of the volume is aligned to measure IR radiation changes for the repeating source and dark reference levels, with the detected IR levels changing with gas absorptance due to volumetric change in carbon dioxide and water vapor concentration inside the sample cell. The lowest (raw) level detector measurements within several separate IR bands provide the demodulated light and dark reference data used to calculate the mixing ratios for both gases.

[0061] The example IRGA used to acquire measurement data presented herein was an altered version of the EC 155 analyzer having a constructure that improves detector thermal performance. In particular, the altered version of the EC 155 analyzer included a detector having an internal volume that was under vacuum. This modification was implemented without further changes to the established EC155 mechanical and opto-electrical design, data handling and output, and sensor control. This example unit is denoted as EC155P in this disclosure. The limited scope of modifications means that side-by-side evaluation of the example EC155P versus the standard EC155 unit is straightforward, simplifying the quantification of sensor performance differences.

[0062] One of two inertial motion measurement packages was employed to log platform motion data in these tests (Parker Lord Microstrain 3DM-GX3 or 3DM-GX5). Either provides dynamic attitude data at a 20 Hz sampling rate coincident with the IRGA measurements. The motion sensor was aligned and mounted on the rigid IRGA test plate adjacent to the analyzers. All platform pitch, roll, and acceleration estimates were derived using standard post processing approaches employed at sea. All recorded datasets include continuous high-rate measurements of all standard EC 155 output variables including cell temperature and pressure, additional low- level EC155 engineering outputs, and motion sensor outputs including three-axis accelerations and angular rotation rates. Most data presented were recorded at a 20 Hz sampling rate. EC 155 and EC155P CO2 and H20 mixing ratio outputs were low-pass filtered prior to data logging using the recommended EC 155 bandwidth of 10 Hz. The sensor error evaluation involved a simple approach of measuring a continuous stream of nearly pure (dry) CO2 reference gas flowing through the system, typically at a controlled 0.7-1.5 1pm flow rate.

[0063] Evaluation of prototype differences with the standard EC 155 sensor was primarily made via direct comparison of time series measurements or derived noise variance of the CO2 and H20 mixing ratio data observed in frequency ranges associated with the platform motion from the simultaneously-recorded motion sensors.

[0064] Simple two-axis and dynamic three axis motion table tests were employed to diagnose, improve, and evaluate EC155 measurement noise associated with platform tilting motion having magnitudes and frequencies expected at sea. In all tests, sensor CO2 and H20 mixing ratio errors were assessed by sampling intake gas having a known and fixed CO2 concentration drawn through the IRGA measurement sample cell at a flow rate typical for field measurements. The CO2 reference gas was dry (effectively no water vapor) and the CO2 concentration was either 500 or 520 ppm. The two axis motion tests were first performed in order to individually evaluate rotation (pitch or roll) effects. The laboratory motion table included a stiff 1.2 x 1.2 meter (m) plate with one free axis of rotation about the center, and where both the EC 155 and EC155P were mounted side by side, centered atop that rotational axis and in the same horizontally-mounted orientation along with the motion sensor. To assess motion impacts for the orthogonal tilt (i.e. roll instead of pitch), the table was physically -rotated on the stand by 90 degrees. The sensor input sample gas was plumbed in series, running through the EC155P and then the EC 155. All measurements were recorded simultaneously using the CSI CR6 data logger.

[0065] A series of rotation tests were conducted at varying pitch and roll levels to assemble IRGA measurement datasets that span a range of expected field tilt amplitudes (from 0-20 deg. in pitch and roll typical of discus buoy platforms), and using an average rotation rate of 0.33 Hz that lies near the fundamental resonance frequency of our specific air-sea flux buoy. The footprint of each EC155 is rectangular (7 x 43 centimeters (cm)), and in the testing framework a roll-impacted measurement implies rotation about the long axis and pitch about the short axis. This two axis (2D) test was fashioned after a motion impact assessment of LI-COR IRGA units performed at NOAA’s Earth System Research Laboratories in 2010 by L. Bariteau.

[0066] FIGS. 6A-D illustrates pitch-induced error observed in both CO2 (FIGS. 6A and 6C) and water vapor (FIGS. 6B and 6D) when using the standard EC 155 in these 2D table tests. FIGS. 6 A and 6B illustrate CO2 and H2O, respectively, molar mixing ratio variation with time, while platform pitch (represented by trace 606 in FIG. 6A and trace 612 in FIG. 6B) is modulated. FIGS. 6A and 6B show a short test segment with a few platform oscillations is shown. In FIG. 6A, trace 602 (CO2mr) represents the CO2 mixing ratio data at the raw 20 Hz rate, and trace 604 (CO2mr LP) represents the CO2 mixing ratio data after low pass filtering. In FIG. 6B, trace 608 (H20mr) represents the H2O mixing ratio data at the raw 20 Hz rate, and trace 610 (H20mr LP) represents the H2O mixing ratio data after low pass filtering. In both FIGS. 6A and 6B, the mean C02 and H20 levels have been subtracted to focus on the relevant small-scale signal variations. The platform rotation rate was roughly 0.35 Hz.

[0067] FIGS. 6C and 6D provide scatter plots of the low-passed mixing ratio data versus the instantaneous tilt angle for CO2 (FIG. 5C) and H2O (FIG. 5D) measurements. Pitch variations are +/- 15 degrees.

[0068] The results presented in FIGS. 6A-D clearly show high correlation between platform tilt and mixing ratio measurements. This test was for repeated pitch motions of +/- 14 degrees . Higher frequency (> 1 Hz) CO2 sensor noise is evident and the measured root-mean-square (rms) noise level is 0.047 ppm. The FIG. 6A pitch-induced signal amplitude estimated using the smoothed curve is near 0.12 ppm peak-to-peak, a factor of at least 2 above the noise. The EC 155 motion-induced signal is clear. The variations are also consistent with, but in this case smaller than, the 0.5-2.0 ppm signals noted in some previous field studies. A similar tilt-related increase above the noise level is observed for H20, as shown in FIG. 6B. The high correlation between the motion and IRGA error suggests linearity; however, FIGS. 6C and 6D show an apparent systematic hysteresis, more evident in the CO2 than the water vapor in this case.

[0069] FIGS. 7A-D show similar data but for roll rotations (tilting the EC155 side to side). FIGS. 7 A and 7B illustrate CO2 and H2O, respectively, molar mixing ratio variation with time, while platform roll (represented by trace 706 in FIG. 7A and trace 712 in FIG. 7B) is modulated. In FIGS. 7A and 7B, both the raw 20 Hz (traces 702, CO2mr; and 708, H20mr) and low-pass filtered (traces 704, CO2mr LP; and 710, H20mr LP) data are shown for CO2 and H2O, respectively. In both FIGS. 7A and 7B, the mean CO2 and H20 levels have been subtracted.

[0070] FIGS. 7C and 7D provide scatter plots of the low-passed mixing ratio data versus the instantaneous roll angle for CO2 (FIG. 7C) and H2O (FIG. 7D) measurements. Roll variations are +/- 15 degrees.

[0071] In this case, the roll-induced CO2 signal amplitude is similar, but the hysteresis differs with CO2 error vs. pitch results. As seen in FIG. 7C, the CO2 variation with the roll is more linear. In both pitch and roll cases it is evident that EC 155 water vapor and CO2 motion- induced errors do not track with the induced motions in the same manner.

[0072] It was observed that hysteresis between the mixing ratio and tilt angle time series data becomes increasingly nonlinear once 3D motions and more varied platform motion frequencies are allowed. This implies that simple tilt-related data correction approaches would be problematic even on less dynamic platforms, for example those subject to mostly pitching motions such as a glider or ship or for an aircraft to roll. [0073] A similar fixed plate configuration was used for three axis motion tests intended to more closely simulate field measurement conditions. The two sensor test plate was mounted directly onto the center of a 2 m discus buoy platform. A CO2 reference gas tank was strapped to the buoy and the entire platform was then floated in an 8 m deep ocean instrument test tank located in the Chase Engineering Laboratory at the Univ, of New Hampshire. Both periodic and irregular platform motions were induced by coordinated manipulation of the buoy from multiple sides with the buoy centered in the large tank. The approach allowed simulated time series data collection with platform pitch and roll tilting rates near the nominal buoy resonant frequency (f = 0.33 Hz) and with variation in the mean level of tilt. Tests with mean pitch and roll variations in the range of 3 to 15 degrees were performed reflective of low to high wind conditions measured in recent buoy deployments, where a root mean squared (rms) tilt angle, atilt, of 8 degrees is nominal for wind speeds of 9 m/s. Mixed (or confused) sea conditions more typical of actual adverse field situations were also achieved with this setup by more random forcing of the platform. Typically two to three minutes of continuous data were collected for a given motion test (e.g. atilt= 5 degrees in roll). This permits characterization of motion-induced noise at different mean tilt angles.

[0074] The EC 155 provides high rate measurements of the temperature associated with the thermoelectrically-cooled (TEC) IR sensor detector. FIGS 8A and 8B show standard model EC 155 data from short 20 second measurement segments during separate buoy pitch (FIG. 8 A) and roll (FIG. 8B) motion tests (f ~ 0.35 Hz) in the deep water tank. The time series data presented in FIGS. 8A and 8B is similar to FIGS. 6A, 6B, 7A, and 7B, but only showing CO2 IRGA data and with the addition of time variation in EC 155 detector temperature after subtracting its mean level. Average pitch and roll variations are +/- 10 to 12 degrees. FIG. 8A corresponds to is pitch-only motion and FIG. 8B corresponds to roll-only motion. In FIG. 8A, trace 802 (CO2mr) represents the CO2 mixing ratio data at the raw 20 Hz rate, trace 804 (CO2mr LP) represents the CO2 mixing ratio data after low pass filtering, trace 806 (Detector Temp) represents the detector temperature, and trace 808 (Pitch) represents the pitch. In FIG. 8B, trace 810 (CO2 mr) represents the CO2 mixing ratio data at the raw 20 Hz rate, trace 812 (CO2mr LP) represents the CO2 mixing ratio data after low pass filtering, trace 814 (Detector Temp) represents the detector temperature, and trace 814 (Roll) represents the roll.

[0075] Both tests show EC 155 detector temperature data (traces 806, 810) has frequency variations with amplitude of 0.1-0.2°C that are visibly elevated in the roll tests (FIG. 8B). The variations are anti correlated with tilt, but slightly out of phase with the motion. There is also an apparent detector temperature correlation, but also a phase shift with respect to the CO2 measurement change, most evident in the smoothed CO2 signal.

[0076] This motion-correlated change in CO2 and H20 data was observed versus detector temperature in all platform tilt and acceleration tests, but with varying amplitude and phase shifts. Empirically, it was clear that there was fairly high nonlinearity between the control temperature, the motion, and the trace gas measurements. This is likely related to the observed hysteresis between mixing ratio and platform tilt variation shown in FIGS. 6C, 6D, 7C, and 7D.

[0077] The EC155P was modified to address this issue. Results from side-by-side tests of EC155P vs. EC155 performance are shown in FIGS. 9A and 9B. FIGS. 9A and 9B present time series data, for both the EC 155 and the EC155P, showing band-pass filtered variation in CO2 and detector temperature data after subtracting mean levels. Pitch test data are shown in FIG. 9A and roll test data is shown in 9B. Variations are +/- 10 to 12 degrees. In FIG. 9A, trace 902 (CO2 EC155) represents the CO2 mixing ratio data for the EC155, trace 904 (CO2 EC155P) represents the CO2 mixing ratio data for the EC155P, trace 906 (DetT_EC155) represents the detector temperature of the EC 155 detector, and trace 908 (DetT_EC155P) represents the detector temperature of the EC155P detector. Similarly, in FIG. 9B, trace 910 (CO2 EC155) represents the CO2 mixing ratio data for the EC155, trace 912 (CO2 EC155P) represents the CO2 mixing ratio data for the EC155P, trace 914 (DetT_EC155) represents the detector temperature of the EC 155 detector, and trace 916 (DetT_EC155P) represents the detector temperature of the EC155P detector.

[0078] The dramatic decrease in EC155P TEC temperature variation relative to the EC155 is clear. In fact, the EC155P appears to be 0.0 as there is effectively no measurable high rate TEC variation in the example unit, while the EC 155 again shows 0.1 °C variations. This test data also demonstrates an improvement in motion -related EC155P CO2 mixing ratio signal. While not completely removed, the CO2 signal variation at the tilting frequencies is reduced by at least a factor of 4 to 5. There is also a clear phase shift in the motion impacts on CO2 between the EC 155 and EC155P indicating a fundamental change between the two sensors. One additional observation is that larger EC155 TEC and CO2 variations are observed under pitch motions than for roll, consistent with FIGS. 8A and 8B. Similar impacts are observed in the H20 channel data (not shown). This complete attenuation of TEC temperature variation correlated with sensor motion was observed across the full range of tilt motion tests conducted.

[0079] The EC155P results confirm the determination disclosed herein that improved detector stabilization leads to reduced motion contamination in IRGA CO2 and H2O measurements. [0080] To further present EC155P performance change with respect to the standard EC 155 unit, MIE calculations were performed using Equations (1) and (2) described above. In some examples, the MIE calculation was performed in the spectral domain over the motion frequency pass band encountered in the buoy wave tank tests ( fLo = 0.2 to fHi = 0.6Hz ) described herein. Example spectra for one mixed sea motion test set are shown in FIG. 10. FIG. 10 illustrates autospectra of CO2 measurements during both static platform and mixed sea motion tests. The platform atilt for this segment was 10 degrees. Frequency band pass limits for MIE calculations are shown with dashed lines. In FIG. 10, trace 1002 (EC155PMotion) represents the autospectrum of CO2 measurements performed using the example EC155P unit during mixed sea motion tests. Trace 1004 (EC155Pstatic) represents the autospectrum of CO2 measurements performed using the example EC155P unit during static platform tests. Trace 1006 (EC 155 Motion) represents the autospectrum of CO2 measurements performed using the standard EC155 unit during mixed sea motion tests, and trace 1008 (EC155static) represents the autospectrum of CO2 measurements performed using the standard EC 155 unit during static platform tests. The large EC155 CO2 signal increase in the pass band versus the static case is evident, while the level for the example IRGA (EC155P) is much closer to its noise floor. Note that static-case EC155P noise level slightly exceeds the EC 155, even in the pass-band. As one measure of the difference, the rms CO2 noise level above 1 Hz for the EC155 is 0.047 while the EC155P is 0.068 ppm. This is a known limitation of the example unit that is unrelated to motion improvement goals and could be improved in the future.

[0081] Summary of side by side sensor measurements under varied motion is provided in FIGS. 11 A, 11B, 12A, and 12B including separate pitch, roll, and mixed sea segments. FIGS. 11A and 11B illustrate a summary of CO2 motion-induced error during tank measurements. Each test segment had a differing mean tilt amplitude (atilt ). Pitch or roll motion test results are shown in FIG. HA for both the EC155 and EC155P. FIG. 11B shows the mixed sea motion results under increasing tilt. MIE, the effective motion-related error factor above the noise floor, is defined in Equation (1) described above. Buoy tilting motions are always present and thus the platform never provides atilt levels below 3-4 degrees.

[0082] Referring to FIGS. 11 A and 1 IB, the largest sensor error is seen for pitch motion in the EC155, with MIE increasing from 4-11 for tilt amplitudes of 5-15 degrees. This is 3-4 times greater error than seen for EC 155 roll motion error. EC 155 error shows a quasi-linear increase with increasing tilt amplitude for pitch, roll or mixed motions. The difference between the EC155 and the EC155P is apparent in all cases. The EC155P has much lower MIE that never exceeds 1.1 and there is no evident MIE increase with increasing atilt. The mixed sea test data FIG. 1 IB represents the net effect of anticipated buoy tilt impacts on CO2 error in the field. One measure of improvement of the example IRGA is the ratio of the EC155 to EC155P signal for a given motion test segment, i.e. MIEECIH/MIEECIHP. Using this, the observed EC155P improvement versus the EC155 is 3.6-6.2 times for atilt below 8 degrees and as much as 10.4 for the highest tilt. The average improvement factor is 5.2.

[0083] Water vapor channel data are shown in FIGS. 12A and 12B for the same test series as the data presented in FIGS. 11 A and 1 IB. Pitch or roll motion test results are shown in FIG. 12A for both the EC 155 and EC155P, and FIG. 12B shows the mixed sea motion results under increasing tilt. The data presented in FIGS. 12A and 12B demonstrate overall EC155P improvement by a factor of 2-8, but the results differ somewhat from the CO2 data. As shown in FIG. 12A, the EC155 pitch-induced H20 response is similar to the CO2 data in FIG. 11 A. But the roll-induced signal is significantly elevated compared to the CO2. Second, H20 measurement error in the example IRGA is also evident and it also increases with atilt, though at a much lower overall level than the EC155. Pitch test EC155P MIE ranges from just above 1.3 at atilt of 5.5 degrees to 3.2 at atilt above 14 degrees. EC155P roll error is roughly 3 times smaller than pitch error. The overall H20 improvement of the example IRGA versus the EC 155 is again quantified using the mixed sea test results shown in FIG. 12B. These results indicate that the average EC155P improvement level is 3.5, ranging from 1.6-8.5 across the varied motion amplitude tests.

[0084] Thus, aspects and examples demonstrate that detector modifications (for example, replacing a standard infrared detector assembly with an infrared detector assembly having an evacuated internal volume) to a chopper-wheel type IRGA reduces the motion-induced error due to platform tilt by a factor of 3.6 to 10.4 in the CO2 channel and by 1.6 to 8.5 in the water vapor channel. The EC155P unit also showed no measurable increase in CO2 error as platform tilt amplitudes were increased, or when varying the orientation of the applied rotations. This significant level of improvement suggests that the closed-path IRGA approach to trace gas measurements may yet be able to attain CO2 field measurement precision approaching that of the cavity ring down spectrometer and permit accurate CO2 mass flux measurements when the air-sea pCO2 disequilibrium is well below the presently assumed IRGA EC flux baseline level of 40-50 //Atm level. Data presented herein indicate that platform motion impacts on a standard IRGA (EC155) measurement error have several clear characteristics. Error increases with platform tilt amplitude, it varies with the tilt direction relative to sensor mounting orientation, it varies non-linearly with platform tilt, and it shows a correlation with sensor detector temperature control data.

[0085] While most of the presented data focused on CO2 measurements, these four characteristics were similarly observed in the H20 data. Each of the first three characteristics have been mentioned or alluded to in previous literature discussing platform motion effects. In particular, data presented herein that demonstrates significantly enhanced error amplitude under pitch as compared to roll rotations is consistent with the understanding that one should optimize the sensor mounting orientation to limit error on any particular platform or field deployment. The example EC155P unit discussed herein removes or significantly reduces nearly all of these error-related characteristics.

Additional Examples

[0086] The following examples pertain to further embodiments, from which numerous permutations and configurations will be apparent.

[0087] Example 1 is an infrared gas analyzer that includes a measurement module and a detector assembly coupled to the measurement module. The measurement module includes an infrared light source configured to emit an infrared light signal. The detector assembly includes an infrared sensing element and a sealed detector housing having an optical window transparent to the infrared light signal and an internal volume that is under vacuum. The infrared sensing element is disposed within the internal volume of the detector housing and configured to detect the infrared light signal via the optical window.

[0088] Example 2 is an infrared gas analyzer comprising a measurement module including an infrared light source, and a detector assembly coupled to the measurement module. The detector assembly includes a sealed detector housing having an internal volume that is under vacuum, a sensing element, a temperature sensor, and a thermoelectric cooler, wherein the sensing element, the temperature sensor, and the thermoelectric cooler are disposed within the internal volume of the detector housing, and wherein the sensing element and the temperature sensor are coupled to the thermoelectric cooler.

[0089] Example 3 includes the infrared gas analyzer of Example 2, wherein the infrared gas analyzer is a closed-path infrared gas analyzer.

[0090] Example 4 includes the infrared gas analyzer of one of Examples 2 or 3, wherein the measurement module includes a housing having a first optical window, wherein the detector housing has a second optical window, and wherein the detector assembly is positioned with respect to the measurement module, the infrared light source is positioned within the housing and with respect to the first optical window, and the sensing element is positioned with respect to the second optical window, such that light emitted from the infrared light source is directed toward the sensing element via the first and second optical windows.

[0091] Example 5 includes the infrared gas analyzer of Example 4, further comprising a coupling mechanism that couples the detector assembly to the measurement module, wherein the coupling mechanism includes one or more mechanical coupling elements configured to hold the detector assembly in position with respect to the measurement module.

[0092] Example 6 includes the infrared gas analyzer of one of Examples 4 or 5, wherein the sensing element is positioned centered with respect to the second optical window.

[0093] Example 7 includes the infrared gas analyzer of any one of Examples 2-6, wherein the internal volume of the detector housing is evacuated to less than 2E9 Torr.

[0094] Example 8 includes the infrared gas analyzer of any one of Examples 2-7, wherein the thermoelectric cooler is configured to cool the sensing element to a temperature of approximately -40 degrees Celsius.

[0095] Example 9 includes the infrared gas analyzer of any one of Examples 2-8, wherein the temperature sensor is a thermistor.

[0096] Example 10 includes the infrared gas analyzer of any one of Examples 2-9, further comprising an assembly housing having at least one gas inlet port, wherein the measurement module and the detector assembly are disposed within the assembly housing.

[0097] Example 11 is a detector assembly for an infrared gas analyzer, the detector assembly comprising a sealed detector housing having an internal volume that is under vacuum, the sealed detector housing including an optical window that is transparent to infrared radiation, an infrared sensing element disposed within the internal volume of the sealed detector housing and positioned to detect an infrared light signal via the optical window, a thermoelectric cooler disposed within the internal volume of the sealed detector housing and configured to cool the infrared sensing element, and a temperature sensor coupled to the thermoelectric cooler and configured to provide a temperature measurement, wherein the thermoelectric cooler is configured to regulate a temperature of the infrared sensing element to a selected temperature based on the temperature measurement.

[0098] Example 12 includes the detector assembly of Example 11, wherein the internal volume of the sealed detector housing is evacuated to less than 2E9 Torr.

[0099] Example 13 includes the detector assembly of one of Examples 11 or 12, wherein the selected temperature is -40 degrees Celsius.

[0100] Example 14 includes the detector assembly of any one of Examples 11-13, wherein the sensing element and the temperature sensor are mounted to the thermoelectric cooler. [0101] Example 15 includes the detector assembly of any one of Examples 11-14, wherein the temperature sensor is a thermistor.

[0102] Example 16 includes the detector assembly of any one of Examples 11-15, wherein the sensing element is positioned centered with respect to the optical window.

[0103] Example 17 is a closed-path infrared gas analyzer comprising a measurement module and a detector assembly coupled to the measurement module. The measurement module includes a housing having a first optical window transparent to infrared radiation, and an infrared light source configured to emit an infrared light signal, the infrared light source being disposed within the housing and positioned to direct the infrared light signal through the first optical window. The detector assembly includes a sealed detector housing having an internal volume that is under vacuum, the sealed detector housing including a second optical window transparent to the infrared radiation, an infrared sensing element disposed within the internal volume of the sealed detector housing and positioned to detect the infrared light signal via the second optical window, a thermoelectric cooler disposed within the internal volume of the sealed detector housing and configured to cool the infrared sensing element, and a temperature sensor coupled to the thermoelectric cooler and configured to provide a temperature measurement, wherein the thermoelectric cooler is configured to regulate a temperature of the infrared sensing element to a selected temperature based on the temperature measurement.

[0104] Example 18 includes the closed-path infrared gas analyzer of Example 17, further comprising an assembly housing having at least one gas inlet port, wherein the measurement module includes a pump configured to draw a gas sample into the assembly housing via the at least one gas inlet port, and wherein the measurement module and the detector assembly are disposed within the assembly housing and arranged such that the infrared light source directs the infrared light signal to the sensing element through the gas sample.

[0105] Example 19 includes the closed-path infrared gas analyzer of Example 18, wherein the gas sample includes at least one of CO2 or H2O.

[0106] Example 20 includes the closed-path infrared gas analyzer of any one of Examples 17-

19, further comprising one or more mechanical coupling elements configured to couple the detector assembly to the measurement module and to hold the detector assembly in alignment with respect to the measurement module.

[0107] Example 21 includes the closed-path infrared gas analyzer of any one of Examples 17-

20, wherein the internal volume of the sealed detector housing is evacuated to less than 2E9 Torr. [0108] Example 22 includes the closed-path infrared gas analyzer of any one of Examples 17-

21, wherein the sensing element and the temperature sensor are mounted to the thermoelectric cooler.

[0109] Example 23 includes the closed-path infrared gas analyzer of any one of Examples 17-

22, wherein the infrared light source is a broadband infrared source.

[0110] Example 24 includes the closed-path infrared gas analyzer of any one of Examples 17-

23, wherein the temperature sensor is a thermistor.

[0111] Example 25 includes the closed-path infrared gas analyzer of any one of Examples 17-

24, wherein the selected temperature is -40 degrees Celsius.

[0112] Having described herein several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of this disclosure. Accordingly, the description and drawings of various embodiments are presented by way of example only. These examples are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. In addition, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Any references to examples, components, elements, or acts of the systems and methods herein referred to in the singular can also embrace examples including a plurality, and any references in plural to any example, component, element or act herein can also embrace examples including only a singularity. References in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. The use herein of “including”, “comprising”, “having”, “containing”, “involving”, and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” can be construed as inclusive so that any terms described using “or” can indicate any of a single, more than one, and all of the described terms.

Claims

1. An infrared gas analyzer comprising: a measurement module including an infrared light source; and a detector assembly coupled to the measurement module, the detector assembly including a sealed detector housing having an internal volume that is under vacuum, a sensing element, a temperature sensor, and a thermoelectric cooler, wherein the sensing element, the temperature sensor, and the thermoelectric cooler are disposed within the internal volume of the detector housing, and wherein the sensing element and the temperature sensor are coupled to the thermoelectric cooler.

2. The infrared gas analyzer of claim 1, wherein the infrared gas analyzer is a closed- path infrared gas analyzer.

3. The infrared gas analyzer of claim 1, wherein the measurement module includes a housing having a first optical window; wherein the detector housing has a second optical window; wherein the detector assembly is positioned with respect to the measurement module, the infrared light source is positioned within the housing and with respect to the first optical window, and the sensing element is positioned with respect to the second optical window, such that light emitted from the infrared light source is directed toward the sensing element via the first and second optical windows.

4. The infrared gas analyzer of claim 3, further comprising a coupling mechanism that couples the detector assembly to the measurement module, wherein the coupling mechanism includes one or more mechanical coupling elements configured to hold the detector assembly in position with respect to the measurement module.

5. The infrared gas analyzer of claim 3, wherein the sensing element is positioned centered with respect to the second optical window.

6. The infrared gas analyzer of claim 1, wherein the internal volume of the detector housing is evacuated to less than 2E9 Torr.

7. The infrared gas analyzer of claim 1, wherein the thermoelectric cooler is configured to cool the sensing element to a temperature of approximately -40 degrees Celsius.

8. The infrared gas analyzer of claim 1, wherein the temperature sensor is a thermistor.

9. The infrared gas analyzer of claim 1, further comprising an assembly housing having at least one gas inlet port; wherein the measurement module and the detector assembly are disposed within the assembly housing.

10. A detector assembly for an infrared gas analyzer, the detector assembly comprising: a sealed detector housing having an internal volume that is under vacuum, the sealed detector housing including an optical window that is transparent to infrared radiation; an infrared sensing element disposed within the internal volume of the sealed detector housing and positioned to detect an infrared light signal via the optical window; a thermoelectric cooler disposed within the internal volume of the sealed detector housing and configured to cool the infrared sensing element; and a temperature sensor coupled to the thermoelectric cooler and configured to provide a temperature measurement, wherein the thermoelectric cooler is configured to regulate a temperature of the infrared sensing element to a selected temperature based on the temperature measurement.

11. The detector assembly of claim 10, wherein the internal volume of the sealed detector housing is evacuated to less than 2E9 Torr.

12. The detector assembly of claim 10, wherein the selected temperature is -40 degrees Celsius.

13. The detector assembly of claim 10, wherein the sensing element and the temperature sensor are mounted to the thermoelectric cooler.

14. The detector assembly of claim 10, wherein the temperature sensor is a thermistor.

15. A closed-path infrared gas analyzer comprising: a measurement module including: a housing having a first optical window transparent to infrared radiation, an infrared light source configured to emit an infrared light signal, the infrared light source being disposed within the housing and positioned to direct the infrared light signal through the first optical window; and a detector assembly coupled to the measurement module and including: a sealed detector housing having an internal volume that is under vacuum, the sealed detector housing including a second optical window transparent to the infrared radiation, an infrared sensing element disposed within the internal volume of the sealed detector housing and positioned to detect the infrared light signal via the second optical window, a thermoelectric cooler disposed within the internal volume of the sealed detector housing and configured to cool the infrared sensing element, and a temperature sensor coupled to the thermoelectric cooler and configured to provide a temperature measurement, wherein the thermoelectric cooler is configured to regulate a temperature of the infrared sensing element to a selected temperature based on the temperature measurement.

16. The closed-path infrared gas analyzer of claim 15, further comprising: an assembly housing having at least one gas inlet port; wherein the measurement module includes a pump configured to draw a gas sample into the assembly housing via the at least one gas inlet port; and wherein the measurement module and the detector assembly are disposed within the assembly housing and arranged such that the infrared light source directs the infrared light signal to the sensing element through the gas sample.

17. The closed-path infrared gas analyzer of claim 15, further comprising: one or more mechanical coupling elements configured to couple the detector assembly to the measurement module and to hold the detector assembly in alignment with respect to the measurement module.

18. The closed-path infrared gas analyzer of claim 15, wherein the internal volume of the sealed detector housing is evacuated to less than 2E9 Torr.

19. The closed-path infrared gas analyzer of claim 15, wherein the sensing element and the temperature sensor are mounted to the thermoelectric cooler.

20. The closed-path infrared gas analyzer of claim 15, wherein the infrared light source is a broadband infrared source. 1