Abstract:
Gas analyzer systems and methods for determining gas flux in a short intake tube configuration without using any Webb-Pearman-Leuning density correction. Gas analyzer systems and methods for measuring concentrations of gasses and in particular dry mole fraction of components of a gas. The systems and method allow for rapid measurement of the gas density and/or dry mole fraction of gases for a number of environmental monitoring applications, including high speed flux measurements. A novel coupling design allows for tool-free removal of a cell enclosing a flow path to enable in field cleaning of optical components.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation of pending U.S. application Ser. No. 12/266,379 filed Nov. 6, 2008 and is incorporated herein in its entirety for all purposes. This application is related to pending U.S. application Ser. No. 12/508,003 filed Jul. 23, 2009 and is incorporated herein in its entirety for all purposes. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to gas analysis, and more particularly to systems and methods for measuring concentrations of gases. 
         [0003]    The increasing carbon dioxide concentration in the atmosphere and the resulting greenhouse effect and climate change have become important topics for scientific research. In order to understand the global carbon balance, it is necessary to determine the rate at which carbon dioxide and energy exchanges between the atmosphere and terrestrial and oceanic ecosystems. A measurement technique called “eddy covariance” has been widely used to determine these rates. The air within a few hundred meters above the earth surface is mostly turbulent, so that turbulent structures (vortices of variable sizes) called “eddies” are responsible for the vertical transport of the most of the gases, including carbon dioxide and water vapor, and also heat and momentum between the surface and the atmosphere. The rates of such transport can be calculated from simultaneous, high-frequency measurements of the vertical component of wind speed, the concentrations of carbon dioxide and water vapor, and the air temperature. 
         [0004]    To measure concentrations of carbon dioxide and water vapor, a gas analyzer can be used to analyze the transmittance of light in appropriate wavelength bands through a gas sample. With some gas analyzers, a sample gas containing unknown gas concentrations of carbon dioxide and water vapor is placed in a sample cell, and a reference gas with zero or known concentrations of carbon dioxide and water vapor is placed in a reference cell. The analyzer measures the unknown gas concentrations in the sample cell from calibrated signals that are proportional to the difference between light transmitted through the sample cell and light transmitted through the reference cell. 
         [0005]    In eddy covariance applications, ambient air that is full of dust and pollen must be moved through the analyzer at high flow rates in order for the analyzer to have the necessary frequency response. Even when the air is filtered, contamination of the sample cells is to be expected especially during long deployments, requiring the analyzer to be returned to the factory for cleaning. This is an expensive and time-consuming process, especially when the analyzer is used in a remote location such as the Amazon basin, the north slope of Alaska, or the deserts of Africa. 
         [0006]    There is a need, therefore, for an improved gas analyzer. In particular, there is a need for gas analyzers that are easy to clean and that provide robust measurement capabilities. 
       BRIEF SUMMARY 
       [0007]    Systems and methods are provided for determining gas flux. The systems and method according to various embodiments allow for determining gas flux in a short intake tube configured gas analyzer without the need for Webb-Pearman-Leuning density corrections. 
         [0008]    Systems and methods are provided for measuring concentrations of gases and in particular dry mole fraction of components of a gas. The systems and method according to various embodiments allow for rapid measurement of the gas density and/or dry mole fraction of gases for a number of environmental monitoring applications, including high speed flux measurements. 
         [0009]    According to various embodiments, systems and methods are provided that enable rapid measurements of gas concentrations simultaneously with rapid measurements of pressure and temperature of sampled gas. In addition, devices according to various embodiments can advantageously use substantially shorter intake tubes as compared to previously existing devices, and substantially lower power consumption. 
         [0010]    Various embodiments also provide unique mechanical-optical design solutions for a gas analyzer that include several novel elements in a tool-free removable gas cell. Various embodiments also allow for measuring the dry mole fraction of a gas in a removable gas cell using temperature and pressure measurement in the gas stream. 
         [0011]    According to one aspect of the present invention, a gas analyzer is provided that typically includes a detector section including a detector, a source section including a light source, and a removable gas cell removably disposed between the source and detector sections. The removable gas cell typically includes a housing structure defining a gas flow channel, e.g., an enclosed gas flow channel, wherein, when attached, emitted light from the light source passes through the gas flow channel to the detector section along a light path. The gas cell also typically includes a gas inlet port, a gas outlet port, wherein the inlet and outlet ports are located on the housing structure, and a first temperature sensor adapted to measure a temperature of gas flowing in the flow channel, and a pressure sensor located at an interior point of the housing structure in the gas flow channel, the pressure sensor adapted to measure a pressure of the gas at an interior point in the flow cell. In certain aspects, the removable gas cell further includes a second temperature sensor located proximal to the outlet port, wherein the first temperature sensor is located proximal to the input port. 
         [0012]    According to another aspect of the present invention, a gas analyzer is provided that typically includes a detector section including a detector, a source section including a light source, and a removable gas cell removably disposed between the source and detector sections. The removable gas cell typically includes a housing structure defining a gas flow channel, e.g., an enclosed gas flow channel, wherein, when attached, emitted light from the light source passes through the gas flow channel to the detector section along a light path. The gas cell also typically includes a gas inlet port, a gas outlet port, wherein the inlet and outlet ports are located on the housing structure, and a first temperature sensor located proximal to the inlet port, and a second temperature sensor located proximal to the outlet port. In certain aspects, the gas analyzer includes a pressure sensor located at an interior point of the housing structure in the gas flow channel, the pressure sensor adapted to measure a pressure of the gas at an interior point in the flow cell. 
         [0013]    In certain aspects, the gas analyzers include an intelligence module coupled with the temperature sensor(s), with the pressure sensor and with the detector. The intelligence module is typically adapted to determine a dry mole fraction of components of a gas within the flow channel based on a detector signal and substantially simultaneous measurements of the pressure of the gas by the pressure sensor and the temperature of the gas by the temperature sensor(s). In certain aspects, the intelligence module corrects for a delay due to spatial separation in the temperature signals received from the first and second temperature sensors, wherein the delay is typically a function of a flow rate of the gas in the flow channel. In certain aspects, the gas analyzers include a first optical window proximal to the detector section and a second optical window proximal to the source section, wherein the first and second optical windows provide an airtight seal for components within the source and detector sections. 
         [0014]    According to yet another aspect of the present invention, a method is provided for measuring dry mole fraction of components of a gas in a flow cell of a gas analyzer. The method typically includes flowing a gas through the flow cell, measuring an absorbance of light of the components of the gas in the flow cell, substantially simultaneously measuring a temperature, T1, of the gas at an input port of the flow cell, substantially simultaneously measuring a temperature, T2, of the gas at an output of the flow cell, and substantially simultaneously measuring a pressure, P, of the gas at an interior point of the flow cell. The method also typically includes determining a dry mole fraction of the components of the gas based on the measured absorbance, P, T1 and T2. In certain aspects, the components of the gas include CO 2  and H 2 O, and wherein the gas is air. 
         [0015]    According to yet a further aspect, a gas analysis system that measures dry mole fraction of components of a gas is provided. The system typically includes a flow cell within which a gas is flowed from an inlet port to an outlet port, a light source configured to transmit light through the flow cell, and a detector subsystem configured to output an absorbance signal representing an absorbance of light by the components of the gas in the flow cell. The system also typically includes a first temperature sensor positioned proximal to the input port of the flow cell, a second temperature sensor positioned proximal to the output port of the flow cell, and a pressure sensor adapted to measure pressure at an interior point of the flow cell. the system further typically includes an intelligence module coupled with the first and second temperature sensors, the pressure sensor, and with the detector subsystem. The intelligence module is typically adapted to determine a dry mole fraction of the components based on the absorbance signal and the substantially simultaneous measurements of the pressure of the gas and of the temperature of the gas by the first and second temperature sensors. In certain aspects, the measurements of the pressure and of the temperature of the gas by the first and second temperature sensors occur within about 0.2 seconds or less of each other. In certain aspects, the measurements of the pressure and of the temperature of the gas by the pressure sensor, and the first and second temperature sensors are taken at a rate of about 1.0 Hz or faster. 
         [0016]    According to another aspect, a removable gas cell adapted to be disposed between light source and detector sections of a gas analyzer system is provided. The removable gas cell typically includes a housing structure defining a gas flow channel, a first opening at one end of the gas flow channel, a second opening at the other end of the gas flow channel, wherein the first and second openings define a light path along which light from a light source passes through the cell to the detector section. The gas cell also typically includes a gas inlet port, a gas outlet port, wherein the inlet and outlet ports are located on the housing structure off of the optical axis, a first temperature sensor located proximal to the inlet port, and a second temperature sensor located proximal to the outlet port. In certain aspects, the gas cell includes a pressure sensor located at an interior point of the housing structure in the gas flow channel, the pressure sensor adapted to measure a pressure of a gas at an interior point in the flow cell. In certain aspects, at least one of the first and second temperature sensors includes a thermocouple positioned at a central point of a flow path defined by the respective port. 
         [0017]    According to still another aspect, a removable gas cell adapted to be disposed between light source and detector sections of a gas analyzer system is provided. The removable gas cell typically includes a housing structure defining a gas flow channel, a first opening at one end of the gas flow channel, a second opening at the other end of the gas flow channel, wherein the first and second openings define a light path along which light from a light source passes through the cell to the detector section. The gas cell also typically includes a gas inlet port, a gas outlet port, wherein the inlet and outlet ports are located on the housing structure off of the optical axis, a first temperature sensor adapted to measure a temperature of a gas flowing in the flow channel, and a pressure sensor adapted to measure a pressure of the gas at an interior point in the flow cell. 
         [0018]    In certain aspects, the removable gas cells include, or can be coupled with, a gas intake tube that couples the atmosphere with the input port. The intake tube can advantageously have a length of less than 2.0 meters and more advantageously less than about 1.0 meters. 
         [0019]    Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  illustrates a gas analyzer including a removable sample flow cell according to one embodiment. 
           [0021]      FIG. 2  illustrates a thermocouple design used in the sample cell inlet and outlet according to one embodiment. 
           [0022]      FIG. 3  is a diagram of a pressure measurement scheme according to one embodiment. 
           [0023]      FIG. 4  illustrates thermal conductivity within a flow cell for an un-insulated (top) flow cell and an insulated (bottom) flow cell. 
           [0024]      FIG. 5  illustrates one embodiment including a sample cell with a cell insert removed. 
           [0025]      FIG. 6  illustrates a temperature and pressure measurement scheme for a gas flow cell according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The present invention provides systems and methods for measuring densities of gasses such as carbon dioxide and water vapor. The systems and methods are particularly useful in turbulent air structures. For example, in certain embodiments, the systems and methods advantageously sample and measure gas concentration, temperature and pressure at high speed and at high bandwidth, and allow for calculation of dry mole fraction of gas components. 
       Operational Overview 
       [0027]    The gas analyzers according to various embodiments generally include a light source, a sample cell, and a detector. The sample cell is removable in certain aspects to facilitate in-field cleaning and repair. The gas analyzers disclosed herein can be used to measure a concentration of one or more gases that have a high absorbance at different wavelength bands. For example, in one application, a gas analyzer can be used to measure a concentration of CO 2  and water vapor (H 2 O) in a sample gas, typically air. Generally, the gas analyzer uses non-dispersive infrared (NDIR) absorption to measure concentration of a gas in a sample cell based on the difference between absorption of infrared radiation passing through the sample cell and a reference cell, or against a calibrated reference signal. Advantageously, the gas analyzers do not require long intake tubes and are capable of taking high speed measurements which enable, inter alia, calculation of dry mole fraction of gas components. 
         [0028]    In operation, a light source transmits light having a spectrum of wavelengths through sample and reference cells. Gases present in the sample cell absorb light at different wavelength bands. For example, CO 2  has a high absorbance at 4.255 μm, and water vapor has a high absorbance at 2.595 μm. Light exiting the sample cell is detected by the detector, which is sensitive to wavelength bands absorbed by the gases (e.g., CO 2  and H 2 O), or two detectors can be used, each sensitive to the wavelength band absorbed by one of the gases. The concentration of the gases in the sample cell can be determined by calculating the difference between absorption in the sample cell and the reference cell or a reference signal. For example, when a reference cell contains a non-absorber gas, the signal detected in the sample cell is compared to the signal detected in the reference cell to provide an absolute measurement of gas concentration in the sample cell. U.S. Pat. Nos. 6,317,212 and 6,369,387, which are each hereby incorporated by reference in its entirety, disclose various features of open and closed path gas analyzers, including optical filter configurations and techniques. 
         [0029]    As described above, contamination of the gas analyzer can result in inaccurate concentration measurements. Contamination can occur in the sample cell and can occur in the sections of the gas analyzer that house the light source and detectors. Additionally, inaccurate concentration measurements can result due to fluctuations in gas temperature. The following embodiments offer solutions to these problems. It should be noted that any of these embodiments can be used alone or in combination with one another. 
       Removable Flow Cell 
       [0030]    According to various embodiments, a gas analyzer includes a packaging scheme that enables simple, tool-free removal of the sample flow cell to facilitate in-field cleaning of optical components such as source and detector optical windows. Such a packaging scheme also advantageously allows for maintaining a desiccant and scrub path (see e.g., U.S. Pat. No. 6,317,212, previously incorporated by reference) and advantageously allows for repeatable set distance between the source and detector. 
         [0031]      FIG. 1  illustrates a gas analyzer  10  including a removable sample flow cell  20  according to one embodiment. Sample cell  20  includes a housing structure having a gas inlet port  25  and a gas outlet port  30 . A cylindrical portion  40  defines a gas flow path which has two openings at either end. The cylindrical portion may be part of the housing structure of cell  20  or it may itself be separable from the housing structure. In operation, gas enters input port  25 , flows through the flow path defined by cylindrical portion  40  and exits outlet port  30 . In one embodiment, a pressure sensor  35  (external portion shown) is positioned to measure the pressure at an internal point within the flow path. 
         [0032]    Removable sample cell  20  is configured to couple with structure  22  of gas analyzer  10 . Structure  22  includes a source portion  60  that houses a light or radiation source and associated electrical and optical components. Structure  22  also includes a detector portion  50  that houses one or more detectors and associated optical and electrical components. A first optical window  70  is provided in one embodiment proximal to source portion  60  as shown in  FIG. 1 . A second optical window (not shown) is provided in one embodiment proximal to detector portion  50 . In typical operation, light (typically IR light) emanating from source portion  60  travels to detector portion  50  along an optical path defined by the first and second optical windows. In one embodiment, an O-ring is provided proximal the first and second optical windows to provide a more robust seal between the sample cell  20  and the housing  22  when in a coupled state. 
         [0033]    When in a coupled state, the gas flow path defined by cylindrical portion  40  substantially aligns with the optical path defined by the first and second optical windows. It should be appreciated that the optical path and the flow path do not need to align, and that only a portion of the gas flow path need be contiguous with the optical path. It should also be appreciated that although cylindrical portion  40  includes an open-ended flow path when decoupled from housing  22 , optical windows could be coupled to or located on sample cell  20  at either end of the flow path in place of or in addition to the first and second optical windows of structure  22 . 
         [0034]    In one embodiment, to couple and uncouple sample cell  20  from the gas analyzer housing structure  22 , one or more thumbscrews (two shown) are provided to increase or decrease the distance of detector section  50  relative to column  23 . When in a coupled state, a user need only activate the thumbscrews, separate detector section  50  a sufficient distance from column  23  and remove sample cell  20 . Similarly, to couple sample cell  20  with the gas analyzer, the user would extend the detector section  50  a sufficient distance, insert cell  20  adjacent to column  23  and activate the thumbscrews to re-engage the detector section  50  with column  23  and simultaneously engage sample cell  20  between detector section  50  and source section  60 . The optical windows and O-rings help ensure an airtight seal for the gas flow path in sample cell  20 . Alternative coupling mechanisms could include a bayonet (such as used in cameras), clamps, clips or other fastening mechanisms. 
         [0035]    Advantageously, the design shown in  FIG. 1  enables easy removal of the sample cell  20  and associated components. The mechanical packaging scheme enables easy, tool-free removal of the sample cell to clean the source and detector optical windows. This enables use of the gas analyzer (e.g., an IR gas analyzer or IRGA) without need to filter the air sample for dust (which lowers the power requirements for air flow). To maintain air-tight seals and allow the sample cell to be removed, the detector enclosure  50  is extended away from the rest of the assembly as shown in  FIG. 1 . Air-tight paths are provided from the detector enclosure  50 , through circulation column portion  23 , to the source enclosure  60  to run CO 2  and H 2 O free air as well as necessary electrical components. It should be appreciated that the gas analyzer could be configured such that attachment mechanisms enable the source section  60  to extend away from column  23  in addition to or in lieu of detector section  50  extending away from column  23 . 
       Temperature and Pressure Measurements 
       [0036]    Knowing the temperature of the sample gas is important for various measurements, such as for calculating the mole fraction from the number density. According to one embodiment, the temperature is measured at the inlet port  25  and at the outlet port  30  of the sample cell  20  in a manner that does not block any of the optical signal (e.g., IRGA signal) in the flow path. The volume average temperature in the cell can be calculated from a relationship between the inlet and outlet temperatures along with the flow rate of the sample gas. In certain aspects, for example, a generic function is T IRGA =f(T in , T out , T block , U), where U is the mean velocity through the cell. In certain aspects, additional temperature sensors may be used, e.g., to measure the block temperature of cylinder  40 , and/or to measure the gas temperature at different points in the flow path. 
         [0037]    In one embodiment, the inlet and outlet temperatures are measured using a disposable thermocouple (e.g., type-E thermocouple). For example, in certain aspects, a thermocouple is strung taught across a printed circuit board with a hole through for the sample gas to flow.  FIG. 2  illustrates a thermocouple design used in the sample cell inlet and outlet ports according to one embodiment. A printed circuit board  80  includes an aperture across which is strung a thermocouple bead  85 . In certain aspects, the printed circuit board is included in structure  20  or mounted on structure  20  such that the aperture is contiguous with the gas flow path of the entry/exit port  25 / 30 . An O-ring  90  provides an air tight gas path. This design allows the thermocouples to be easily replaced as well as making sure the temperature measurement is in the center axis of the gas flow. In general it is desirable to obtain accurate measurements with the same frequency response. For example, it is desirable to match the frequency response of the thermocouple to that of the volumetric frequency response, e.g., physical response of the flow cell. For example, a 0.002″ thermocouple advantageously provides a frequency response to a 15 liter/minute (LPM) flow that is substantially the same as the signal attenuation due to the volume averaging within the IRGA. In one embodiment, the measurements are synchronized with each other to account for slight timing variations and frequency variations. For example, the measurements of gas concentration (e.g., CO 2  and H 2 O), temperature and pressure are advantageously taken within about 0.2 seconds, and more advantageously within about 0.1 seconds of each other in certain embodiments. this enable a calculation of dry mole fraction as will be described more below. These signals may be aligned in real time to account for time variations/delays as will be discussed more below. 
         [0038]    In certain embodiments, a single temperature sensor is used. For example, a single temperature sensor may be located proximal to the inlet port, proximal to the outlet port, or proximal to an internal portion of the flow cell. Temperature measurements can be taken and a volume temperature can be calculated using the single temperature sensor signal and known parameters, such as flow rate, flow cell volume, etc. However, it is appreciated that the use of two (or more) temperature sensors as described above will provide a more robust and accurate temperature for the gas in the flow cell. 
         [0039]    It is also important know the pressure in the flow cell to facilitate calculation of mole fraction from number density.  FIG. 3  is a diagram of a pressure measurement scheme according to one embodiment. In one embodiment, the pressure sensor, e.g., sensor  35  includes a differential pressure sensor In order to maintain the sensor head as compact as possible, in one embodiment, a coupled set of absolute/differential pressure transducers are used to obtain high frequency pressure data from the sample cell (since pressure sensor that measures absolute sensor may be quite bulky). The pressure of the gas at the interior of the flow cell can be determined by adding the differential pressure and the mean pressure. In one embodiment, the pressure sensor is a high speed pressure sensor. Useful pressure sensors include a piezoresistive silicon differential pressure transducer (e.g., MPX2010DP, Freescale Semiconductor Inc. (Motorola)) and a piezoresistive silicon absolute pressure transducer (MPX4115A, Freescale Semiconductor Inc. (Motorola)). Other pressure sensors as would be apparent to one skilled in the art may be used. 
         [0040]    According to one embodiment, the sample flow cell includes an insulating sleeve made out of a low thermal conductivity material and/or pockets of air or vacuum. Useful materials include low-CO 2  absorption plastics (e.g., Teflon PTFE (polytetrafluoroethylene) which can be very useful because all plastics absorb some CO 2  and water vapor, but Teflon is exceptionally low in absorption) and other suitable materials. A double-wall vacuum metal sleeve or other designs may be used. Such sleeves advantageously minimizes the temperature change between the inlet and outlet thermocouples (to maximize accuracy) by decoupling the air flow from the heat dissipation of the gas analyzer, e.g., heat dissipation due to electronics of gas analyzer. In general, the bigger the temperature change, the more the characterization between the volume average temperature as a function of the two measured temperatures is relied on. Thus, the thermal insulation sleeve, when used, should insulate at least a portion of the flow cell from other components of the system.  FIG. 4  illustrates an example of the thermal conductivity within a flow cell for an un-insulated (top) flow cell and an insulated (bottom) flow cell. The top image is a typical metal sample cell, while the lower image shows an insulated cell where the only heat flux comes from the windows on either end. As can be seen, the heat flux is greatly reduced and the error in temperature measurement is also greatly reduced. In both cases, the error is calculated by the actual temperature minus an un-weighted average of the inlet and outlet temperatures. 
         [0041]      FIG. 5  illustrates one embodiment including a sample cell with a cell insert  95  (defining the gas flow path) removed. As can be seen, a position and sealing scheme for the thermocouple boards  80  proximal to both the inlet and outlet ports is also shown. 
       Mole Fraction Measurement 
       [0042]    To determine and output mole fraction, calculations should be performed in real time. To implement this, digital electronics receive various signals and perform calculations in real-time. The high speed temperature and pressure signals are acquired from spatially separated sensors. Also, each sensor typically has its own characteristic frequency response (sometimes depending on the flow rate). According to one embodiment, the timing of the sensors is aligned (e.g., delays removed) and the frequency responses, where necessary, are accounted for to calculate the mole fraction in the center of the sample cell (e.g., without directly measuring temperature in that position). As shown in  FIG. 6 , according to one embodiment, the inlet temperature is measured at point ‘A’, the pressure is measured at point ‘B’, the outlet temperature is measured at point ‘C’, and the gas concentrations are a volume-averaged measurement contained within the dotted outline. In certain aspects, signal alignment occurs in real time as signals are received (e.g., by an intelligence module adapted to process such signals). It should be appreciated, however, that signal processing may be performed later. For example, the signals, or data representing the signals, may be stored and provided to an intelligence module for processing at a later time, after measurements have been taken. 
         [0043]    According to one embodiment, the dry mole fraction is determined using a gas analyzer according to the various embodiments disclosed herein. Advantageously, gas analyzers as disclosed herein are capable of making dry mole fraction measurements at high bandwidth. For example, it is desirable to determine a dry mole fraction of CO 2  or other gas component at a certain frequency response, e.g., 10 Hz. In order to provide this output, the following steps occur according to one embodiment. First, a flow rate is introduced through the IRGA that purges the volume, for example, at about a 10 Hz or greater purge rate. For example, a flow of ambient air into the inlet port, through the flow cell and out the outlet port is initiated at the desired flow rate. The temperature is then measured at points A and C with a frequency response similar to the volume-average (e.g., type ‘E’ thermocouples of 0.002″ diameter provides a similar response to a flow rate of 15 liters-per-minute (LPM) through the IRGA). The pressure (e.g., differential pressure) is measured at point B with a similar frequency response to the other measurements. The temperature measurements are taken substantially simultaneously with each other and with the pressure measurement. The temperatures at A and C are corrected for spatial separation (e.g., T A (t+delay) and T C (t−delay) where the delay is a function of the flow rate and flow path/IRGA geometry). Simultaneously with these measurements, the detector system is determining concentration of gas components (e.g., carbon dioxide and water vapor). In certain aspects, the bandwidth of all the signals (e.g., temperature, pressure, absorbance) is matched to the physical time constant of the flow through the flow cell volume (i.e., flow rate divided by volume). Appendix A illustrates exemplary calculations for determining the dry mole fraction. Once determined, the dry mole fraction may be returned, e.g., displayed or stored for later use. 
         [0044]    It should be understood that, herein and elsewhere, when referring to a mole fraction (e.g., CO 2  mole fraction) that includes water vapor, the term “mole fraction” is typically used; when referring to a mole fraction after water vapor is removed, the term “dry mole fraction” is typically used, and in certain instances the term “instantaneous mole fraction” or “instantaneous dry mole fraction,” may be used to refer to high speed measurements. 
         [0045]    It should be appreciated that the gas analysis processes, including the mole fraction determination processes, may be implemented in computer code running on a processor of a computer system. The code includes instructions for controlling a processor to implement various aspects and steps of the gas analysis processes. The code is typically stored on a hard disk, RAM or portable medium such as a CD, DVD, etc. Similarly, the processes may be implemented in a gas analyzer including an intelligence module, typically having one or more processors executing instructions stored in a memory unit coupled to the processor(s). The intelligence module may be part of the gas analyzer, or part of a separate system directly or indirectly coupled with the gas analyzer. Code including such instructions may be downloaded to the gas analyzer memory unit over a network connection or direct connection to a code source or using a portable medium as is well known. 
         [0046]    One skilled in the art should appreciate that the processes of the present invention can be coded using a variety of programming languages such as C, C++, C#, Fortran, VisualBasic, etc., as well as applications such as Mathematica® which provide pre-packaged routines, functions and procedures useful for data visualization and analysis. Another example of the latter is MATLAB®. 
         [0047]    While the invention has been described by way of example and in terms of the specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.