Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
  • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/28—Investigating the spectrum
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/28—Investigating the spectrum
  • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/27—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands using photo-electric detection ; circuits for computing concentration
  • G01N21/274—Calibration, base line adjustment, drift correction
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
  • G01J3/28—Investigating the spectrum
  • G01J3/42—Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry
  • G01J3/427—Dual wavelengths spectrometry
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
  • G01N2021/3166—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths using separate detectors and filters
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
  • G01N2021/317—Special constructive features
  • G01N2021/3177—Use of spatially separated filters in simultaneous way
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
  • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
  • G01N2021/3196—Correlating located peaks in spectrum with reference data, e.g. fingerprint data
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/59—Transmissivity
  • G01N21/61—Non-dispersive gas analysers

Definitions

• the technical field of the invention is an optical method for analyzing a gas, by implementing a light source of the black body or gray body type and by measuring an absorption of a light wave emitted by the light source. .
• the gas analyzed extends between a light source and a photodetector, said photodetector measurement, the latter being intended to measure a light wave transmitted by the gas to be analyzed, the light wave being partially absorbed by the latter.
• the light source is usually a source emitting in the infrared
• the method used is usually referred to by the term Anglosaxon "NDIR detection", the acronym NDIR meaning No Dispersive Infra-Red.
• NDIR detection the acronym NDIR meaning No Dispersive Infra-Red.
• the usual methods generally comprise a measurement of a light wave, referred to as the reference light wave, emitted by the source, the reference light wave being unabsorbed or negligibly absorbed by the gas analyzed.
• the measurement of the reference light wave makes it possible to estimate the intensity of the light wave emitted by the source, or to estimate the light wave which would be detected by the photodetector of measurement in the absence of absorption by the gas analyzed.
• This technology is referred to as "double beam".
• the comparison between the light wave in the presence of gas and the light wave without gas makes it possible to characterize the absorption of the gas. This involves, for example, determining a quantity of a gaseous species in the gas, according to the technology designated by the term "NDIR by absorption".
• the reference light wave is measured by a reference photodetector. It may be a reference photodetector different from the photodetector of measurement, and arranged to be disposed facing the light source, the reference photodetector being associated with a reference optical filter.
• the reference optical filter defines a reference spectral band, in which the gas to be analyzed does not show significant absorption.
• a measurement photodetector and a reference photodetector are used, the two photodetectors detecting a light wave in the same spectral band, in this case a CO2 absorption spectral band.
• the reference photodetector is arranged closer to the light source than the photodetector. The comparison of the signals respectively measured by the photodetector of measurement and the reference photodetector makes it possible to dispense with the knowledge of the intensity of the light wave emitted by the source.
• Document FR3000548 describes a CO2 sensor comprising a measurement channel, in an infrared spectral band, and a reference channel, in a visible spectral band (0.4 pm to 0.8 pm).
• the reference path is considered uninfluenced by the C0 2 concentration in the measured gas.
• this document describes the use of a function F, representative of the aging of the light source respectively in the visible and infra-red spectral bands.
• the function F is approximated by an identity function: thus, the aging of the infrared light source is considered equal to the aging of the light source in the visible.
• the inventor has found that the use of a reference light wave may have certain disadvantages. It proposes a method for overcoming these disadvantages, so as to improve the accuracy of the measurement.
• a first object of the invention is a method for measuring an amount of a gaseous species present in a gas, the gaseous species being able to absorb light in an absorption spectral band, the method comprising the following steps :
• the gas between a light source and a measurement photodetector, the light source being able to emit an incident light wave, the incident light wave propagating through the gas towards the photodetector of measurement, the measuring photodetector being able to detect a light wave transmitted by the gas, in the absorption spectral band;
• steps b) to d) being implemented at a plurality of measurement times, the method comprising, at each measurement instant:
• step e) comprises taking into account a correction function, representative of a temporal variation of an intensity of the light wave incident in the spectral band of measurement relative to an intensity of the light wave incident in the spectral reference band.
• the light source may comprise a filament raised to a temperature permitting light emission in the illumination spectral band.
• the correction function can be representative of a comparison between:
• the comparison can be expressed as a ratio or a subtraction.
• the correction function is preferably established beforehand during a calibration phase, comprising the following steps:
• test light source facing a test measurement photodetector, and facing a test reference photodetector, the test light source, the test measurement photodetector and the photodetector of test reference being respectively representative of the light source, the photodetector of measurement and the reference photodetector;
• the test light source may be pulsed, with each pulse corresponding to a calibration instant.
• the calibration period can comprise at least 1000 instants of calibration.
• the correction function can be established from a comparison, at different instants of calibration, between:
• the intensity detected by the photodetector of test measurement normalized by an initial intensity detected by the photodetector of test measurement
• the intensity detected by the test reference photodetector normalized by an initial intensity detected by the test reference photodetector.
• initial intensity is meant an intensity measured at an initial time of the calibration period.
• Step e) may comprise, from the reference intensity measured at the measurement instant and the correction function, an estimate of an intensity that would be detected, at the instant of measurement, by the photodetector in the spectral band of measurement, in the absence of gas. It can comprise, from the reference intensity measured at the measurement time and the correction function, a correction of the measurement intensity, the corrected measurement intensity corresponding to a measurement intensity in the measurement intensity. the absence of aging of the light source.
• a second subject of the invention is a device for determining an amount of a gaseous species in a gas, the device comprising:
• a light source configured to emit an incident light wave propagating toward the gas, the incident light wave extending in a spectral absorption band of the gaseous species;
• a measurement photodetector adapted to detect a light wave transmitted by the gas, at different measurement times, in a spectral measurement band and to measure an intensity, called the measurement intensity;
• a reference photodetector configured to measure an intensity, referred to as the reference intensity, of a reference light wave emitted by the light source, in a reference spectral band, at the different measurement times;
• FIG. 1A represents an exemplary device for implementing the invention.
• Fig. 1B schematizes an emission spectrum of a black body light source.
• Figure 2A shows the observed decrease in light intensity emitted by a light source in two different spectral bands.
• Figure 2B illustrates the emissivity loss of the light source in a spectral band of measurement as a function of the emissivity loss of the light source in a reference spectral band.
• FIG. 2C illustrates the emissivity loss of the light source in a measurement spectral band as a function of the emissivity loss of the light source in a reference spectral band, and this considering three different supply voltages of the light source.
• FIG. 3 shows taking into account the reference intensity to correct the emissivity loss of the light source respectively according to a conventional method and according to the invention.
• Figure 4 shows the main steps of a method embodying the invention.
• Fig. 1A is an example of a gas analyzer 1.
• This device comprises an enclosure 10 defining an internal space inside which are:
• a light source 11 able to emit a light wave 12, called incident light wave, so as to illuminate a gas G extending in the internal space.
• the incident light wave 12 extends along an illumination spectral band D.
• the light wave 14 is designated by the term measurement light wave. It is detected by the photodetector 20 in a measurement spectral band D.
• a reference photodetector 20 ref configured to detect a reference light wave 12 ref , in a reference spectral band A ref.
• the reference spectral band A ref is a spectral band in which it is considered that the absorption of the light wave 12 by the gas G is negligible.
• the spectral band reference A ref is different from the spectral measurement band D20.
• the spectral band A 2 o may in particular be wider than the reference spectral band Aref.
• the spectral measuring band D20 may comprise the reference spectral band A re f.
• the gas G comprises a gaseous species G x whose aim is to determine an amount c x (k), for example a concentration, at a measurement instant k.
• This gaseous species absorbs a measurable part of the light in an absorption spectral band A x .
• the light source 11 is able to emit the incident light wave 12, according to the illumination spectral band D, the latter being able to extend between the near ultraviolet and the mean infrared, for example between 200 nm and 10 ⁇ m, and most often between 1 pm and 10 pm.
• the absorption spectral band A x of the gaseous species G x analyzed is included in the illumination spectral band D.
• the light source 11 may in particular be pulsed, the incident light wave 12 being a pulse of duration generally between 100 ms and 1 s.
• the light source 11 may in particular be a filament-type light source suspended and heated to a temperature of between 400 ° C. and 800 ° C. Its emission spectrum, in the emission spectral band D, corresponds to the emission spectrum of a black body.
• the measurement photodetector 20 is preferably associated with an optical filter 18, defining the spectral band measurement D 20 covering all or part of the spectral absorption band A x of the gaseous species.
• the photodetector 20 is a thermopile, able to deliver a signal depending on the intensity of the detected light wave.
• the photodetector may be a photodiode or other type of photodetector.
• the reference photodetector 20 ref is arranged next to the photodetector 20 and is of the same type as the latter. It is associated with an optical filter, called reference optical filter 18 ref .
• 18 ref reference optical filter defines the spectral band reference A ref corresponding to a wavelength range not absorbed by the gas species in question. Bandwidth reference A ref is for example centered around the wavelength 3.91 microns.
• the intensity / (/ c) of the light wave 14 detected by the measurement photodetector 20, called measurement intensity, at a measurement instant k, depends on the quantity c x (k) at the measurement instant, according to Beer-Lambert's relationship:
• - m (c x (k)) is an absorption coefficient, depending on the quantity c x (k) at time / c; - 1 is the thickness of gas crossed by the light wave in the chamber 10;
• I 0 (k) is the intensity of the incident light wave, at time k, which corresponds to the intensity of the light wave, in the spectral measuring band D20, reaching the photodetector 20 in the absence of gas absorbing in the enclosure.
• the expression (1) assumes a control of the intensity / 0 (/ c) of the incident light wave 12 at the measurement instant k.
• FIG. 1B schematizes an emission spectrum of a light source 11 of the black body type, obeying Planck's law:
• L 1, T is the luminance, dependent on the wavelength 1 and the surface temperature T of the black body
• k is Boltzmann's constant, it is the speed of light in the air.
• the emission spectrum S of the light source 11 corresponds to the evolution of the luminance L (/ l, 7) as a function of L, when the light source is brought to a temperature T.
• the temperature T is between 400 ° C and 800 ° C.
• FIG. 1B shows the illumination spectral band D 12 of the light source 1 extending between 1 ⁇ m and 10 ⁇ m. There is also shown by a dotted line, the reference spectral band A ref.
• This type of light source is particularly advantageous, since it makes it possible to modulate the illumination spectrum S by a simple modulation of the temperature T of the source.
• T the temperature of the source.
• the emissivity of a light source of black body or gray body type varies with time, and can in particular undergo a decrease resulting from the aging of the light source.
• the temporal variation of the emission of the light source 11 is taken into account by the reference photodetector 20 ref .
• the latter is arranged to detect a reference light wave 12 ref , representative of the incident light wave 12 emitted by the light source 11.
• the reference light wave 12 ref reaches the reference photodetector 20 ref without interacting with the gas G, or without interacting significantly with it.
• the intensity of the reference light wave 12 ref detected by the reference photodetector 20 ref , at the measurement instant k, is designated by the term reference intensity / re (/ c).
• the device comprises a microprocessor 30, connected to a memory 32 comprising instructions for carrying out the process steps described below.
• the microprocessor 30 is configured to receive a signal representative of the intensity / re (/ c) of the reference light wave 12 ref , measured by the reference photodetector 20 ref at each measurement instant k.
• the microprocessor 30 estimates the intensity / 0 (/ c) from / re / (/ c).
• the microprocessor 30 is configured to receive a signal representative of the reference intensity / re (/ c), then to perform a correction of the measured intensity I (k).
• the corrected intensity is noted I * (k).
• the latter corresponds to the intensity that would be measured by the photodetector without the aging of the light source.
• the absorption absqt) of the incident light wave can then be obtained by the expression:
• the corrected intensity I * (k) is obtained from / re (/ c), applying a correction function:
• the inventor has found that the aging of the light source 11 affects differently the reference spectral band A ref and the spectral band of measurement D 2 o ⁇ Contrary to what is suggested in document FR3000548, the aging in the Measurement spectral band can not be considered as similar to aging in the spectral reference band.
• the inventor has carried out an experimental calibration described in connection with FIGS. 2A to 2C. It used a test measurement sensor 20 'and a test reference sensor 20' ref respectively similar to the measuring and reference sensors described in connection with Figure IA.
• the gas analyzed was a known gas, in this case CO 2 at a concentration of 400 ppm.
• the experimental parameters were as follows:
• Measurement filter 18 Heimann filter F4.26-180, centered on a wavelength of 4.26 ⁇ m.
• Reference filter 18 ref Heimann filter F3.91-90, centered on a wavelength of 3.91 mih.
• Photodetectors 20 and reference 20 ref thermopile Heimann HCM Cx2 Fx.
• the measurement filter 18 defines a deliberately narrow D20 spectral measurement band, so as to highlight the aging observed by the inventor. It will be understood that the invention applies to other spectral bands of measurements D20, in particular wider than the reference spectral band A ref .
• FIG. 2A represents the time evolutions: of a measurement intensity (k), measured by the test measurement photodetector 20 'in the measurement spectral band D20;
• Notations (k) and I ' re f (k) denote the fact that these intensities are measured during a calibration phase, using test sensors, and a test light source, using a known gas.
• the calibration phase makes it possible to learn the aging of the light source 11, which is of the same nature as the test light source 11 '.
• the measurement intensity (k) and the reference intensity / ' re / (/ c) decrease with time, which was expected. This corresponds to the aging of the light source 11. It is also observed that the respective decays in the measurement spectral band D20 and in the reference spectral band A ref are different. This means that the aging of the light source 11 in the measurement spectral band D20 is different from the aging of the light source 11 in the reference spectral band A ref . Thus, the ratio varies with time k. It means that the aging of the
• FIG. 2B shows the loss of emissivity EL 20 in the measurement spectral band (ordinate axis) as a function of the emissivity loss in the reference spectral band EL re f (x-axis).
• the emissivity losses, expressed in%, in each spectral band are respectively obtained at each instant k according to the expressions:
• FIG. 2C shows the curves EL 20 as a function of EL re f respectively for each potential V.
• the test described with reference to FIGS. 2A to 2C can be considered as a calibration test, during which the differential aging of the light source 11 in the measurement spectral band D20 is learned with respect to the reference spectral band D Gb ⁇ . It is performed with a test sensor, whose components are similar to those fitted to the sensor for the analysis of an unknown gas.
• This calibration test makes it possible to determine a correction function d characterizing the relative emissivity variation in the two spectral bands D20 and A ref .
• the function of correction d comprises a comparison, at each instant k, of the reference intensity with the intensity in the spectral band of measurement.
• the correction function d can be such that:
• A is the slope of a line obtained by applying a linear regression from the data shown in Figures 2B and 2C.
• A is a scalar value, representing the differential aging of the light source in each spectral band.
• FIG. 3 shows an example of time evolution of a measurement intensity I ⁇ k). This change corresponds to that measured in FIG. 2A.
• a second correction (curve 2), corresponding to an implementation of the invention, by applying the correction function d, variable as a function of time, to / re / (/ c), so that in such a way that :
• Step 100 illumination of the gas at a moment k;
• Step 110 measurement of the reference intensity / re (/ c), in the reference spectral band A ref , by the reference photodetector 20 ref .
• Step 120 measuring the intensity I (k) of the radiation 14 transmitted by the gas, in the measurement spectral band D 20 , by the photodetector 20.
• Step 130 estimation of an intensity / (/ c) which would be detected by the photodetector 14, in the measurement spectral band D 20 , in the absence of gas in the chamber.
• the estimate is made taking into account the correction function ⁇ 5 (/ c), and applying the expression:
• Step 150 from the absorption, estimate an amount c x (k) of a gas species G x from the ratio by applying the expression ().
• Step 160 repeating steps 100 to 150, incrementing the measurement instant k, or output of the algorithm.
• This embodiment is advantageous because following the calibration, only the value of the differential aging A is to be retained, so as to be able to apply the correction function d to each measurement of the reference intensity / re (/ c) measured at each moment of measurement k.
• correction function d can be such that
• the value I (k) of the intensity measured by the measurement sensor is corrected, taking into account the correction function d.
• a corrected intensity I * (k) is obtained.
• the absorption is obtained according to the expression:
• the calibration phase makes it possible to evaluate the relative decay, over time, of the intensity of the illumination radiation produced by the light source in the reference spectral band and the spectral band of measured.
• the correction function includes a comparison of the decay in each spectral band.
• the use of the correction function ⁇ 5 (/ c) makes it possible to take into account the variation of the decay of the intensity of the illumination radiation 12 in the two spectral bands, so as to obtain an estimate of the intensity / 0 (/ c) which would be measured by the photodetector of measurement in the absence of gas;
• the invention can be implemented for the detection of a quantity of gaseous species G x whose absorption spectrum D20 is in the spectral band D20.
• the latter may be narrow, as in the experimental example described above. It can also be broad, so as to include, for example, the absorption spectral bands A x of several different gaseous species.

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