FR2653559A1 - DEVICE FOR DETERMINING, BY MOLECULAR FLUORESCENCE, THE CONCENTRATION OF A COMPONENT TO BE MONITORED IN A GAS MIXTURE IN CIRCULATION. - Google Patents

Abstract

Device for determining the concentration of a component to be monitored in a gas mixture circulating inside a main duct (10). The determination is carried out by measuring the intensity of fluorescence emitted by the component excited by a light beam. This determination requires a calibration which according to the invention is not carried out at the same time as the determination but with the same measuring system. Application to the monitoring of toxic components contained in a gas mixture released into the atmosphere.

Description

The present invention relates to a device for the determination,

  by molecular fluorescence, from the concentration of a component to

  monitor in a circulating gas mixture.

  It applies in particular to the monitoring of the presence of toxic components in a gaseous mixture

released into the atmosphere.

  A gaseous mixture resulting for example from a

  Material reprocessing is most often rejected.

  in the air. This is why we ensure that no toxic gas is present in the mixture, even in very low concentration. An extremely sensitive concentration measuring device shall enable the detection of the exceeding of a concentration threshold

  fixed very low for security reasons.

  FIG. 1 represents a known device for measuring the concentration of a component in a gaseous mixture by molecular fluorescence.

in circulation.

  The gas mixture circulates in a duct

  main 10 (in the direction indicated by the arrow).

  Under the action of a suction pump 12, a part of the gaseous mixture is sucked into a d4rivE pneumatic circuit comprising a measuring cell 14. The debit and the pressure of the gaseous mixture are measured by a flowmeter 22 and a manometer 24 seats

at the entrance of cell 14.

  A light beam, coming from a suitable laser 16, passes through the measuring cell 14 from one side to the other. Its wavelength is chosen as it

  preferentially excites the component to be monitored.

  This light beam is modulated by an optical modulator 26 ("Chopper" in English terminology)

  which allows synchronous detection.

  The fluorescent light emitted outside the cell 14 via a side window 18 is filtered by a filter to eliminate the scattered light from the

excitation beam.

  A photomultiplier 28 powered by a power source 30 collects the fluorescence light and delivers on an output an electrical signal proportional to the detected light intensity. A second cell 34 filled with a gaseous mixture of known composition and containing the component to be monitored is placed in the path of the excitation light beam coming from the measuring cell 14. A system 35 is slaved to the pressure gauge 24 and allows make the pressure inside the second cell 34 equal to that prevailing in the

measuring cell 14.

  The excitation beam is absorbed in the second cell 34 and gives rise to a fluorescence light emission from the component

  identical to the component to be monitored.

  This fluorescence light emitted outside the cell 34 via a side window 33 is filtered by a filter 36 eliminating stray light from the excitation beam scattering and then collected by a photomultiplier 38 fed by the power source 30. The latter delivers on an output an electrical signal proportional to the intensity

detected light.

  The electrical signals coming from the two photomultipliers 28, 38 are delivered on two inputs of processing means 32. The measurement of the fluorescence luminous intensity of the component whose concentration is known makes it possible to deduce the concentration of the component to be monitored from the luminous intensity emitted by the latter. The measurement made from the second cell

  38 constitutes a calibration of the measuring device.

  This known device has many disadvantages. Its sensitivity does not allow the determination of concentrations lower than 1012 molecules. / cm3 in an industrial environment, which is not sufficient in the case where one must monitor highly toxic components and whose discharge standards

are extremely low.

  Insufficient sensitivity leads to long response times: the detected signal being very weak, the determination of the concentration is obtained by means of successive measurements. The number of these measurements is all the greater as the signal is weak and drowned in noise. During the determination, the concentration of the component may vary significantly, thus distorting the

result of the measurement.

  For the calibration to be effective, the two photomultipliers 28, 38 must have similar characteristics, which leads to adjustment complications and calibration difficulties. This known device does not make it possible to distinguish parasitic fluorescence lights coming from other components constituting

optionally the gaseous mixture.

  The present invention overcomes these various disadvantages. Concentration measurement and calibration are done using the same cell,

at different times.

  Calibration can be performed at regular intervals but also, thanks to the quick and easy transition from "measurement" mode to "calibration" mode, when a concentration threshold is crossed; the operation of the measuring device is thus controlled and the reliable measurement can be used to trigger

alarm devices.

  The sensitivity of the device according to the invention allows the monitoring of a component

  appearing at very low concentrations.

  More specifically, the present invention relates to a device for determining the concentration of a component to be monitored in a circulating gas mixture inside a main conduit comprising: a measuring cell capable of circulating the mixture gaseous at a known flow rate and a pressure, means for exciting a fluorescence of said component to be monitored inside the measuring cell, means for measuring the fluorescence luminous intensity delivering on an output an electrical signal, means for processing said electrical signal, these means being connected by an input to the

  output of the intensity measuring means.

  Calibration measurements are made from the fluorescence light emitted by a component identical to said component to be monitored but contained in known concentration in a mixture

gaseous calibration.

  The calibration measurements and the determination of the concentration of the component to be monitored are carried out at different times, using the

same measuring system.

  Calibration and measurement are performed with the same tools, which confirms the validity of the measurements and improves the accuracy

of determination.

  According to a preferred embodiment, the device according to the invention further comprises: a pneumatic sampling circuit comprising a suction pump, this sampling circuit being connected by an inlet and an outlet to the main duct; a pneumatic circuit of measurement comprising said measuring cell and a suction pump, said measuring circuit being connected on the one hand to the pneumatic sampling circuit and, on the other hand, to the main duct in such a way that the gaseous mixture sucked from the pneumatic withdrawal circuit is rejected. in the main duct, - a pneumatic calibration circuit comprising means for supplying a calibration gas mixture containing said component in known and variable concentration at will, this pneumatic calibration circuit being connected to the pneumatic sampling circuit by a bypass suitable for isolating the pneumatic sampling circuit of the main duct, the melan geometry of circulating calibration

  then in the pneumatic measuring circuit.

  To switch from normal operation to "calibration" mode, it is sufficient to turn off the gas mixture coming from the main line and to blow the composition calibration mixture

  known in the pneumatic measuring circuit.

  According to an alternative embodiment of the present invention, the sampling circuit comprises a pyrolyzer. At will, it is possible to dissociate the compounds present in the gas mixture and

  possibly containing the component to be monitored.

  Without the pyrolyzer, the amount of component to be monitored present as a compound would escape

to the measure.

  Preferably, the means for supplying the calibration gas mixture consists of a cryogenic exchanger. According to a preferred embodiment, a light trap prohibits any return inside the measuring cell to a light beam having

  crossed the latter longitudinally.

  Preferably, the pneumatic sampling circuit and the pneumatic measuring circuit are made of materials having no affinity

  chemical with said component to be monitored.

  In a preferred embodiment, the means for measuring the light intensity consist of a multichannel spectrometer. The use of this analyzer makes it possible to take into account only the fluorescent light coming from the component to be monitored and to eliminate parasitic and near-wavelength fluorescence due to other components.

contained in the gaseous mixture.

  Advantageously, the measuring cell comprises two angled windows of Brewster angle

  relative to the longitudinal axis of the cell.

  In any case, the features and advantages of the invention will appear better after

  the following description given for explanatory purposes

  and in no way limiting. This description refers to

  to the accompanying drawings in which: - Figure 1, already described, schematically shows a known device for determining the concentration of a component to be monitored in a circulating gas mixture; FIG. 2 diagrammatically represents a device according to the invention for determining the concentration of a component to be monitored in

a gaseous mixture in circulation.

  Figure 2 schematically represents a

device according to the invention.

  The gaseous mixture (represented diagrammatically by an arrow) containing the component to be monitored circulates in a main duct 10. A pneumatic sampling circuit is connected in parallel to the duct 10. A suction pump 40 entrains a portion of the gaseous mixture in the cooling circuit. sampling at a volume flow of 1 l / mm at the

atmospheric pressure.

  A valve 42 placed at the inlet of the sampling circuit allows its isolation or adjustment of the pressure of the gas mixture flowing in this circuit. The latter and the debit are controlled by means of a manometer 46 and a

flowmeter 44.

  The sampling circuit also comprises a pyrolizer 48, made of quartz for example. At will, we can dissociate the compounds from the mixture and

  possibly containing the component to be monitored.

  The piping and the various elements of the sampling circuit are made of material which has no chemical affinity with the toxic component to be monitored. This avoids any accumulation

of this component in the circuit.

  The piping as well as the pump 40 and

  the isolation valve 42 are for example Teflon.

  The pyroletter 48 is for example made of quartz.

  The debit-meter 44 is for example glass and Teflon.

  A pneumatic measuring circuit is connected on the one hand to the pneumatic sampling circuit and on the other hand to the main pipe 10 downstream of

  the output of the sampling circuit.

  This measuring circuit comprises a flowmeter, a valve 52, a pressure gauge 54, a measurement gauge 56 and a suction pump 59. Preferably, a pressure of less than Pa is placed in the cell 56. be regulated so that the excitation of the fluorescence is effected in the best conditions

possible.

  Advantageously, the power of the pump 59 is sufficient to obtain in the cell 56 a pressure of the order of 0.1 mbar with a flow rate

  volume in the cell 56 of 100 cc / min.

  Under the effect of the pump 59, a part of the gaseous mixture circulating in the circuit of

  sample is sucked into the measuring circuit.

  The debit-meter 50 and the pressure gauge 54 serve to control the flow and pressure inside the measuring cell 56. The pressure inside

  of the cell 56 is adjusted with the aid of the valve 52.

  The measuring cell 56 comprises two windows 58, 60 inclined for example Brewster angle relative to the longitudinal axis of the cell 56 so as to avoid any parasitic reflection in

  the cell 56 when a light beam passes through them.

  The measuring cell 56 is flanked by a side window for observing the fluorescent light emitted by the component to be monitored

in cell 56.

  In the same way as above, the constituent materials of the measurement circuit do not have a chemical affinity with the component to be monitored. The d-bitmeter 50 and the pressure gauge 54 are identical to the flowmeter 44 and the pressure gauge 46 of the sampling circuit. The valve 52 is for example Teflon. The measuring cell 56 is for example Teflon; the windows 58, 60 and the porthole

62 are for example quartz.

  A laser 16, which may for example be a dye laser, delivers a light beam of narrow spectral width and able to selectively excite the component to be monitored. An optical modulator 26 modulates the light beam, which allows

  synchronous detection of the fluorescence light.

  A lens 64 focuses the light beam in an optical fiber 66 guiding the light beam towards the measuring cell 56. After propagating in the fiber 66, the beam is focused by a lens 68 inside the measuring cell 56 in which he enters through the window 58 and

  o It excites the component to be monitored.

  The absorption of the beam is not total; it is even weak in most cases since the component to be monitored, which alone absorbs the beam, is present only at a very low concentration. The light beam thus passes through the cell and leaves it via the

second window 60.

  A light trap 70 completely absorbs the light beam having passed through the cell 56 so as to avoid any return of this beam inside the cell 56 which can create parasitic light. Following its excitation by the light beam, the component to be monitored contained in the circulating gas mixture inside the cell 56 produces a fluorescence light, part of which is emitted externally by

through the porthole 62.

  A lens 7Z placed near the porthole 62 puts in the form of a parallel beam Fluorescence light. This beam passes through an interference filter 74 which makes it possible to eliminate most of the light diffused by the excitation light beam.

  A lens 76 focuses the filtered beam.

  in an optical fiber which guides it to means for measuring the fluorescence luminous intensity 80 advantageously constituted by a multichannel spectrometer. Such a spectrometer has a dispersive element which allows the spread of the light beam

  depending on the wavelengths. The beam spread.

  is detected. by a strip of photodiodes each corresponding to a narrow range of wavelengths. The multichannel spectrometer therefore simultaneously delivers electrical signals proportional to the light intensities corresponding to each

range of wavelengths.

  The fluorescence wavelength of the component to be monitored being known, it is thus possible to filter (electrically by taking into account only the signal of interest) parasitic light at other wavelengths: in particular, the light residue originating from the diffusion of the excitation beam and the lights of parasitic fluorescence emitted by other components present in the gas mixture. The electrical signal corresponding to the fluorescence intensity of the component to be monitored is delivered to an input of the microcomputer-type processing means 82 which determines the concentration from the measured intensity and

  calibration curves stored in a memory.

  1 1 As we will see later in the

  description, the calibration is done by

  via the measurement circuit already described.

  This calibration is performed a first time during the commissioning of the present device and re-fetched from time to time. It can also be triggered automatically by the microcomputer 82 when the concentration of component to monitor exceeds a threshold set in advance. In this case, and to save time, it is possible to control only one particular operating point without performing a complete calibration. It thus controls the reliability of the measurement almost instantaneously because, as we will see, the transition from "measurement" mode to

"calibration" is simple and fast.

  Calibration consists in measuring the fluorescent light intensities emitted by a component identical to the component to be monitored and contained in a calibration gas mixture of known concentration, the pressure itself being determined. Various measurements are made by varying these different parameters and calibration curves are recorded in a memory of the microcomputer. Intermediate values between

two curves are extrapolated.

  To carry out calibration measurements, the sampling circuit is isolated from the main duct 10 and connected to a pneumatic calibration circuit by a bypass ("bypass").

Anglo-Saxon terminology).

  The bypass consists of three valves 42, 86, 88 for example Teflon manually controlled or automatically by the

  microcomputer 82 to which they are connected.

  Isolation valve 42 isolates the entrance

  the main line sampling circuit 10.

  The pneumatic calibration circuit comprises means for providing a calibration gas mixture containing said component in known concentration and variable at will. In the exemplary embodiment shown in FIG. 2, this means consists of

a cryogenic exchanger 90.

  When the valve 42 isolates the sampling circuit of the main duct, the valve 86 is open and releases a mixture gas selected for example from air, argon or nitrogen contained under pressure in a reserve 92. The gas released circulates in a conduit 94 immersed in a cryogenic fluid 96

  contained in a hermetically sealed tank.

  Inside the tank, the temperature of the cryogenic fluid 96 is determined by that of a thermistor 92 immersed in the fluid 96 connected to a power source 100 controlled by the microcomputer 82. The fluid temperature

96 can range from -100 C to 10 C.

  Inside the tank, the conduit 94 has a portion of twisted-e form in which crystals 102 of component identical to the component to be monitored. The vapor pressure of the

  crystals being known, one can vary at will.

  and in a determined manner the concentration of the gaseous component in the mixing gas flowing in the pipe 94 simply by varying the temperature of the

cryogenic fluid 96.

  The conduit 94 is connected to the sampling circuit and when the valve 42 is closed and the valves 86 and 88 are open, a calibration gas mixture of known composition and concentration circulates in the pre-collection circuit and in the

measuring circuit.

  Thus, calibration curves at different concentrations are carried out using the same measuring device as for determining the concentration. This avoids the difficulties of adjustments and inaccuracies. The return to "measurement" mode is simply done by closing the valves 86 and 88 and

the opening of the valve 42.

  The device according to the invention has a high sensitivity and makes it possible to determine extremely low concentrations. The measurement automatically checked if a threshold is exceeded is reliable and allows the triggering of alarm devices. A pyrolyzer furthermore makes it possible to determine the concentration of the component to be monitored normally contained in a compound.

which is usually negated.

  It goes without saying that the invention is not limited to the embodiment described and

  represent; she admits all variants.

Claims (8)

claims   Apparatus for determining the concentration of a component to be monitored in a circulating gas mixture within a main conduit (10) comprising: - a measuring cell (56) capable of circulating the gas mixture to a known flow rate and pressure; means for inducing fluorescence of said component to be monitored inside the measuring cell; means for measuring the fluorescence luminous intensity delivering on an output an electrical signal; processing means (82) of said electrical signal, these means (82) being connected by an input to the output of the intensity measuring means (80), - calibration measurements being made from the light fluorescence emitted by a component identical to said component to be monitored but contained in known concentration in a calibration gas mixture, characterized in that the calibration measurements and the determination of the concent components to be monitored are performed at different times, using the same system. measurement.   2. Device according to claim 1, characterized in that it further comprises: - a pneumatic sampling circuit comprising a suction pump (40), this sampling circuit being connected by an inlet and an outlet to the main duct (10) a pneumatic measuring circuit comprising said measuring cell (56) and a suction pump (59), this measuring circuit being connected on the one hand to the pneumatic sampling circuit and on the other hand to the main duct (10). so that the gaseous mixture sucked from the pneumatic sampling circuit is rejected in the main conduit (10)   a pneumatic calibration circuit comprising means (90) for supplying a gaseous calibration mixture containing said component in known concentration and variable at will, this pneumatic calibration circuit being connected to the pneumatic sampling circuit by a bypass suitable for isolate the pneumatic sampling circuit from the main duct, the calibration gas mixture then circulating in the circuit pneumatic measurement.   3. Device according to claim 2, characterized in that the pneumatic circuit of   sampling comprises a pyrolyzer (48).   4. Device according to claim 2, characterized in that the means (90) for providing the calibration gas mixture consists of a cryogenic exchanger. 5. Device according to claim 2, characterized in that a light trap (70) prohibits any return inside the measuring cell to a light beam having passed through the latter. longitudinally.   6. Device according to claim 1, characterized in that the pneumatic sampling circuit and the pneumatic measuring circuit are made of material having no affinity   chemical with said component to be monitored.   7. Device according to claim 1, characterized in that the means (80) for measuring the luminous intensity of fluorescence consist of in a multichannel spectrometer.   8. Device according to claim 1, characterized in that the measuring cell (56) comprises two windows (58, 60) inclined at Brewster angle with respect to the longitudinal axis of the cell (56).