FR2459968A1 - DEVICE FOR CONTROLLING GAS WITH ABSORPTION CELL - Google Patents

Abstract

DEVICE FOR MONITORING THE PRESENCE OR CONCENTRATION OF A GAS, IN PARTICULAR OZONE, IN A SAMPLE ACCORDING TO THE ABSORPTION OF RADIATION BY THE GAS CONTAINING AN INPUT PORT 12 INTO WHICH THE SAMPLE IS CONTINUOUSLY PROVIDED, THE Orifice INPUT BEING CONNECTED TO A CHAMBER 14 CONTAINING MATERIAL FOR REMOVING GAS, A SOURCE OF RADIATION 40 OF PREDETERMINED WAVELENGTH AND AT LEAST ONE SAMPLE CELL 24 PROVIDED IN THE PATH OF RADIATION FROM THE SOURCE. IT INCLUDES A REFERENCE CELL 20, MEANS CONNECTING AN OUTLET OF THE CHAMBER TO THE REFERENCE CELL TO CAUSE A CONTINUOUS CIRCULATION OF THE SAMPLE LESS THE SPECIFIC GAS THROUGH THE REFERENCE CELL, A VALVE 16 TO PERIODICALLY CONNECT THE OR INPUT TO THE SAMPLE CELL AND MEANS TO DETECT AND COMPARE THE ABSORPTION OF RADIATION BY THE CONTENTS OF THE REFERENCE CELL AND OF THE SAMPLE CELL.

Description

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  The present invention relates generally to absorption cell gas control devices and concerns in particular the compensation of errors resulting from the successive measurements made by such absorption cell gas control devices and their delivery to zero without loss of

data acquisition time.

  The presence or concentration of a gas or vapor in a sample is frequently determined by its characteristic absorption or attenuation of wavelength radiation

  special. For any given gas or vapor to be measured or

  The sample is irradiated with an energy of a wavelength which is substantially absorbed by the gas or vapor to be controlled. Another determining factor in the choice of wavelength is naturally the inability of other gases, which are supposed to be present in the sample, to be absorbed significantly.

  the radiation of the chosen wavelength.

  The range of wavelengths used in the

  that absorption is relatively wide; gases such as ozone or sulfur dioxide and vapors such as acetone and benzene substantially absorb radiation whose wavelengths are in the ultraviolet range. On the other hand, wavelengths in the infra-red domain are easily absorbed by gases such as NO2, CO2 and H2S and by

water vapor.

  Although the present invention is primarily concerned with ozone measurements, it is also useful for the measurement of other gases and vapors provided modifications are made.

  radiation source and other components.

  In a basic arrangement for controlling the presence or concentration of a specific gas or vapor, a comparison is made between the transmission of the appropriate wavelength radiation through a sample cell initially containing a sample whose specific gas has been removed and the transmission of the same energy through the same cell containing the same sample whose specific gas has not been removed. The theoretical basis for such measurements is found in the Beer-Lambert law (eklc) in which for the particular case of ozone control

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  I = photointensity detected from the radiation having passed through a sample of ambient medium containing ozone, I0 = photointensity detected from the radiation having passed through the sample of ambient medium whose ozone has been removed, k = coefficient of absorption (at 254 monomers for ozone) c = concentration of ozone 1 = length of the absorption chamber, and

e = base of natural logarithms.

  Usually, in known systems, the radiation of a suitable light source passes through a cell or sample chamber which first contains a sample of ambient medium from which the ozone has been removed and the measurement is made. a sample whose ozone has not been removed replaces the first sample in the cell, the radiation passes through the cell again and the I measurement is made. The difference in the transmission detected by appropriate photoelectric elements provides a

  measuring the concentration of ozone in the sample.

  It is also customary to place a reference detector in a position to directly see the light source, and the reference detector output can be used to compensate for fluctuations in the output of the light source.

source.

  Measurements made in this way have yielded

  useful, but the accuracy of these results is not satis-

  doing for different reasons. For example a sample can pass through an appropriate chamber containing a purifier or other

  mechanism for removing ozone. Although the removal room

  In addition, the concentration of other gaseous or vapor components that also absorb radiation at 254 nm can vary over time. So, if we

  makes successive measurements of errors can occur.

  In some processes, an absorption cell is used.

  tion and, in addition to the time periods between the measurement of the transmission through the sample from which the ozone has been removed and the measurement of the transmission through the sample containing the ozone, an acquisition time of data is lost because it is necessary to purge or flush the system with the help of

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  the sample minus the ozone in order to establish a "zero" base before the measurements are made. Thus, for example one can lose up to half a minute as a result of purge, reset and single measurement operations. Therefore, in a given half-minute period, less than 10% of the available time

  is used for the actual measurement.

  The present invention therefore proposes to eliminate

  or to minimize errors resulting from the presence of gas or vapor with radiation absorption characteristics

  similar to those of the gas or steam that we wish to control

  l. The present invention also aims to avoid errors

  resulting from the time between measuring the absorption by the sample

  containing the gas or vapor to be tested and the sample

  whose gas or vapor to be controlled has been removed.

  Finally, the present invention proposes to provide a

  accurate zeroing with a minimal loss in time of acqui-

  the data in the system in which the data is

gas control.

  For the implementation of the present invention

  uses the basic absorption cell concepts listed below.

  above. However, a reference absorption cell is placed between the radiation source and the reference detector. A sample, whose gas, such as ozone, to control has been removed, continuously passes through the reference absorption cell and the absorption is continuously recorded by the reference detector. A substantially continuous flow of the ambient medium allows the suppression of the effect of gases or vapors other than ozone that absorb light at 254 nm in real time. In addition,

  the reference detector measures the composite of the variations of

  light intensity and the photointensity detected from the radiation passing through the reference cell is measured at the same time as the photointensity detected from the radiation passing through a sample cell containing ozone. In addition, the design of the device which has the function of removing ozone from the sample is optimized to provide a minimal flush time. This is achieved by reducing its volume as much as possible and, if necessary, by providing an accumulator of comparable size and shape, through which the surrounding environment

  pass simultaneously as it passes through the

sitive removing ozone.

  In order to provide more up-to-date

  a second absorption cell can be added to the

  operative part. The additional cell is determined to operate in a 50% duty cycle with a phase shift of 1800 relative to the base cell. While one of the cells provides data the other cell can be flushed or purged and

reset.

  Other advantages and characteristics of the invention

  will appear on reading the following description of modes of

  embodiment of the invention with reference to the accompanying drawing, in which: FIG. 1 is a schematic diagram of an embodiment of the ozone control device, FIG. 2 is a schematic diagram of a second embodiment of FIG. Ozone control device, and Figure 3 is a block diagram of the part

electronic device.

  In Figure 1 there is shown an ambient inlet port 12 communicating directly with a chamber 14 which contains an ozone-removing catalyst. The catalyst removing the ozone may be in the form of a fixed bed filter

  metal oxide, although other systems can be used.

  my ozone removal. The inlet orifice 12 may be connected to an accumulator (not shown) of the same size and shape as the chamber 14 or may communicate directly with an input of a three-way electromagnetic valve 16. The chamber 14 communicates with a second inlet of the valve 16. The chamber 14 is also connected by a line 18 to a reference cell 20. From the outlet of the valve 16, another line 22

  extends to a sample cell 24.

  An output line 26 extending from the reference cell 20 is connected to a similar line 28 extending from

  sample cell 24. A line 30 connects the combined outputs

  The pump 36 continuously discharges the contents of the device through an outlet 38. The various solid lines and the elements described constitute a representation of a flowmeter 32 whose output is connected by a needle valve 34 to a pump 36. simplified mechanism used for the flow of gas through the control device according to the invention.

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  In addition to the solid lines indicating the circulation

  gas through the device, lines

  broken represent the path of radiation, in the form of light in the case of ozone control. The light is preferably generated by a low pressure mercury vapor lamp 40 whose output is a line which contains about 95% of the total energy at 254 nm and is caused to reflect on a first mirror 42 and a second mirror 44. From the mirror 42 the light is diverted to longitudinally cross the reference cell 20 to a reference detector 46. The reference detector 46 may be any one of various known devices such as

  that a vacuum photodiode detector insensitive to solar light

  but sensitive to radiation at 254 nm. The output intensity of the detector 46 is determined by the intensity of the selected wavelength radiation striking the detector. In this way,

  an effective monochromator is formed for the detection of ozone.

  At the same time the light of the source 40 is reflected by the

  mirror 44 to longitudinally traverse the sample cell

  24 to a sample detector 48 which may be similar

  in all respects to the reference detector 46.

  In addition to the solid lines indicating the flow of gas and the dashed lines indicating the light paths of the dashed lines illustrate the path of the electrical signals. A signal in the form of a current is

  taken from the reference detector 46 and is supplied to an electro-

  digital reference meter and voltage / frequency converter 50. The output of the element 50 is connected to a microcomputer 52 which may comprise a standard 8-bit microprocessor and others

elements described below.

  At the same time a comparison signal is generated in

  the sample detector 48 and supplied to a digital electrometer

  The output of the element 54 is also connected to the unit 52. In addition to providing data for the display or other use at a terminal 60, the microprocessor of FIG. unit 52 is programmed to

  provide an appropriate output signal to control the function

  electromagnetic valve 16.

  Details concerning the electrical operation of the device of FIG. 1 are given in FIG. 3. The signal taken from the reference detector 46 is first converted into

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  a voltage in an electrometer 82 and then in a frequency by the voltage / frequency converter 92. The output of the converter 92

  is counted by a counter 72 for a specific period of time.

  determined by the microprocessor 90.

  At the same time that the comparison signal from the sample detector 48 is processed similarly by the electrometer 84, the voltage-frequency converter 9.4 and the

counter 74.

  The microprocessor 90 is programmed in a memory

  (ROM) 85 and interfaced with a periph-

  95 to the electromagnetic valve 16 to control its operation. The microprocessor 90 is also programmed to store data of the counters 72 and 74 in a random access memory (RAM) 87. The microprocessor also calculates the

  concentration of ozone which is provided in the form of

  output logic at a terminal 91 and as a digital output on a

  display device 97 or at an output terminal 98.

  In operation the gas to be controlled enters the ori-

  input fuse 12 and the three-way valve 16 is set by the

  microprocessor control program 90 to allow the listening

  gas through the ozone removal chamber 14, through the cell 24, and finally for its exhaust through the outlet 38 through the continuously operating pump 36. The flow control can be set at the needle valve 34 as a function of the readings obtained from the flowmeter 32. After the complete rinsing of the system by the sample minus the ozone the

  The system can be set to zero in the manner described below.

  below. The readings are obtained from the reference detector 46 which supplies a current of intensity varying with the absorption of the radiation in the reference cell 20 and in a synchronous manner with the sample detector 48 which supplies a current of varying intensity as a function of the absorption of the radiation in the sample cell 24 in which the ambient medium has been introduced in a manner to be described below. These intensities are converted into voltages by the reference electrometer 82 and the sample electrometer 84, respectively, and the voltages

  are converted into frequencies by the voltage / frequency converter

  reference 92 and the voltage / frequency converter for

  94. Frequencies are then counted syn-

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  by the reference counter 72 and the sample counter

  74 for a period of approximately one second, controlled by the microprocessor 90. These values are stored in memory

  in the random access memory (RAM) 87 of the microcalcula-

  used. The ambient medium is introduced into the device either directly from the inlet port 12 or through an accumulator (not shown) of the same size and general shape as the chamber 14. Although the chamber 14 has very small dimensions, a certain period of time is required, and, to ensure that the ambient medium and the ambient medium minus the ozone come from the same sample, the accumulator can be used to delay the flow of the ambient medium in the same way as the

  Chamber 14 delays the circulation of the environment less ozone.

  Whether or not an accumulator is used, a microprocessor signal

  90 and the peripheral interface adapter 94 switches the valve 16 after the reset mode to allow the ambient medium to flow from the inlet port 12 through the valve 16 and the line 22 to the cell 24. After a suitable delay to allow the sample cell to be fully charged with ambient medium, the intensity is measured by the sample detector 48 as a function of the absorption of the sample.

  radiation of the lamp 40 in the sample cell 24.

  In synchronism, a measurement is also made of the intensity coming from the reference detector 46 as a function of the absorption of the radiation of the lamp 40 in the reference cell 20. The output of the sample detector 48 and that of the detector 46 are then converted into digital form as described above and stored in the memory at

  direct access 87 with the previously stored values.

  During the cycle, the temperature and the pressure are measured by appropriate transducers 75 and 77 respectively. The voltage outputs of these transducers are converted into

  frequencies by voltage / frequency converters 96, and

  98, and counted by counters 76, and respectively

  78, controlled by the microprocessor 90. These values are also

  memorized in memory (RAM) 87.

  The actual calculation according to the Beer-Lambert law is carried out as follows. The ratio I / Io, also known as the transmittance T, Io being the intensity that would have been determined by the sample detector 48 at the instant when I was

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  determined can be calculated from a stored intensity

  measured in the reset mode by the sample detector

  lon 48 divided by the intensity determined by the detector of

  reference 46 in the reset mode multiplied by the inten-

  determined by the reference detector 46 during the

sample measurement.

  In an example of ozone control device ok at 254 nanometers and 308 atm_1 cm-1 at standard temperatures and pressures (2730K and 1 atm), and 1 is the length of the cell, for example 78.8 cm the concentration of ozone can be calculated from the following equations: -log (I / I0) = klc (1) c = -1 / kl box (I / Ib) (2)

  To determine c in ppm (parts per million) we multiply

  fold equation (2) by 10. Concentration at pressures and temperatures other than standard pressures and temperatures

  is related to concentration at standard pressures and temperatures

  darts by the law of perfect gases. Thus equation (2) becomes c (ppm) = -106 / kl x (760) / P x T / 273 box (I / I0) (3) where P is expressed in millimeters and T in degrees Kelvin. equation

  (3) is automatically resolved by the microcomputer.

  The temperature transducer 75, and respectively the pressure transducer 77 are, as indicated, interfaced with

  the microcomputer to continuously correct the changes

  of these variables. The pressure transducer is, preferably

  at the exit of sample cell 24 and the

  Pressure transducer can be located in the middle of this cell.

  In FIG. 2, most of the circuit and the device are similar to those of FIG. 1. The same inlet orifice 12 and the ozone removal chamber 14 are used, as are the flowmeter 32. the needle valve 34, the pump 36 and the outlet port 38. A mercury lamp 40 as well as mirrors 42 and 44 are also used, the mirror 42 reflecting light through the reference cell 20 to a reference detector 46 whose output signal passes through a digital electrometer and converter 50 to be memorized

in a microcomputer 52.

  In the same way, the mirror 44 reflects light through the sample cell 24 to the detector 48 whose output is integrated and converted into frequency in the element 54 and

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  is also brought to the microcomputer 52.

  However, in addition to the sample cell 24, a second sample cell 24a is used. The radiation of the mercury lamp 40 strikes directly the sample cell 24a without being reflected by the mirrors. A sample detector 48a operates in connection with the sample cell 24a and

  the output signal of the sample detector 48a passes through

  to the electrometer and converter 54a to the microcomputer 52. Due to the presence of an additional sample cell, detector and electro-convertor, some additional components are required. For example although the sample entering the inlet port 12 and passing through

  the chamber 14 reaches the three-way valve 16a, contrary to

  to the device of Figure 1, the chamber 14 is not alone-

  in communication with the reference cell 20 by the

  line 18 but it is also connected to the entrance of the second

  In addition, the inlet port 12 is not only connected to the valve 16 but is also connected to an inlet of the valve 16a. A

  exit line extends from the valve 16 to the sample cell.

  24 as in the device of Figure 1 and a second

  line extends from an outlet of the valve 16a to the sample cell.

24a.

  The electrical connections to operate valves 16 and 16a are also slightly more complicated in

  measurement or connections are made from the microcalcu-

  52 to both the valves 16 and 16a. Also the micro-

  The processor of element 52 is also somewhat differently programmed to provide the more complex control signals necessary to operate or switch the

three-way valves 16 and 16a.

  As indicated, in order to achieve the set target of an ozone concentration accuracy of the order of parts per billion, it is desirable that the system be reset to zero as often as possible. In the device of FIG. 2, a preliminary purge is first performed. Then a reading of I0 is obtained from the output of the reference detector 46 and

  stores it in the microcomputer 52.

  The microprocessor of the element 52 is then programmed

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  to perform a number of operations. First of all,

  signal is sent to trigger the valve 16 and allow the

  from the ambient sample to the sample cell.

  24 for about 20 seconds, the readings being made and the data collected every second. In a synchronous way, the sample minus the ozone is allowed to flow and sweep the

  sample cell 24a for a period of nine seconds.

  So for two seconds, readings are made of the

  transmission of the sample minus the ozone in the sample cell.

  24a to allow resetting. A subsequent circulation

  re in the sample cell 24a then takes place for nine

  seconds. At this moment, cell 24a is ready for introduction.

  sample of the surrounding environment.

  When the ambient sample is then introduced into the sample cell 24a, the valves are

  triggered by the microprocessor 90 to allow the sample

  less ozone to be introduced to flush the exchange cell.

  24. The resetting of the sample cell 24 then

  place while the sample cell 24 controls the concentration

  ozone in the ambient sample. By performing the switching as described, the two sample cells operate in a phase-shift relationship of substantially 180. while

  that one of the cells is rinsed and reset, the other one

  kills the control, measurements are made every second. Thus, there is no interruption of measurements, which can be very important in some applications such as instruments

embedded in aircraft.

  A change of 0.001 ppm in ozone corresponds to a

  transmittance change of one part for fifty thousand.

  To detect an ozone concentration of + or - 0.001 ppm, the

  I0 value can not change more than 1 part per 100,000.

  The output intensity of the lamp is difficult to control by

  conventional means and the location of the reference detector.

  in the output path of the lamp results in the output intensity of the lamp being controlled by the detector

  simultaneously with the output of the sample detector.

  in operation. The microcomputer is fed with this information to correct any change in the value of I0 that may have occurred between the time it is actually

  measured and when it is used to calculate the concentration

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  a few seconds later.

  It has previously been indicated that ozone is not the

  only gas or vapor that absorbs light at 254 nm. If the concentration

  These other products do not vary, their effect on the accuracy of the measurement is substantially suppressed because they are present when measurements are made on the contents of both the sample and reference cells. However, if the concentration of these other gases varies over time, it could affect the measurement of the location of the cell.

  reference between the radiation source and the reference detector.

  ence minimizes this possible error. Any change in 10 due to the change in the concentration of these other gases is internally controlled and corrected in the same way as the changes

output intensity of the lamp.

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Claims (9)

  A device for controlling the concentration of a specific gas or vapor in a sample of ambient medium according to the absorption of radiation of a predetermined wavelength by said specific gas, said device having an inlet port (12) wherein said ambient sample is continuously fed, said inlet port (12) being connected to a   chamber (14) containing a material for removing said specific gas   a source (40) of radiation of said predetermined wavelength and at least one sample cell (24) arranged   in the path of radiation from said source (40),   characterized in that it comprises in combination a reference cell (20) also disposed in the radiation path from said source (40), means (18) connecting an outlet from said chamber (14) to said reference cell (20) to cause a continuous circulation of said ambient sample minus the specific gas through said cell of   reference (20), valve means (16) for periodically connecting   said inlet port (12) to said sample cell   (24) to cause intermittent circulation of said   sample of ambient medium through said sample cell   lon (24) and means for detecting and comparing the absorption of said radiation by the contents of said reference cell   (20) and said sample cell (24).   2. Device according to claim 1, characterized in that said valve means (16) for connecting in a manner   periodically said inlet port (12) to said sample cell   (24) comprises a three-way valve (16) having a first input connected to said input port (12), a second input connected to said output of said chamber (14) and an output connected to said sample cell (24), a control circuit (52) and means electrically connecting said control circuit (52) to said valve (16), said ambient sample and said ambient sample minus the   specific gas that can be directed to the said sample cell.   (24) for predetermined periods.   3. Device according to claim 2, characterized in that it comprises a second sample cell (24a) also disposed in the path of said radiation and a second valve (16a) with three channels, said second valve (16a). -13- 2459968   having a first input connected to said input port (12) and a second input connected to said output of said chamber (14), said second valve (16a) having an output connected to said second sample cell (24a) and means electrically connecting said control circuit (52) to said second valve (16a), the ambient sample being alternately directed to said first sample cell   (24) and said second sample cell (24a).   4. Device according to claim 2, characterized in that said specific gas or vapor is ozone and that the material in said chamber (14) comprises metal oxides for removing ozone from said sample of ambient medium therethrough, said apparatus further comprising a continuously operating pump (36) connected to said reference and sample cells (20, 24) for causing said ambient sample to flow therethrough, said control circuit (52) providing signals to said said valve (16) for controlling the flow time of the ambient sample containing ozone from said inlet (12) and the flow time of said ambient sample minus the ozone of said chamber (14) to said sample cell (24). 5. Device according to claim 4, characterized by   the fact that said control circuit (52) comprises a microprocessor   (90) for providing signals to said valve (16), which valve connects said ozone removal chamber (14) to said sample cell (24) to allow flow therethrough for its rinsing, the ambient sample minus the ozone said microprocessor further providing signals to said valve (16) for connecting said inlet port (12) to said sample cell (24) to allow flow of   the ambient sample through it.   Device according to claim 3, characterized by   the fact that said control circuit (52) comprises a micro-   processor (90) for providing signals to said first and   second valves (16, 16a), whereby, alternately, samples   ambient samples and ambient samples minus ozone are directed into a substantially out of phase relationship of 1800 to said first and second sample cells (24, 24a). -14- 2459968   7. Device according to claim 1, characterized in that it comprises an accumulator connected between said inlet port (12) and said valve means (16), said accumulator being of similar volume and shape to said chamber ( 14), whereby the admission of the ambient sample to said device is subject to a delay comparable to that which is   subjected to the admission of the ambient medium minus said specific gas.   8. A device for controlling the concentration of ozone in a sample of ambient medium according to the absorption of radiation of a predetermined wavelength by ozone, said device comprising an inlet orifice (12) in which the sample of ambient medium is continuously admitted, said inlet orifice (12) being connected to the inlet of a chamber (14) containing a material for removing ozone, a source (40) of   radiation of approximately 254 nanometers and a cell of exchange   trillon (24) disposed in the path of radiation from said source (40), characterized in that it comprises a cell   reference (20) also arranged in the beam path   from said source (40), means (18) connecting an output of said chamber (14) to said reference cell (20) to cause a continuous flow of said ambient sample minus ozone through said reference cell (20), a three-way valve (16) connecting said inlet port (12) to said sample cell (24) in one of its positions and connecting said chamber (14) to said sample cell (24). )   in a second position, a microcomputer (52) electrically connected   said three-way valve (16) to control its   according to a predetermined program to bring a listening   periodically of said ambient medium minus the ozone and a periodic flow of said ambient sample through the sample cell (24), and means (46, 48) for detecting absorption of said radiation by the contents of said sample medium (24);   said reference cell (20) and said sample cell   the latter means (46, 48) being connected to, and providing input to, said microcomputer (52) for a calculation at   inside, in order to provide output data on the ozone concentration.   Device for controlling the concentration of ozone in an environmental sample according to claim 8, characterized in that it further comprises a second three-way valve (245a), a second sample (24a) also disposed in the path of said radiation, said second valve (16a) being arranged to connect said inlet port (12) to said second sample cell (24a) in one of its positions and to connect said chamber (14) to said second sample cell (24a) in a second position, and means (48a) for detecting radiation absorption in   said second sample cell (24a), said microcalcule   the driver (52) also being connected to said second three-way valve (16a) for controlling its operation according to a predetermined program, said first sample cell (24) and said second sample cell (24a) being alternately connected to said chamber (14) for purging it, and   said inlet port (12) for measuring the absorption of the   internally, said microcomputer (52) also being connected to receive, store and calculate the output signal for said reference cell detecting means (46),   said detection means (48) of first sample cell   and second sample cell detecting means (48a) also in accordance with said predetermined program for providing a substantially continuous output signal representing the concentration of ozone in said ambient sample.