GB2054843A

GB2054843A - Absorption cell gas monitor

Info

Publication number: GB2054843A
Application number: GB8020326A
Authority: GB
Original assignee: Thermo Electron Corp
Current assignee: Thermo Fisher Scientific Inc
Priority date: 1979-06-25
Filing date: 1980-06-20
Publication date: 1981-02-18
Also published as: IT8022887A0; FR2459968A1; DE3023625A1; JPS567039A; NL8003651A; IT1131364B; CA1140772A; JPS567038A

Abstract

An absorption cell gas monitor for measuring the concentration of a specific gas such as ozone in an ambient atmosphere comprises a test cell (24) through which is passed alternately an ambient sample from an inlet port (12) and containing the specific gas and an ambient sample from which the specific gas has been removed in a chamber (14), and a reference absorption cell (20) through which is passed continuously ambient sample from which the specific gas has been removed. Radiation from a source (40) having a wavelength absorbed by the specific gas is directed through each cell (20) and (24) and is detected by a detector (46), (48) respectively, the transmitted intensifies being compared in a microcomputer (52). Fluctuations in the intensity of the source (40) are taken into account and errors arising from changes with time in the concentrations of gases can be avoided by delaying the ambient sample which is fed to the test cell (24) by the same amount as the sample which is fed to the reference cell (20) is delayed by the chamber (14). <IMAGE>

Description

SPECIFICATION Absorption cell gas monitor This invention relates to absorption cell gas monitors for monitoring the concentration of a specific gas or vapour in an ambient atmosphere, and is particularly concerned with the compensation for errors resulting from sequential measurements, and the zeroing of such monitors without loss of dataaquisition time.

The presence or concentration of a gas or vapour in a sample is frequently determined by its characteristic absorption or attenuation of radiation of a particular wavelength. For any given gas or vapour to be measured or monitored, the sample is irradiated by energy of a wavelength which is significantly absorbed by the given gas or vapour, and the transmission loss suffered by the radiation is determined. An important factor affecting the choice of wavelength is, of course, the inability of other gases or vapours which are expected to be present in the sample significantly to absorb radiation of the chosen wavelength. The range of wavelengths which may be used in absorption cell gas monitors is rather wide.For example, gases such as ozone and sulphur dioxide, and vapours such as acetone and benzene significantly absorb radiation of wavelengths in the ultraviolet region, whereas wavelengths in the infrared region are readily absorbed by gases such as NO2, CO2 and H2S, and by water vapour.

The present invention is primarily concerned with ozone monitoring, but it is also applicable to absorption cell gas monitors for other gases and vapours, provided appropriate selection of the radiation source and other components of the monitor is made.

In a basic arrangement for monitoring the presence or concentration of a specific gas or vapour in a sample, a comparison is made between the transmission of radiation of a suitable wavelength through an absorption cell containing a sample from which the specific gas has been removed and the transmission of similar radiation through the cell when it contains a sample from which none of the specific gas has been removed.The theoretical basis for such arrangements is found in the Beer Lambert Law: i/l = el- hlC1 where = = the intensity of radiation transmitted by a sample containing the specific gas; 1o = the intensity of radiation transmitted by the sample having the specific gas removed; k = the absorption coefficient of the specific gas at the wavelength of the radiation; c = the concentration of the specific gas in the sample; I = the length of the absorption cell; and e = the natural logarithmic base.

Generally, in known absorption cell gas monitors, radiation from a suitable light source is passed through an absorption cell or chamber which first contains an ambient sample from which the specific gas has been removed and 1o is measured using a suitable photoelectric device. Then, a sample from which the specific gas has not been removed replaces the first sample in the cell, and radiation is again passed through the cell and I is measured. The difference in transmission, as detected by the photoelectric device, provides a measure of the concentration of the specific gas in the sample. In some such monitors a reference detector is placed in a position to view directly the light source, and the output of the reference detector is used to compensate for fluctuations in the output of the source.

Measurements made in the manner described have produced some usefui results, but the accuracy of those results suffers for several reasons. For example, the sample may be passed through a suitable chamber containing a scrubber or other means for removing the specific gas, but other gaseous and vaporous components which absorb radiation at the particular wavelength may remain. If the concentration of such other components varies with time, then errors may arise as a result of making measurements sequentialiy.

Also, in procedures in which a single absorption cell is used, in addition to the time delay between the measurement of transmission through the sample from which the specific gas has been removed and the measurement of transmission through the sample containing the specific gas, data-gathering time is lost because it is necessary to purge or flush the cell with the sample from which the specific gas has been removed to establish a "zero" base before the measurements are made. Typjcally, one might spend as much as a half-minute in the process of purging, zeroing and taking a single measurement, and in a given half-minute less than 10% of the available time is spent in actual measurement.

One of the objects of the present invention is to eliminate or minimize errors resulting from the presence in the samples of gases or vapours having similar radiation-absorbing characteristics to the specific gas or vapour being monitored.

To this end, according to the invention, an absorption cell gas monitor for monitoring the concentration of a specific gas or vapour in an ambient atmosphere comprises means for continuously drawing a sample of the ambient atmosphere into an inlet port, a chamber connected to the inlet port and containing material which removes the specific gas or vapourfrom the ambient sample passing through the chamber, a pair of absorption cells, one of which functions as a reference cell and is connected to the chamber so that there is a continuous flow through the reference cell of ambient sample from which the specific gas or vapour has been removed, and the other of which functions as a test cell, valve means for periodically connecting the inlet port to the test cell in addition to the chamber to cause an intermittent flow of the ambient sample through the test cell, a source or radiation of a predetermined wavelength which is absorbed by the specific gas or vapour arranged so that radiation from the source passes through both the reference cell and the test cell, and means for detecting and comparing the absorption of the radiation by the contents of the reference cell and the test cell.

In the monitor in accordance with the present invention, the basic absorption cell theory described earlier is followed. However, by inserting a reference absorption cell between the source of radiation and a reference detector, passing an ambient sample from which the specific gas to be monitored has been removed continuously through the reference cell, and continuously recording the reference detector output, the effect in real time of gases or vapours other than the specific gas or vapour in the sample can be cancelled. Moreover, the reference detector measures the composite of variations in radiation source intensity and the radiation transmitted through the reference cell at the same instant as that transmitted through the test cell is measured.

Preferably the design of the chamber which functions to remove the specific gas or vapour from the sample is optimized to have a minimum throughput time. This can be accomplished by reducing its volume as much as possible. If desired, an accumulator of comparable size and shape to the chamber may be provided for the passage of the ambient sample containing the specific gas or vapour from the inlet port to the test cell. In this way the samples passing through the reference and test cells will have been drawn into the inlet port at substantially the same time, and errors resulting from delay between measurements will be avoided.

For the purpose of obtaining more frequent updated measurements, a second test absorption cell may be incorporated in the system and operated in the same way as the first test cell but 1 800 out of phase therewith. Thus, while one of the test cells is providing data, the other test cell is being flushed or purged and zeroed, thereby maintaining accurate zeroing with minimum loss of data acquisition time.

Two examples of an absorption cell gas monitor in accordance with the invention for monitoring the concentration of ozone in an ambient atmosphere will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the first example; Figure 2 is a schematic diagram of the second example; and, Figure 3 is a block circuit diagram of the electronic control and measuring portion of the monitor illustrated in Figure 1.

In Figure 1, a port 12 for the sample intake from the ambient atmosphere is shown communicating directly with a chamber 14 which contains an ozone-removing catalyst. The ozoneremoving catalyst may be in the form of a fixed bed filter of metallic oxides, although other ozoneremoving systems may be used. The inlet port 1 2 may also be connected to an accumulator (not shown) of the same general size and shape as the chamber 14 or may communicate directly with one inlet of a three-way solenoid valve 1 6. The chamber 14 communicates with a second inlet of the valve 16, and is also connected by a line 18 to a reference cell 20. From the outlet of the valve 1 6 another line 22 runs to a sample cell 24.

An outlet line 26 running from the reference cell 20 is joined by a similar line 28 running from the sample cell 24. A line 30 connects the combined outlets to a flow meter 32, the outlet of which is connected through a needle valve 34 to a pump 36. The pump 36 continuously exhausts the contents of the system through an outlet 38. The various solid lines and the elements described constitute a simplified showing of the mechanism involved in the flow of gas through the apparatus of the monitor.

In addition to the solid lines indicating the gas flow through the apparatus, dotted lines show the path of radiation, in the form of light in the case of ozone monitoring. The light is generated preferably by a low-pressure mercury vapor lamp 40, the output of which is a line which contains about 95% of the total energy at 254 nm and is trained upon a first mirror 42 and a second mirror 44. From the mirror 42, light is deflected through the length of the reference cell 20 to a reference detector 46. The reference detector 46 may be any of several known devices such as a solar-blind vacuum photodiode detector sensitive to 254 nm radiation. The output current of the detector 46 is determined by the intensity of radiation of the preselected wavelength impinging upon the detector. In this fashion, an efficient monochromator for the detection of ozone is formed. At the same time, light from the source 40 is reflected by the mirror 44 through the length of 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 gas flow and the dotted lines indicating light paths, dashed lines show the path of electrical signals. A signal in the form of current is derived from the reference detector 46 and is coupled to a reference digital electrometer and voltage-to-frequency converter 50.

The output of the element 50 is passed to a microcomputer unit 52 which may include a standard 8-bit microprocessor and other elements described below. At the same time, a comparison signal is generated in the sample detector 48 and coupled to a sample digital electrometer and voltage-to frequency converter 54. The output of the element 54 is also coupled to the unit 52. Besides providing data for display or other use at a terminal 60, the microprocessor of element 52 is programmed to provide a suitable output to control the operation of the solenoid valve 1 6.

Greater detail on the electrical operation of the instrument of Fig. 1 is provided in Fig. 3. The signal derived from the reference detector 46 is first converted to a voltage in an electrometer 82 and then to a frequency by the voltage-to-frequency converter 92. The output of the converter 92 is counted by a counter 72 for a specific length of time determined by the microprocessor 90.

At the same time, the comparison signal from the sample detector 48 is similarly handled by the electrometer 84, the voitage-to-frequency converter 94 and the counter 74.

The microprocessor 90 is programmed in a read-only memory 85 and interfaced through a peripheral interface adaptor 95 to the solenoid valve 1 6 to time its operation. The microprocessor 90 is also programmed to store data from the counters 72 and 74 in the random access memory 87. The microprocessor also computes the concentration of ozone which is supplied as output analog data at a terminal 91 and as digital output on a display device 97 or at an output terminal 98.

In operation, gas to be monitored enters the input port 12 and the three-way valve 1 6 is set by the timing program of the microprocessor 90 to permit the flow of gas through the ozone-removing chamber 1 4 through the cell 24 and ultimately to be exhausted through the outlet 38 by the continuously operating pump 36. Control of the flow may be adjusted by means of the needle valve 34 in accordance with readings obtained from the flow meter 32. After the system has been completely flushed by the sample-minus-ozone, the system may be zeroed in the manner outlined below.

Readings are derived from the reference detector 46 which provides a current varying in accordance with the absorption of radiation in the reference cell 20 and synchronously from the sample detector 48 which provides a current varying in accordance with the absorption of radiation in the sample cell 24, into which ambient has been introduced in the manner outlined below. These currents are converted to voltage by the reference electrometer 82 and the sample electrometer 84 respectively and the voltages are converted into frequencies by the reference voltage-to-frequency converter 92 and the sample voltage-to-frequency converter 94. The frequencies are then synchronously counted by the reference counter 72 and the sample counter 74 for a period of time which may be approximately one second as controlled by the microprocessor 90.These values are stored in a random access memory (RAM) 87 of the microcomputer system to be used later on.

Ambient is introduced into the system either directly from the inlet port 12 or through an accumulator (not shown) of the same general size and shape as the chamber 14. Even though the chamber 14 is held to very small dimensions, a finite time delay is encountered and to assure that both the ambient and the ambient-minus-ozone are from the same sample, the accumulator can be used to delay the flow of ambient just as the chamber 14 delays the flow of ambient-minus-ozone. Whether or not as accumulator is used, a signal from the microprocessor 90 and peripheral interface adaptor 95 switches the valve 16 after the zeroing mode to permit ambient to flow from the input port 12 through the valve 1 6 and the line 22 to the sample cell 24.Again, after a suitable delay to permit the sample cell to be fully charged with ambient, current is measured by the sample detector 48 as a function of the absorption of radiation from the lamp 40 in the sample cell 24.

Synchronously, a measurement is also made of the current from the reference detector 46 as a function of absorption of radiation from lamp 40 in the reference cell 20. The output of the sample detector 48 and reference detector 46 are then converted to digital form as described above and stored in the random access memory 87 along with the previously stored values; During the cycle the temperature and pressure are measured by suitable transducers 75 and 77 respectively. The voltage outputs of these transducers are converted to a frequency by the voltage-tofrequency converters 96 and 98 respectively and counted by counters 76 and 78 respectively under timing control of microprocessor 90. These values are also stored in the RAM 87.

The actual computation in accordance with the Beer-Lambert Law is carried out in the following manner. The ratio l/lo, also known as the transmittance T, 1c being the intensity that would have been determined by the sample detector 48 at the same instant I was determined, can be computed from the stored intensity measured in the zeroing mode by the sample detector 48 divided by the intensity determined by the reference detector 46 in the zeroing mode multiplied by the intensity determined by the reference detector 46 during the sampiing mode.

In a typical ozone-monitoring system where k at 254 nanometers is 308 atm-' cm~~' at standard temperature and pressure (2730K and 1 atm) and I is the length of the cell, e.g., 78.8 cm, the concentration of ozone can be computed from the following equations: --log, (I/lo) = klc (1) c =I/kl lose (I/lo) (2) To determine c in ppm (parts per million) multiply equation (2) by 106. The concentration al pressures and temperatures other than STP are related to the concentration at STP by the ideal gas law.

Thus, equation (2) becomes: c(ppm) = (-1 06/kl) x (760/P) x (T/273) loge (I/io) (3) where P is in millimeters and T in degrees Kelvin. Equation (3) is solved automatically by the microcomputer.

The temperature transducer 75 and the pressure transducer 77, respectively, are, as noted, interfaced with the microcomputer to continuously correct for changes in these variables. The pressure transducer is preferably located at the outlet of the sample cell 24 and the temperature transducer may be at the mid-point of that cell.

In Fig. 2, much of the circuitry and apparatus is similar to that of Fig. 1. The same input port 12 and ozone-removing chamber 14 are used as are the flow meter 32, the needle valve 34, and pump 36 and the outlet port 38. A mercury lamp 40 with mirrors 42 and 44 is also used, the mirror 42 reflecting light through the reference cell 20 to a reference detector 46, the output of which passes through a digital electrometer 50 and converter for storage in a microcomputer 52.

In like fashion, the mirror 44 reflects light through the sample cell 24 to the detector 48, the output being integrated and converted to frequency in the element 54 and also passed to the microcomputer 52.

However, in addition to a sample cell 24, a second sample cell 24a is utilized. Radiation from the mercury lamp 40 impinges directly on the sample cell 24a rather than being deflected by mirrors. A sample detector 48a operates in conjunction with the sample cell 24a and the output of the sample detector 48a is passed through the electrometer and converter 54a to the microcomputer 52.

Because of the additional sample cell, detector, and electrometer-converter, some additional components are required. For example, although the sample entering the inlet port 12 and passing through the chamber 14 goes to the three-way valve 16a, unlike the apparatus of Fig. 1, the chamber 14 is not only in communication with the reference cell 20 through the line 18, it is also connected to the inlet of the second three-way solenoid valve 1 6 through the line 1 9. Moreover, the inlet port 12 is not only connected to the valve 1 6, it is also connected to an inlet of the valve 1 6a. An outlet line runs from the valve 16 to the sample cell 24 as in the apparatus of Fig. 1 and a second line runs from an outlet of the valve 1 6a to the sample cell 24a.

Electrical connections for operating the valves 1 6 and 1 6a are also slightly more complicated in that connections are made from the microcomputer 52 to both the valves 1 6 and 1 6a. Also, the microprocessor of the element 52 is somewhat differently programmed to provide the more complex timing signals needed to operate or switch the three-way valves 1 6 and 1 6a.

As has been noted, to achieve the goal of accuracy in the range of ozone concentrations of parts per billion, it is desirable that the system be re-zeroed as often as possible. In the apparatus of Fig. 2, preliminary purging is first conducted. Then a reading of 1c derived from the output of the reference detector 46 is taken and stored in the microcomputer 52.

The microprocessor of the element 52 is programmed then to perform a number of operations.

First, a signal is sent to trigger the valve 1 6 and permit the flow of ambient sample to the sample cell 24 for about 20 seconds, readings being taken and data gathered every second. Synchronously, sample minus-ozone is permitted to flow and flush the sample cell 24a for a period of nine seconds. Then, for two seconds, readings are taken of transmission through the sample-minus-ozone in the sample cell 24a to permit re-zeroing. A further flush of the sample cell 24a then takes place for nine seconds. At this point, the cell 24a is ready for the introduction of ambient sample. As the ambient sample is then introduced to the sample cell 24a the valves are triggered by the microprocessor 90 to permit sampleminus-ozone to be introduced to flush sample cell 24.Re-zeroing of the sample cell 24 then takes place while sample cell 24a is monitoring the concentration of ozone in the ambient sample.

By switching in the fashion described, the two sample cells operate in an essentially 1 800 out-ofphase relationship. While one cell is being flushed and re-zeroed, the other is monitoring measurements taking place every second. Thus, there is no black-out of measurements, which can be of great importance in certain applications such as in airborne instruments.

A change of 0.001 ppm in ozone corresponds to a change in transmittance of 1 part in 50,000. To detect ozone concentration within + or -0.001 ppm, the value of 1c cannot change by more than 1 part in 100,000. Lamp output intensity is difficult to control by any conventional means and the placement of the reference detector in the path of the output of the lamp results in the output intensity of the lamp being monitored by the reference detector simultaneously with the output of the operating sample detector. The microcomputer is supplied with this information to correct for any changes in 1c which may have occurred between the time it is actually measured and the time it is used to compute concentration some seconds later.

It has previously been noted that ozone is not the only gas or vapor which absorbs light at 254 nm.

If the concentration of these other substances does not vary, their effect on the accuracy of measurement is essentially cancelled because they are present when measurements are made of the contents of both the sample and reference cells. However, if the concentration of these other gases does vary with time, it would affect the measurement of loB The placement of the reference cell between the source of radiation and the reference detector minimizes this potential error. Any changes in 1c due to variation of the concentration of these other gases are monitored and internally corrected in the same manner as changes in lamp output intensity.

Claims

1. An absorption cell gas monitor for monitoring the concentration of a specific gas or vapour in an ambient atmosphere, comprising means for continuously drawing a sample of the ambient atmosphere into an inlet port, a chamber connected to the inlet port and containing material which removes the specific gas or vapour from the ambient sample passing through the chamber, a pair of absorption cells, one of which functions as a reference cell and is connected to the chamber so that there is a continuous flow through the reference cell of ambient sample from which the specific gas or vapour has been removed, and the other of which functions as a test cell, valve means for periodically connecting the inlet port to the test cell in addition to the chamber to cause an intermittent flow of the ambient sample through the test cell, a source of radiation of a predetermined wavelength which is absorbed by the specific gas or vapour arranged so that radiation from the source passes through both the reference cell and the test cell, and means for detecting and comparing the absorption of the radiation by the contents of the reference cell and the test cell.

2. A monitor according to claim 1, in which the valve means comprises a three-way valve having a first inlet connected to the inlet port, a second inlet connected to the outlet of the chamber, and an outlet connected to the test cell, and a timing circuit which controls the valve so that the test cell receives alternately for predetermined periods ambient sample directly from the inlet port and ambient sample from which the specific gas or vapour has been removed by the chamber.

3. A monitor according to claim 1, comprising a further absorption cell which functions as a second test cell and which is disposed so that radiation from the radiation source passes through it, and further valve means for periodically connecting the inlet port to the second test cell to cause an intermittent flow of the ambient sample through the second test cell which alternates with the flow of the ambient sample through the first test cell, the absorption detection and comparison means also detecting and comparing the absorption of the radiation by the contents of the reference cell and the second test cell.

4. A monitor according to claim 2, comprising a further absorption cell which functions as a second test cell and which is disposed so that radiation from the source passes through it, and a second three-way valve having a first inlet connected to the inlet port, a second inlet connected to the outlet of the chamber. and an outlet connected to the second test cell, the timing circuit controlling the second valve so that the second test cell also receives alternately for predetermined periods ambient sample directly from the inlet port and ambient sample from which the specific gas or vapour has been removed by the chamber, the ambient sample from the inlet port being directed alternately to the first and second test cells, and the absorption detection and comparison means also detecting and comparing the absorption of the radiation by the contents of the reference cell and the second test cell.

5. A monitor according to claim 4, in which the first and second valves are operated substantially 180 out-of-phase with each other so that each of the first and second test cells receives ambient sample from the inlet port while the other receives ambient sample from which the specific gas or vapour has been removed by the chamber.

6. A monitor according to claim 1, in which an accumulator is connected between the inlet port and the valve means, the accumulator being similar in volume and shape to the specific gas removing chamber whereby the ambient sample which is supplied to the reference cell from the specific gas removing chamber while ambient sample is supplied to the test cell will have been drawn in through the inlet port at substantially the same time as the ambient sample which is supplied to the test cell.

7. A monitor according to any one of the preceding claims, in which the specific gas to be monttored is ozone, the material in the chamber comprising metallic oxides for removing ozone from the ambient sample which passes through the chamber, and the means for continuously drawing ambient sample into the inlet port is a continuously operating pump connected to the reference and test cells downstream from the cells.

8. An absorption cell gas monitor for monitoring the concentration of ozone in an ambient atmosphere, comprising means for continuously drawing a sample of the ambient atmosphere into an inlet port, a chamber having an inlet connected to the inlet port and containing material for removing ozone from the ambient sample which passes through the chamber, a pair of absorption cells, one of which functions as a reference cell and is connected to the outlet of the chamber so that there is a continuous flow of the ambient sample from which ozone has been removed through the reference cell, and the other of which functions as a test cell, a three-way valve connecting the inlet port to the test cell in one position thereof and connecting the chamber to the test cell in a second position thereof, a microcomputer electrically connected to the three-way valve for controlling its position in accordance with a predetermined program to cause periodic flows of ambient sample from the inlet port and ambient sample from the chamber through the test cell, a source of radiation having a wavelength of substantially 254 nanometers and arranged so that radiation from the source passes through the reference cell and through the test cell, and detection means for detecting the absorption of the radiation by the contents of the reference cell and the test cell, the detection means being connected to and providing inputs to the microcomputer for computation therein to provide output data on the concentration of ozone in the ambient sample.

9. A monitor according to claim 8, comprising a further absorption cell which functions as a second test cell and which is disposed so that radiation from the source passes through it, a second three-way valve arranged to connect the inlet port to the second test cell in one position thereof and to connect the chamber to the second test cell in a second position thereof, and means for detecting the absorption of radiation by the contents of the second test cell, the microcomputer being connected to the second three-way valve to control its operation in accordance with a predetermined program whereby the first and second test cells are connected alternately to the chamber and alternately to the inlet port, and the microcomputer also being connected to receive, store and compute the outputs from the detection means of the reference cell and the first and second test cells in accordance with the predetermined program to provide a substantially continuous output representing the concentration of ozone in the ambient sample.

10. A monitor according to claim 1, substantially as described with reference to Figures 1 and 3 or to Figure 2 of the accompanying drawings.