Ann. occvp. Hyg. Vol. 18, pp. 45-51. Perg«mon Pren 1975. Printed In Great Brtutn
INFRARED TECHNIQUES FOR THE MEASUREMENT OF CARBON MONOXIDE K.
F.
LUFT
Forschungsinstitut des Steinkohlenbergbauvereins, Esscn-Kray, West Germany
I M P O R T A N C E AND PHYSICAL P R I N C I P L E S OF I N F R A R E D GAS ANALYSIS AMONG methods for measuring CO in the atmosphere, i.r. gas analysis is of particular
importance. There are at present more than 50 000 i.r. gas analysers in use all over the world, of which a high proportion are used for this purpose. This development is due to the increasing importance of CO not only as a component of combustion gases, in particular of motor cars, but also in other branches of industry. For example, the measurement of CO traces is an excellent tool for the early detection of mine fires and, in West German coal mines alone, nearly 1000 i.r. gas analysers have been installed to measure traces of CO in the mine air. The faculty of matter for absorbing electro-magnetic radiation in certain wavelength ranges has been the starting point for the development of numerous physical methods of analysis. Of particular interest is the spectral range extending from visible to the near and intermediate infrared where practically all substances display characteristic absorption spectra. Moreover, this range is relatively easily accessible to physical measuring techniques. N6n-elementary gases produce strong absorption bands in this range, which are due to the interaction between radiation and the vibrations and rotations of the molecules. Figure 1 illustrates the position of the absorption bands of
100
Wavelength [|jm] 1 10 100
X-rays juttraviol Visi-
Infrared
ble
) Microwaves
Absorption CH4 C0 2
3
4
CHt CO
5 6 7 8 Wavelength |(jm]
9
10
FIG. 1. Location of i.r. absorption bands within the electromagnetic spectrum. 45
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
Abstract—The importance of the i.r. technique, particularly for measuring traces of carbon monoxide in air, is mentioned and the physical principles are explained. Selective response is obtained without spectral dispersion by the use of a pneumatic detector cell filled with gas of the same type as that to be measured. A double beam system is normally employed to give zero stability and, more recently, a two-layer detector cell has been introduced which further improves the stability and selectivity. Further promising developments are briefly outlined.
46 •
K. F. LUFT
some gases in a very simplified form. The CO spectrum is shown in greater detail in Fig. 2, which was obtained by a high-resolution spectograph. The position of the centre of this band, at 4-66 (xm, is determined by the vibration of the C and O atoms in relation to each other; the structure of the band is due to the impact of rotation. In reality, the distance between the rotation lines, compared with their width, is much larger; to reveal the real band structure, a spectrograph with a much higher resolving power would be required. Whereas the diatomic CO molecule shows a single fundamental band, composed of two branches, the spectra of polyatomic molecules become more and more complex according to the increasing number of degrees of freedom.
4,9
4.8
R-branch
4.7 4.6 Wavelength [pm]
4,5
FIG. 2. Rotation fine structure of the CO absorption band at 4-66 (im.
Although it is difficult to determine exactly the band structure of gases with the spectrographs so far available, the extinction values measured at a great number of different wavelengths furnish sufficient information for solving even complex analytical problems in i.r. spectroscopy. Although i.r. spectroscopy has made its way in the analytical laboratories, its application to industrial control remains restricted to exceptional cases, not only on account of cost, but also because the problems to be solved are usually different. Often it is sufficient to measure a certain component of a gas mixture of which the qualitative composition is known. Therefore, attempts were made to determine the concentration of an i.r. absorbing component of a mixture by a simple integral absorption measurement, without the help of spectral dispersion systems. A precedent was given by the colorimetric analysis of liquids within the visible spectral range. In its simplest form such a colorimeter is composed of a light source, a cell filled with a sample of the substance to be analysed, and a detector, such as a photo-cell, to measure the intensity of radiation after its passage through the cell. It was found, however, that it is not possible for this simple photometric technique to be transferred without modifications to the i.r. analysis of gases. There are hardly any problems of gas analysis where the mixture contains only one i.r. absorbing component. As a rule, the measurements of CO traces in the atmosphere have to be made in the presence of much higher concentrations of CO 2 or water vapour. The simplification of such problems, if at all feasible, by elimination of interfering components, e.g. with chemical absorbents, leads inevitably to the loss of valuable features of the otherwise purely physical measuring method.
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
P-branch
Infrared techniques for the measurement of carbon monoxide
Source
Sample cell
i ' \
• CO o N 2 * 0 2 » C 0 ;
IK,
0 2 4 6 8
Detector chamber
Wavelength[pml
Fio. 3. Principle of the non-dispersive i.r. gas analyser based on selective detection.
filled with the gas to be determined (CO). The right-hand side of the diagram shows, schematically, the spectral distribution of radiation emitted by the source. Since both nitrogen and oxygen are non-absorbing in the i.r. range, the radiation passes both cells unweakened (apart from losses by reflection from the windows) except for the parts which fall into the CO and CO 2 absorption ranges, referred to in the following as CO or CO 2 radiation. These are weakened in the sample cell pro rata to the CO and CO 2 contents as shown in the second curve. The remaining CO radiation is almost completely absorbed by the CO-filling in the detector cell (bottom curve) and thereby heats up. the detector gas; the other radiation ranges do not contribute to this rise in temperature. Thus, the amount of heating is a measure of the CO radiation still remaining after the preliminary absorption in the sample cell, and depends on the CO concentration in the sample cell. The amount of heating can be measured in different ways. The most common method is the pneumatic technique which we introduced from the very beginning: the rise in pressure caused by the heating of the gas being measured with a condenser microphone. Some figures will indicate the order of magnitude of the effects involved. Assuming that the power input to the source of radiation is about 5W, the portion of the radiation
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
INFRARED ANALYSER WITH SELECTIVE DETECTOR Thus, the idea suggested itself to modify the simple photometer arrangement by the insertion of optical niters, as is done in other spectral ranges, to achieve selectivity. Among the various possibilities, the method of selective detection has proved particularly efficient. This method, the beginnings of which can be traced back to the research work of Tyndall, Bell and RSntgen, was developed to technical perfection a short time before the second world war in the laboratories of BASF by Lehrer and myself. The success of the non-dispersive i.r. gas analyser developed at that time helped the rapid propagation of this measuring technique after the war. As it is impossible in a short lecture to survey all the many types of instrument now existing, I shall restrict my comments mainly to my own studies and some of the instruments to which they have led; for the rest, I refer to the relevant literature (e.g. HILL and POWELL, 1968). Briefly, the principle of the selective measuring technique is to use as detector the component of interest in the gas mixture. This is shown schematically in Fig. 3. Radiation from a heat source, e.g. an incandescent wire spiral, passes through a sample cell containing the gas mixture to be analysed (air + CO), and into a detector
48
K. F. LUFT
which is effective for the selective heating of the detector gas does not exceed 10 mW. A concentration of 100 ppm CO in a 200 mm-long sample cell attenuates the CO radiation by about 1/100. The changes in temperature and pressure in the detector cell which are caused by the absorption amount to about 0 0003°C and 0 005 mm water gauge, respectively. By means of the pneumatic detector, the change in pressure is converted into a change in capacity. Its value, too, is extremely small, but the advent of modern electronics has made it possible to obtain a reliable and stable output signal which can easily be indicated, recorded and teletransmitted. Figure 4 shows schematically the design of the first commercial i.r. gas analyser which was introduced in the chemical industry in 1938 under the trade name URAS (LUFT, 1943). Later, it found application in other fields of technique and science, and Sample cell
Detector Indicator
Amplifier Reference cell Chopper
FIG. 4. I.R. double beam analyser with pneumatic detector.
served as a model for the development of numerous other instruments. It is a doublebeam instrument; the second, reference, beam permits a symmetrical set-up of the measuring system and gives the instrument improved stability. The double optopneumatic detector responds to the differential selective absorption of the radiation in the two beams, the difference being due only to the presence of the component to be measured, e.g. CO, in the sample cell. Another feature is the simultaneous modulation of the two beams using a rotating chopper. This enables the very small temperature and pressure effects caused by selective absorption of the radiation to be distinguished from other temperature and pressure changes. At present, the large majority of all non-dispersive i.r. analysers operate on this principle. UNOR ANALYSER
In view of the particular requirements to be met by such an instrument in coal mines, a new type was devised some 15 yr ago in the physics department of BergbauForschung in Essen; this has become known under the trade name UNOR. The direct incentive to its development was a prize competition sponsored by the then High Authority of the European Community for Iron and Steel with a view to improving safety in mines. The particular difficulties involved in such underground measuring problems, due inter alia to the high methane, moisture and dust content of the mine air, have been successfully overcome by the UNOR analyser (LUFT et al., 1967), shown diagrammatically in Fig. 5. Its essential feature is an opto-pneumatic detector with
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
Source
Infrared techniques for the measurement of carbon monoxide
m
the two detector cells arranged in series in the same beam path so as to measure, with a condenser microphone, the pressure difference caused by the unequal absorption of radiation. With appropriately-shaped cells, in particular a longer rear cell than front cell, it can be arranged that the signals arising in the two cells are equal and the Synchronous motor Single infra -red source
Flowmeter Beam-combining tube Detector front chamber Detector rear, chamber Connecting tube
Reference beam path Connecting lube
Diaphragm
j j Chart IS recorder fce|_Output il~meter
Transistorized amplifier
Fto. 5. Schematic diagram of the UNOR analyser.
differential signal emitted by the detector is zero at zero CO in the sample cell. Additional zero stabilization is attained by combining the single-beam two-layer detector with a photometer operating on the principle of alternate chopping of the two beams. When the intensity of radiation is the same in both the sample and the reference beam paths, i.e. at zero sample, the radiation entering the detector remains more or less constant so that there is no (chopped) output signal even though there may be some imbalance in the two-layer detector. Only when the equilibrium of the radiations is changed by the occurrence of CO in the sample cell, is a signal corresponding to the CO content generated in the detector. Figure 6 illustrates the spectral distribution of absorption in the two gas layers of the detector, with restriction to one line of the CO absorption band shown in Fig. 2. The absorption pattern is given for only half the width of such a line, on the left without, on the right with, preliminary absorption by CO in the sample cell. As shown in the figure, most of the radiation intercepted in the front detector cell comes from the centre of the band, whereas in the rear cell it comes mostly from the flanks. The length ratio of the two cells is so adjusted that the signals generated in both of them are more or less equal; thus, the differential signal is zero. If any preliminary absorption takes place in the sample cell, due to the presence of CO, as indicated on the right side of the figure, its effect is restricted to the absorption in the front cell; there is practically no effect on the rear cell. Thus, a differential signal which depends on the preliminary absorption or, in other words, on the CO content of the gas to be measured, is emitted from the detector. Neutral changes of radiation by contrast, independent of the wavelength, exert the same influence on the signals from both cells so that the differential signal remains unchanged. Thus, the zero of a single-beam two-layer
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
Optical tubes
50
K. F. LUFT
detector is much less affected by changes in radiation which might be due to the ageing of the emitters, contamination of the cells etc., than in the case of a two-beam detector where such changes take full effect. The better zero stability of the single-beam detector has in fact been confirmed in particular under the severe conditions obtaining in coal mines. Absorbed radiation
no CO in sample cell
with CO in sample cell detector cell sample cell second detector cell
/ first
It should be emphasised also that the second important merit of i.r. gas analysers, viz. their selectivity in the presence of interfering gases, is enhanced by the in-series arrangement of the detector cells. As a rule, selectivity is restricted chiefly by the fact that the bands of the gas to be measured are overlapped by the absorption bands of other gases. In a two-layer detector there is a certain probability that the interfering absorptions are distributed between the zones of maximum absorption of both cells and will neutralize each other, at least in part, in particular if the lines of a spectrum are considered as a whole. For many problems the higher selectivity obtained in this way is quite sufficient, so that additional measures such as drying of the gases can be dispensed with. If the selectivity is insufficient it can be improved by inserting a suitable interference filter into the beam path. This is the case in coal mines when it is required to measure a few ppm of CO in sealed-off, non-ventilated panels where the concentration of CH 4 and CO 2 can reach high levels. Also, when CO-concentration measurements are to be carried out for air pollution control, the i.r. measuring technique meets practically all requirements. Using sample cells some hundred mm in length, measuring ranges down to 0-20 ppm CO can be covered. The development of this technique is far from complete. Recently, for instance, we succeeded in developing a small, portable instrument which consumes so little power that it is intrinsically safe and can be operated with a self-contained battery (LUFT and LANGNER, 1974). Much of the progress made during recent years was due to the advent of semiconductor electronics, and the use of lasers and laser diodes is likely to give stimulus to further improvements. By concentrating the radiation in the narrow spectral ranges of absorption lines or bands it will be possible to measure still smaller gas traces. A first promising step in this direction is the report that pollutions of the air by nitric oxides down to 0 01 ppm have been measured using tunable radiation from a laser and an opto-pneumatic detector (KREUZER and PATEL, 1971). Compared with the present potential of the i.r. measuring technique this means an increase in sensitivity by a factor near to 100.
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
FIG. 6. Absorption in the two gas layers of the detector cell of the UNOR.
Infrared techniques for the measurement of carbon monoxide
51
CHARACTERISTIC FEATURES A N D F U T U R E DEVELOPMENT OF THE INFRARED TECHNIQUE
REFERENCES Hnx, D . W. and POWELL, T. (1968) Non-dispersive Infrared Gas Analysis in Science, Medicine and Industry. Hilger, London. KREUZER, L. B. and PATEL, C. K. N . (1971) Science 173, 45.
LUFT, K. F. (1943) Z. tech. Phys. 24, 97 LUPT, K. F., KESSELER, G. and ZORNER, K. H. (1967) Chemie-Ingr-Tech. 39, 937.
LUFT, K. F. and LANONER, W.-D. (1974) GlUckauf110, 125.
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
In conclusion, let me summarize the characteristic features which distinguish the i.r. technique from the other measuring techniques discussed at this conference. The mode of operation is purely physical, there are no chemical changes of the substance to be examined or within the measuring system. Hence, changes in performance can only be caused by the normally slow ageing of the mechanical or electronic structural elements; consequently, the zero and the sensitivity of the measuring system are very stable. The instruments have a long life and require little maintenance. Simply by choosing the proper length of the sample cell, the instruments can be used to measure high concentrations as well as small traces of gas. The gas in the sample cell can be changed easily and rapidly, making possible measurement times of some seconds. The high selectivity, in particular when using interference filters, enables the instruments to measure reliably CO traces in the atmosphere, also in the presence of high concentrations of interfering gases. It is true that these excellent measuring properties have to be paid for by a relatively higher amount of instrumentation which, however, shows clearly a trend toward simplification, owing to the development of semiconductor electronics. As far as I can see, the i.r. measuring technique will continue to hold its ground in the future, against competing measuring techniques, especially in the important field of measuring CO in the atmosphere.
INFRARED TECHNIQUES FOR THE MEASUREMENT OF CARBON MONOXIDE K.
F.
LUFT
Forschungsinstitut des Steinkohlenbergbauvereins, Esscn-Kray, West Germany
I M P O R T A N C E AND PHYSICAL P R I N C I P L E S OF I N F R A R E D GAS ANALYSIS AMONG methods for measuring CO in the atmosphere, i.r. gas analysis is of particular
importance. There are at present more than 50 000 i.r. gas analysers in use all over the world, of which a high proportion are used for this purpose. This development is due to the increasing importance of CO not only as a component of combustion gases, in particular of motor cars, but also in other branches of industry. For example, the measurement of CO traces is an excellent tool for the early detection of mine fires and, in West German coal mines alone, nearly 1000 i.r. gas analysers have been installed to measure traces of CO in the mine air. The faculty of matter for absorbing electro-magnetic radiation in certain wavelength ranges has been the starting point for the development of numerous physical methods of analysis. Of particular interest is the spectral range extending from visible to the near and intermediate infrared where practically all substances display characteristic absorption spectra. Moreover, this range is relatively easily accessible to physical measuring techniques. N6n-elementary gases produce strong absorption bands in this range, which are due to the interaction between radiation and the vibrations and rotations of the molecules. Figure 1 illustrates the position of the absorption bands of
100
Wavelength [|jm] 1 10 100
X-rays juttraviol Visi-
Infrared
ble
) Microwaves
Absorption CH4 C0 2
3
4
CHt CO
5 6 7 8 Wavelength |(jm]
9
10
FIG. 1. Location of i.r. absorption bands within the electromagnetic spectrum. 45
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
Abstract—The importance of the i.r. technique, particularly for measuring traces of carbon monoxide in air, is mentioned and the physical principles are explained. Selective response is obtained without spectral dispersion by the use of a pneumatic detector cell filled with gas of the same type as that to be measured. A double beam system is normally employed to give zero stability and, more recently, a two-layer detector cell has been introduced which further improves the stability and selectivity. Further promising developments are briefly outlined.
46 •
K. F. LUFT
some gases in a very simplified form. The CO spectrum is shown in greater detail in Fig. 2, which was obtained by a high-resolution spectograph. The position of the centre of this band, at 4-66 (xm, is determined by the vibration of the C and O atoms in relation to each other; the structure of the band is due to the impact of rotation. In reality, the distance between the rotation lines, compared with their width, is much larger; to reveal the real band structure, a spectrograph with a much higher resolving power would be required. Whereas the diatomic CO molecule shows a single fundamental band, composed of two branches, the spectra of polyatomic molecules become more and more complex according to the increasing number of degrees of freedom.
4,9
4.8
R-branch
4.7 4.6 Wavelength [pm]
4,5
FIG. 2. Rotation fine structure of the CO absorption band at 4-66 (im.
Although it is difficult to determine exactly the band structure of gases with the spectrographs so far available, the extinction values measured at a great number of different wavelengths furnish sufficient information for solving even complex analytical problems in i.r. spectroscopy. Although i.r. spectroscopy has made its way in the analytical laboratories, its application to industrial control remains restricted to exceptional cases, not only on account of cost, but also because the problems to be solved are usually different. Often it is sufficient to measure a certain component of a gas mixture of which the qualitative composition is known. Therefore, attempts were made to determine the concentration of an i.r. absorbing component of a mixture by a simple integral absorption measurement, without the help of spectral dispersion systems. A precedent was given by the colorimetric analysis of liquids within the visible spectral range. In its simplest form such a colorimeter is composed of a light source, a cell filled with a sample of the substance to be analysed, and a detector, such as a photo-cell, to measure the intensity of radiation after its passage through the cell. It was found, however, that it is not possible for this simple photometric technique to be transferred without modifications to the i.r. analysis of gases. There are hardly any problems of gas analysis where the mixture contains only one i.r. absorbing component. As a rule, the measurements of CO traces in the atmosphere have to be made in the presence of much higher concentrations of CO 2 or water vapour. The simplification of such problems, if at all feasible, by elimination of interfering components, e.g. with chemical absorbents, leads inevitably to the loss of valuable features of the otherwise purely physical measuring method.
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
P-branch
Infrared techniques for the measurement of carbon monoxide
Source
Sample cell
i ' \
• CO o N 2 * 0 2 » C 0 ;
IK,
0 2 4 6 8
Detector chamber
Wavelength[pml
Fio. 3. Principle of the non-dispersive i.r. gas analyser based on selective detection.
filled with the gas to be determined (CO). The right-hand side of the diagram shows, schematically, the spectral distribution of radiation emitted by the source. Since both nitrogen and oxygen are non-absorbing in the i.r. range, the radiation passes both cells unweakened (apart from losses by reflection from the windows) except for the parts which fall into the CO and CO 2 absorption ranges, referred to in the following as CO or CO 2 radiation. These are weakened in the sample cell pro rata to the CO and CO 2 contents as shown in the second curve. The remaining CO radiation is almost completely absorbed by the CO-filling in the detector cell (bottom curve) and thereby heats up. the detector gas; the other radiation ranges do not contribute to this rise in temperature. Thus, the amount of heating is a measure of the CO radiation still remaining after the preliminary absorption in the sample cell, and depends on the CO concentration in the sample cell. The amount of heating can be measured in different ways. The most common method is the pneumatic technique which we introduced from the very beginning: the rise in pressure caused by the heating of the gas being measured with a condenser microphone. Some figures will indicate the order of magnitude of the effects involved. Assuming that the power input to the source of radiation is about 5W, the portion of the radiation
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
INFRARED ANALYSER WITH SELECTIVE DETECTOR Thus, the idea suggested itself to modify the simple photometer arrangement by the insertion of optical niters, as is done in other spectral ranges, to achieve selectivity. Among the various possibilities, the method of selective detection has proved particularly efficient. This method, the beginnings of which can be traced back to the research work of Tyndall, Bell and RSntgen, was developed to technical perfection a short time before the second world war in the laboratories of BASF by Lehrer and myself. The success of the non-dispersive i.r. gas analyser developed at that time helped the rapid propagation of this measuring technique after the war. As it is impossible in a short lecture to survey all the many types of instrument now existing, I shall restrict my comments mainly to my own studies and some of the instruments to which they have led; for the rest, I refer to the relevant literature (e.g. HILL and POWELL, 1968). Briefly, the principle of the selective measuring technique is to use as detector the component of interest in the gas mixture. This is shown schematically in Fig. 3. Radiation from a heat source, e.g. an incandescent wire spiral, passes through a sample cell containing the gas mixture to be analysed (air + CO), and into a detector
48
K. F. LUFT
which is effective for the selective heating of the detector gas does not exceed 10 mW. A concentration of 100 ppm CO in a 200 mm-long sample cell attenuates the CO radiation by about 1/100. The changes in temperature and pressure in the detector cell which are caused by the absorption amount to about 0 0003°C and 0 005 mm water gauge, respectively. By means of the pneumatic detector, the change in pressure is converted into a change in capacity. Its value, too, is extremely small, but the advent of modern electronics has made it possible to obtain a reliable and stable output signal which can easily be indicated, recorded and teletransmitted. Figure 4 shows schematically the design of the first commercial i.r. gas analyser which was introduced in the chemical industry in 1938 under the trade name URAS (LUFT, 1943). Later, it found application in other fields of technique and science, and Sample cell
Detector Indicator
Amplifier Reference cell Chopper
FIG. 4. I.R. double beam analyser with pneumatic detector.
served as a model for the development of numerous other instruments. It is a doublebeam instrument; the second, reference, beam permits a symmetrical set-up of the measuring system and gives the instrument improved stability. The double optopneumatic detector responds to the differential selective absorption of the radiation in the two beams, the difference being due only to the presence of the component to be measured, e.g. CO, in the sample cell. Another feature is the simultaneous modulation of the two beams using a rotating chopper. This enables the very small temperature and pressure effects caused by selective absorption of the radiation to be distinguished from other temperature and pressure changes. At present, the large majority of all non-dispersive i.r. analysers operate on this principle. UNOR ANALYSER
In view of the particular requirements to be met by such an instrument in coal mines, a new type was devised some 15 yr ago in the physics department of BergbauForschung in Essen; this has become known under the trade name UNOR. The direct incentive to its development was a prize competition sponsored by the then High Authority of the European Community for Iron and Steel with a view to improving safety in mines. The particular difficulties involved in such underground measuring problems, due inter alia to the high methane, moisture and dust content of the mine air, have been successfully overcome by the UNOR analyser (LUFT et al., 1967), shown diagrammatically in Fig. 5. Its essential feature is an opto-pneumatic detector with
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
Source
Infrared techniques for the measurement of carbon monoxide
m
the two detector cells arranged in series in the same beam path so as to measure, with a condenser microphone, the pressure difference caused by the unequal absorption of radiation. With appropriately-shaped cells, in particular a longer rear cell than front cell, it can be arranged that the signals arising in the two cells are equal and the Synchronous motor Single infra -red source
Flowmeter Beam-combining tube Detector front chamber Detector rear, chamber Connecting tube
Reference beam path Connecting lube
Diaphragm
j j Chart IS recorder fce|_Output il~meter
Transistorized amplifier
Fto. 5. Schematic diagram of the UNOR analyser.
differential signal emitted by the detector is zero at zero CO in the sample cell. Additional zero stabilization is attained by combining the single-beam two-layer detector with a photometer operating on the principle of alternate chopping of the two beams. When the intensity of radiation is the same in both the sample and the reference beam paths, i.e. at zero sample, the radiation entering the detector remains more or less constant so that there is no (chopped) output signal even though there may be some imbalance in the two-layer detector. Only when the equilibrium of the radiations is changed by the occurrence of CO in the sample cell, is a signal corresponding to the CO content generated in the detector. Figure 6 illustrates the spectral distribution of absorption in the two gas layers of the detector, with restriction to one line of the CO absorption band shown in Fig. 2. The absorption pattern is given for only half the width of such a line, on the left without, on the right with, preliminary absorption by CO in the sample cell. As shown in the figure, most of the radiation intercepted in the front detector cell comes from the centre of the band, whereas in the rear cell it comes mostly from the flanks. The length ratio of the two cells is so adjusted that the signals generated in both of them are more or less equal; thus, the differential signal is zero. If any preliminary absorption takes place in the sample cell, due to the presence of CO, as indicated on the right side of the figure, its effect is restricted to the absorption in the front cell; there is practically no effect on the rear cell. Thus, a differential signal which depends on the preliminary absorption or, in other words, on the CO content of the gas to be measured, is emitted from the detector. Neutral changes of radiation by contrast, independent of the wavelength, exert the same influence on the signals from both cells so that the differential signal remains unchanged. Thus, the zero of a single-beam two-layer
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
Optical tubes
50
K. F. LUFT
detector is much less affected by changes in radiation which might be due to the ageing of the emitters, contamination of the cells etc., than in the case of a two-beam detector where such changes take full effect. The better zero stability of the single-beam detector has in fact been confirmed in particular under the severe conditions obtaining in coal mines. Absorbed radiation
no CO in sample cell
with CO in sample cell detector cell sample cell second detector cell
/ first
It should be emphasised also that the second important merit of i.r. gas analysers, viz. their selectivity in the presence of interfering gases, is enhanced by the in-series arrangement of the detector cells. As a rule, selectivity is restricted chiefly by the fact that the bands of the gas to be measured are overlapped by the absorption bands of other gases. In a two-layer detector there is a certain probability that the interfering absorptions are distributed between the zones of maximum absorption of both cells and will neutralize each other, at least in part, in particular if the lines of a spectrum are considered as a whole. For many problems the higher selectivity obtained in this way is quite sufficient, so that additional measures such as drying of the gases can be dispensed with. If the selectivity is insufficient it can be improved by inserting a suitable interference filter into the beam path. This is the case in coal mines when it is required to measure a few ppm of CO in sealed-off, non-ventilated panels where the concentration of CH 4 and CO 2 can reach high levels. Also, when CO-concentration measurements are to be carried out for air pollution control, the i.r. measuring technique meets practically all requirements. Using sample cells some hundred mm in length, measuring ranges down to 0-20 ppm CO can be covered. The development of this technique is far from complete. Recently, for instance, we succeeded in developing a small, portable instrument which consumes so little power that it is intrinsically safe and can be operated with a self-contained battery (LUFT and LANGNER, 1974). Much of the progress made during recent years was due to the advent of semiconductor electronics, and the use of lasers and laser diodes is likely to give stimulus to further improvements. By concentrating the radiation in the narrow spectral ranges of absorption lines or bands it will be possible to measure still smaller gas traces. A first promising step in this direction is the report that pollutions of the air by nitric oxides down to 0 01 ppm have been measured using tunable radiation from a laser and an opto-pneumatic detector (KREUZER and PATEL, 1971). Compared with the present potential of the i.r. measuring technique this means an increase in sensitivity by a factor near to 100.
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
FIG. 6. Absorption in the two gas layers of the detector cell of the UNOR.
Infrared techniques for the measurement of carbon monoxide
51
CHARACTERISTIC FEATURES A N D F U T U R E DEVELOPMENT OF THE INFRARED TECHNIQUE
REFERENCES Hnx, D . W. and POWELL, T. (1968) Non-dispersive Infrared Gas Analysis in Science, Medicine and Industry. Hilger, London. KREUZER, L. B. and PATEL, C. K. N . (1971) Science 173, 45.
LUFT, K. F. (1943) Z. tech. Phys. 24, 97 LUPT, K. F., KESSELER, G. and ZORNER, K. H. (1967) Chemie-Ingr-Tech. 39, 937.
LUFT, K. F. and LANONER, W.-D. (1974) GlUckauf110, 125.
Downloaded from http://annhyg.oxfordjournals.org/ at Université Laval on June 28, 2015
In conclusion, let me summarize the characteristic features which distinguish the i.r. technique from the other measuring techniques discussed at this conference. The mode of operation is purely physical, there are no chemical changes of the substance to be examined or within the measuring system. Hence, changes in performance can only be caused by the normally slow ageing of the mechanical or electronic structural elements; consequently, the zero and the sensitivity of the measuring system are very stable. The instruments have a long life and require little maintenance. Simply by choosing the proper length of the sample cell, the instruments can be used to measure high concentrations as well as small traces of gas. The gas in the sample cell can be changed easily and rapidly, making possible measurement times of some seconds. The high selectivity, in particular when using interference filters, enables the instruments to measure reliably CO traces in the atmosphere, also in the presence of high concentrations of interfering gases. It is true that these excellent measuring properties have to be paid for by a relatively higher amount of instrumentation which, however, shows clearly a trend toward simplification, owing to the development of semiconductor electronics. As far as I can see, the i.r. measuring technique will continue to hold its ground in the future, against competing measuring techniques, especially in the important field of measuring CO in the atmosphere.
