Infrared Instrument for Industrial Product Control N. C. JAMISON, T. R. ROHLER, AND 0. G. KOPPIUS Philips Laboratories, Znc., Zrvington-on-Hudson, N . Y . tor bolometer served as the heat-sensitive element; samples in the two beams were compared six times per second and the difference in absorption was recorded. The sensitivity of the instrument was great enough to permit operation even with strongly absorbent filters. The operation of the instrument under plant conditions without the necessity for frequent optical adjustment is attributed to the rugged construction and the method of mounting the optical components.
. i n infrared differential comparator with no dispersion was built at the request of the Rubber Reserve fellowship of the 3lellon Institute of Industrial Research. The instrument was designed to be used for the control of the purity of various streams in the synthetic rubber industry. Good stability was obtained without thermostatic control by means of a Bicker system using a single source of radiation, symmetrical optical paths, a single radiation detector and alternating current amplification. A thermis-
A
SUAIBER of investigators have built simple nondispersive infrared instruments for product control in the chemical and petroleum industries (2-5, 8, 10-13). However, the fact that there are a large number of possible commercial applications has not been generally recognized and the development of satisfartory industrial control equipment has not progressed rapidly
symmetry of optical paths is desirable. I t is also imperative that the sample cells in the tvio paths be held a t the same temperature; otherwise, it is possible that the indication of an elaborate comparison instrument, when working a t its stability limit, may be the result of a small temperature difference between the t x o samples rather than a difference in absorption. In order to maintain the samples a t the same temp?rature, not only should the materials which enter the absorption cells be a t the same temperature, but the cells should be placed symmetrically with refercncw to the heat sources ivithin the instrument.
(9).
The infrared instrument described in this report was developed a t the reque'st of the Rubber Reserve fellowship of the AIellon Institute of Industrial Research. The problem was to construct an instrument to be used for the control of the purity of various streams in the synthetic rubber industry. The initial interest was primarily in the control of the misture of butadiene and recycled butadiene ( 5 ) . Subsequently, a large number of similar problems have arisen which are not necessarily connected with the rubber industry. For many such problems an instrument with no dispersion is entirely satisfactory. Familiarity with the nature of such infrared absorption problems is assumed; this paper concerns the instrument itself.
SAMPLE CELL
REQUIREMENTS AND GENERAL DESCRIPTION OF INSTRUMENT
The requirements to be satisfied by such an instrument may be briefly stated as follows: SAMPLE CELL
1. The instrument must have adequatc sensitivity; the signal should be well above noise level even for operation with very htrongly absorbant filters. 2. The drift over a period of hours should be practically negligihle and the sensitivity should remain constant. 3. The instrument should be rugged enough to be used in a production plant without the necessity for frequent adjustment. 4. The instrument should be operable by nontechnical personnel.
Figure 1.
The stability of such an instrument depends upon a number of factors, the most important of which are variations in the radiation from the source or Sources, variations in the sensitivity of the radiation receiver or receivers, drift and gain changes in the amplifier, and the effect of temperature changes. With regard to stability, a splitrbeam instrument is superior to a single-beam instrument only in so far as the effects of source and receiver fluctuations may be reduced. In order to take full advantage of the split-beam arrangement, it is necessary that a single source and a single receiver be used. A single sou-ce means that the radiation in both beams emanates from the same area of the source. This condition is not realized in all doublebeam instruments. Also the images produced by the two beams should coincide as nearly as possible a t a single receiver. The use of a single receiver imposes the condition that the radiation be chopped; this is not a disadvantage inasmuch as it is desirable to use alternating current amplification. In order that the drift owing to temperature change be small,
I IKU
Schematic Diagram of Optical and Electrical Components
The instrument described herein is a split-beam instrument employing a single source of radiation, a single radiation receiver, chopping, alternating current amplification, and symmetrical optical paths. It contains only one off-asis heat source, the preamplifier. In a later laboratory moliification of this instrument, the preamplifier was placed on the axis directly over the bolometer. Figure 1 shows schematically the optical and elcctrical portions
of the instrument. The source of radiation at A , which consists of a flat strip of Nichrome ribbon, is s-t :it the focus of a spherical mirror, B. The collimated beams :ire then rcflected by the double
mirrors a t C , pass through limiting apertures (which are not shown), and are chopped alternativrly l)y sectors mounted on the same shaft. The beams are then reflected from the mirrors D and D', again from E and E', and are incident on the double mirror at F ; then they pass to the spherical mirror, G , and to the radiation detector, H. With this nrr:ingement, a length of about 20 cm. is available for the introductioti of fiample cells. In the later laboratory modification of the instrument, the positions of
551
ANALYTICAL CHEMISTRY
552
the double mirror, F, and the spherical mirror, G, were interchanged. This permitted the mirrors E m d E' t o be located a t the extreme end of the base casting, and in this manner the space for the introduction of sample cells was increased t o about 40 cm. Reversing the optics in this manner required that the beam cross section be changed from the original segment of a
mum flux in the other beam requires a Sector rotation of 12". It is impossible to chop in such a manner that the percentage increase of flux in one beam is exactly equal to the percentage decrease of flux in the other beam during the changeover period; hence, there will always be some undesired changwver signal. The shading mechanism was designed to provide means for decreasing the flux in either beam with a minimum contribution to the undesired changeover signal. The edge of the shading plate which intercepts the beam has the same curvature as the limiting aperture and the plate is pivoted at such a point that the percentage of the maximum flux through the shaded and unshaded apertures is the same for any position of the sector. The effect of shading a beam in this manner is approximately the same as would he achieved by introducing a ueutral filter. The aperture of either beam may thus he varied from 50 to lW%, which is ample for operational purposes. Detecting System. Means must be provided for indicating which of the two h e m incident upon the bolometer has the greater power and, with adequate accuracy, the magnitude of the difference in the powers. Also, in a splibbeam instrument in which the beams are chopped, means for discriminating between the desired signal and that signal which results from mechanical
Figure 2 shows the actual instrument with the cover plates removed. In the source ohamber can be Seen the spring-loaded worm gears which control the shaders. T o the right in the Souroe chamber is a vernier adjustment for balance whioh consists of a mechanism for moving a h e wire in and out of the beam. On the left side there is a mechanism for checking the over-all 8en8itivity, whereby a h e wire may be inserted into one beam to produce a known decrease in flux. There are fittings for the inlet and outlet of fluids. The three cover plates may be removed without disturbing the mechanical controls or the electrical connections. The middle cover plate gives access to the absorption cell chamber. Figure 3 shows the instrument delivered to the Mellon Institute. CamplPte stand-by optical and electronic units were provided. DESCRIPTION O F VARIOUS COMPONENTS
Mirror Mounts. The method of mounting the mirrors in this instrument, while probably not new, at least differs from that ordinarily encountered. All mirrors are held in position by springs in suoh a manner that shacks whioh may displaoe the mirrors will not necessitate a readjustment of the instrument. A description ~~
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which i r e sopositionez thatthe motion of theA& ror is restricted but the mirror is not held tightly in
the &mor. These ball points establish the location of the center of curvature of the spherical mirror regardless of vertiosl or horizontal displacements. There are also three ball points which are spring loaded against the hack of the mirror. The points of contact of these balls and those at the front surface of the mirror are on lines passing through the center of curvature of the mirror. In this way, distortion of the mirror is avoided. Aperture of Instrument. The limiting apertures are segments of circles of diameter 2.625 inches cut by chords displaced 0.375 inch (0.952 cm.) from the center. The spherical mirror has a radius of curvature of 200 mm. and hence the speed of the optical system is about j l 2 . 5 , taking into account that the power in only one beam is incident on the receiver a t any one time. In the later laboratory modification of the instrument, the beams are of circular cross section havine a diameter of 0.93R
source has operated in the field for more than 10,000 hours without any apparent change; the useful life of the source cannot be estimated a t the present time. The Nichrome strip (',X I, 0,008 & inch) is wide compared to the bolometer strip in order that the imperfect images of paints in the source will overlap a t the bolometer and prnrlim uniform illumination over an area which is larger than that of the bolometer strip. The 10 watts necessary to heat the source are supplied by a transfarmer connected to a line voltage regulator. Radiation Detector. An air-cooled thermistor bolometer (Sett Bolometer Type V652) made by the Bell Telephone Laboratories serves a8 a radiationZ I detector and 1meets~ the ~rey+is?pentspf sen-~ ~ ~ ~
Detector Chamber
Absorption
Cell Chamher
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Chopping and Shading Mechanism. The methodof chopping thetwo beams and themeansfor shading the beams are illustrated in Figure 4. The change in maximum flux in one beam to maxi-
Figure 2.
Optical Unit with Cover Plates Removed
V O L U M E 2 3 NO. 4, A P R I L 1 9 5 1
553 the output voltage; this is readily obtained from a smll gener :or geared to the shaft which carries the beam-chopping sectors. hasesensitive detectors ( 1 , 7) are well known; the polarity of the rect current output of the network depends upon the relative lase of the bolometer signal with respect to the generator output. It may appear that the importance of the undesired signal which :curs w o n chanaina _ _ the oDtical path from one side to the other IS been overemphasiaed; however, this signal may he large as compared to the signal which carries the desired information. The undesired, or changeover signal appears twice for each rotation of the choppers. The signals may be of the =me or of opposite polarity. If they were of the same polrtrity with identical wave shapes, B virtual impossibility, there would be no contribution to the signal a t tbe fundamental frequency, six cycles per second, and the selective amplifier would provide sufficient attenuation for the suppression of this signal. The circuit used is effective in suppressing the ever-present 6-cycle component of the changeover signal because this signal differs in phase from the desired signd by approximately 90'; hence the contribution of the change-over signal to the direct current output of the phase-sensitive network m a y be made small. This is done by adjusting the phase of the generator output to differ from that of the nndesired simal 0 ' . The coarse Dbase adiustment is . ~ hv ~" -9 ~. ~ made by rotating the housing of the generator and the fine adjustment by means of a phaseshifting network. ~
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ANALYTICAL CHEMISTRY ACKNOWLEDGMENT
The authors wish to thank J.
G. Black for his contributions
Figure 6.
during the early stages of dc.velopment of the instrument, 0. S. Duffendack for his continued interest, and Michael Poleshuk, G. W. Anderson, and the staff of the instrument shop, headed by R. P. Johnston, for their excellent work in the fabrication of the instrument.
Chart Indicating Stability of I n s t r u m e n t
temperature. For many applications, the drift aould not bo troublesome; however, it is desirable to thermostat the instrument if it is to be subjected t o wide variations in temperature. The chart reproduced in Figure 5 shows a drift of about +O.Ol% aver a &hour period; the run was made in a laboratory room in which the temperature was poorly regulated. The shorbtime variations account for the width and fuzainess of the record shown in Figure 6. These variations amount to about *0.002% or *20 p.p.m. change in flux m one beam. The source of this noise component has not been established; tests have shown, however, that it is not electrical noise in the detecting system. Consequently, the flux may be reduced by strongly absorbant filters in both beams without any reduction in the signal-t+noise ratio. Many industrial uses of the instrument require such filters for sati8frtctory operrttion. The application of instruments of this type to the solution of chemical problems is discussed in another paper ( 6 ) .
LITERATURE CITED
(1) Farren, L. I., Wireless Eng.. 23,330 (1946). ( 2 ) Fastie. , l n ” *W. \ G.. and Pfund, A. H., J . Optiml Soc. Am., 37, 762 ,AT*,,.
(3) Fowler. R. C.. Reo. Sci. Imlnmzents. 20, 175 (1949). (4) Jamison, N. C.,Kohler, T. R.. and Koppius, 0. G., J . Optical SOC.Am., 38, 1099 (A) (1948). 1.5) . Kivenson.. G.. . Steinback. R. T.. and Rider. M., 163.. 38, lW6 (1948). (6) Koppius. 0.G.. ANAL.CHEM..23,554 (1951). (7) Lev, M.,Elec. Cormnun.. 18. 206-28 (1940). (8) Luft. K. F.. Z. tech. physik, 24. 97 (1943). (9) Martin. G. A,, Instwmmzts. 22, 1102 (1949). (10) Pfund, A. H., Science. 90,236 (1939). (11) Phmd, A. H.,and Gemmill, C. L., Bull. Johns Hc 67,61 (1940). (12) Schmiok, H., U. S. Patent 1,758,088.May 13. I€ (13) Wright, N.. and Herseher, L. W,. J. Optic01 Soc (1946). R E C E W EOotober ~ 2.
1950.
Analysis of Mixtures with Double-Beam Nondispersive Infrared Instrument 0. G . KOPPIUS P h i l i p Laboratories, Inc., Iruington-on-Hudson, N. Y. The general problem of the analysis of mixtures by a nondispersive double-beam infrared instrument has heen investigated because there appears nowhere in the literature a n y thorough description of methods of sensitizing an instrument of t h i s type or conditions under which it must he used. By utiliri n g the wmpounds occurring in any given mixture as filtering material i n a suitable arrangement of cells, it i s shown t h a t adequate sensitivity, accuracy, and freedom from interference can he obtained
A .
NALYSIS of complex mixtures for one or more components
by infrared spectroscopy has heen developed during recent years as a highly valuable technique. The method is partiqularly useful because the absorption spectrum is a unique property of a compound and, in the absence of chemical reaction, is retained upon admixture with other substances. A wave length-absorp tion plot therefore serves to identify the substances presont, and from the intensity of absorption at properly selected wave lengths, one may determine, by using the well-known Lambert and Beer relationships, the quantity of substances present in the sample having absorption a t those wave lengths. Conventional application of the technique requires the use of an infrared instrument having means for dispersing the radiation employed. Consideration is given herein to the possibility of
for m a n y cases of practical analytical interest. The investigation wvecs t h r e e ranges of oonoentration of interfering components: (1) a wide range s t a r t i n g from zem; (2) a small range starting from zem; (3) a range between two fixed values of eonoentration. Examples are presented using methane, carbon dioxide, and propane for the interfering wmponents for parts 1a n d 2, a n d propane, propylene, and ethylene for part 3. These results clearly indicate t h e potentialities of t h i s type of instrument. using nondispersive instruments for the analysis and control of the composition of mixtures. A number of nondispersive instruments have heen described recently (f-lS). In general, they have advantages over dispersive instruments in simplicity, ruggedness, lower casts, and better suitability for plant or control laboratory application. Some general considerations for the application of tho instrument have been published (I, S, 4, 8, 18). A number of the references describe specific applications. However, there appears nowhere in the literature m y thorough description of methods for sensitizing the instrument, conditions under which it must be usod, or factors influencing precision and actecuraxy. This paper describes a means of visualizing the phrnomena occurring in the nondispersive analyzer and derives B general theory for i k use in the snalysis of mixtures. A non-
