INSTRUMENTATION Automatic Optical Pyrometers Measure High Temperatures r y HE PROBLEM of measuring very high J- temperatures is of continuing interest in present day science and technology. For many years, the optical pyrometer of the disappearing filament type has been the standard for the measurement of high temperatures. I t was natural that attempts would be made to substitute photoelectric methods for the more precise comparison of brightness of the target and the center portion of the filament of the standard lamp. Recently, we raised the question of what had happened to commercial developments of this sort. One answer has been supplied to us by H. L. Daneman of Leeds and Northrup, who has furnished us with a description of an elegant automatic optical pyrometer. The L & Ν automatic optical pyrom eter, Models 8640 and 8641 are man ufactured bv the Leeds and Northrup Co. of 4907"Stenton Ave., Philadelphia 44, Pa. A general view of the instru ments is given in Figure 1, with Model 8640 on the left. The operating prin ciple of both instruments is illustrated in the schematic diagram of Figure 2. The optical system consists of two simi lar optical paths, one for the target, the other for the standard lamp, that con verge to a point on the photocathode of the multiplier phototube. The two paths are alternately interrupted 90 times a second by a motor-driven disk having three 60-degree slots displaced 120 degrees from each other. At any given instant, one slot permits the phototube to "see" light from one beam while the disk blocks light from the other beam. A reasonably good square wave form is produced by this method of light modulation and switch-over time from one beam to the other is rel atively short. The phototube output consists of two d.c. signal components, one propor tional to the brightness of the target, the other proportional to the brightness of the standard lamp. The a.c. com ponent of these two signals is an error signal which is amplified and then de modulated. The demodulated signal, which is proportional to the a.c. error signal, causes the integrator to change the lamp current in the direction that will reduce the error signal. When the target and lamp brightnesses are the same, the error signal is zero and the integrator holds the lamp current at a • Circle No. 35 on Readers' Service Card
BY RALPH H.
MÜLLER
Figure 1. L & Ν automatic optical pyrometers, Models 8640 (left) and 8641 constant value that is related to the target temperature. The synchronous demodulator is trig gered by two phototransistors which are energized by separate lamps whose light is interrupted 90 times a second by the modulator disk. The output of one phototransistor is also used to gencrate two square wave voltages re quired for operation of the integrator. Inasmuch as the radiant energy of the target increases very rapidly as the temperature increases, the system gain must be decreased for higher tempera tures. This is accomplished by the automatic gain control circuit which samples the standard lamp current and automatically reduces the system gain as the lamp current increases. The temperature can be determined by measuring the standard lamp cur rent with a current recorder, or alter natively by connecting an external pre cision resistor in series with the stand ard lamp and measuring the voltage drop across it with a millivolt recorder, or, for the highest precision, with a type K-3 potentiometer. The instru ment utilizes the same standard lamp that has been performance proven in
Figure 2.
the manually balanced L & Ν optical pyrometer for almost thirty years. The lamp employs a thin ribbon fila ment positioned between two flat win dows mounted so the filament is per pendicular to the optic axis. A tem perature vs. lamp current calibration curve is supplied with the equipment. The 8641 Mark 1 precision automatic is the lower priced version of these py rometers and, although less sensitive to small changes in temperature than the 8640 high precision automatic, it provides a level of precision and sensi tivity which exceeds that of most com mercially available instruments. A tur ret viewer (8640 only) permits view ing the target image on a ground glass screen or through a Ramsden eyepiece. The 8641 utilizes the latter only." Some of the characteristics of the two instruments, stated with respect to the 8640 and 8641 are: range: 775° to 5800° C , in four ranges; and 775° to 2800° C. in three ranges. Res olution: 0.2° C. for one-second bal ance at the gold point (1063.0° C.) and 0.5° C , respectively. The re sponse time for both models is approxi mately one second for 99% of reading
Schematic of the automatic optical pyrometer VOL. 38, NO. 9, AUGUST 1966
93 A
INSTRUMENTATION
WANTED NEW, CHALLENGING APPLICATIONS FOR GOW-MAC DETECTORS to serve YOU at . . . TEMPERATURES:
-200° tô 500°C
PRESSURES:
0.1 to 10 atmospheres
FLOW RATES:
1 to 1000 ml/min.
SERVICE:
intermittent or continuous corrosive or contaminated gases
with SEPARATION by Chromatographic Columns
Packed or Capillary Laboratory or prep scale
COME AND SEE US AT THE CHEMICAL EXHIBITION OF THE A.C.S. SEPT. 13-15 BOOTH 111 NEW YORK HILTON
WRITE TODAY FOR GOW-MAC'S NEW ISSUE
Gas Analysis since 1935
GOW
MAC
INSTRUMENT CO.
100 KINGS ROAD, MADISON, NEW JERSEY 07940 TELEPHONE: (201) 377-3450
Circle No. 153 on Readers' Service Card 94 A
ANALYTICAL CHEMISTRY
or automatic operation. The effective wavelengths are approximately the same, 6500 and 6450 angstroms and the radiation band passes are approximately 200 and 350 angstroms. Standard optics permit viewing distances from 12 inches to infinity and 20 inches to infinity. In the high precision, automatic, objective lens assemblies are available for special target requirements. For example, a target diameter as small as 0.01 inch can be viewed at a target distance of approximately 3 inches. Some useful applications in the research laboratories are listed by the manufacturer, such as: studying the melting points of oxides or metals; measuring high speed transient temperatures; using as a backup for thermocouples to correct for drift; using as a comparison device for calibration of secondary standards and radiation sensors for measuring plasma temperatures; measuring fast temperature changes in induction furnaces ; measuring material temperature during changes of state in an arc imaging furnace; calibrating lamps used as star simulators, etc. This and similar instruments will be a boon to many investigators. We know of one superbly equipped metallurgical laboratory where all measurements in optical pyrometry were conducted by one technician who had no particular technical background but the ability to match target and filament brightness better than anyone on the research staff. Carefully conducted objective tests and intercomparisons with thermocouples showed that his setting errors were two or three times smaller than other observers. But despite his calmness and patience, his visual acuity would wane in time. We know a professional color matcher in New York who enlisted our interest in developing a precise photoelectric photometer to perform his tasks. We were proud of our achievement, but humiliated by the fact that he infallibly could spot differences in hue which neither our eyes nor instrument could detect. Our offering was accepted with gratitude because it would supply acceptable data twentyfour hours per day whereas his daily stint was confined to three widely spaced one-hour intervals of visual matching, a performance accepted by his employers without question. In a different category, the late A. H. Pfund of Johns Hopkins developed a hardness tester for paint and varnish films. As he remarked ruefully, "It was not much better than the thumbnail test of an inspecter but was widely adopted because we have not yet learned to breed a race possessing sensitive and calibrated thumbnails."
