INSTRUMENTATION BY RALPH H. MÜLLER
Precise Temperature Control ERHAPS we have P wrong scientific
been reading t h e a n d engineering journals, b u t we seem to find few references to the precise control of thermostats. As far as instrumental errors arising from t e m p e r a t u r e effects a r e concerned, we have, in most cases, learned how t o correct for these a u t o matically. However, there a r e m a n y phenomena which require a known and carefully controlled environmental temperature. I t is not particularly exciting to hear of thermostats guaranteed to control to within a few hundredths of a degree Centigrade. During our r e search apprenticeship in t h e early twenties, we took it for granted t h a t good temperature control meant ±0.001° C, particularly in precise measurements of electrolytic conductivity or other properties with high temperature coefficients. Vacuum tube control of thermostats was already in vogue (J. J . Beaver, Columbia University, 1921). During t h a t decade, m a n y advances were made, including phaseshifted t h y r a t r o n s for direct heating and minimization of hunting about the desired setting. Elegant devices a r e available today for the assembly of precise and sensitive thermostats, although quite a few factors must be considered in the overall behavior of the system. A recent note emphasizes an important principle enunciated 70 years ago. J. G. Becsey and J . A. Bierlein of the Aerospace Research Laboratories describe a Simple Gouy Regulator for On-Off Thermostatic Control [Rev. Sci. Instr. 38, No. 4, 556 (1967)]. T h e authors, in working with a mercury control thermometer with a screw adjusted contact, minimize inertial effects in this sensing element by mechanically oscillating the screw adjustment about its set point. This they achieve with a small synchronous motor geared down a n d coupled by a link and pulley wheel to the screw adjustment. The mercury contact is thus interrupted a t a constant frequency and amplitude so t h a t a crude proportional control is achieved b y pulse-width modulation. T h e y give recordings showing a fivefold improvement in regulation—i.e., from ±0.020 to ±0.004° C. T h e basic principle was
enunciated by M . Gouy in J. Phys. [3rd series] 6, 479 (1897). I n our time, the same was accomplished b y t y ing the mercury regulator to t h e frame of the motor, which provided stirring of the bath, with a string or a piece of wire. T h e theory of automatic control is more than adequate for such p r o b lems. M o r e frequently, i t is applied t o industrial installations a n d often forgotten in the research laboratory. We think it would still be a challenge to a graduate student to be required t o set u p a thermostat for ±0.001° C performance with a n evaluation of short and long t e r m regulation, determining lag coefficients, response t o transients, and similar criteria. I t might be much more informative t h a n pushing buttons and waiting for data to appear on a recorder or come out of a computer. I t would be useful t o have an u p t o date discussion of t h e uses of silicon controlled rectifiers [SCR's] in instrumentation. The SCR is a solid state device capable of performing the functions of the older t h y r a t r o n . There is a voluminous literature on industrial uses of SCR's ; they are employed in the control of motors in rolling mills and operation in the megawatt range is not unusual. Like thyratrons, small amounts of power in the input of a n SCR can control very large amounts of power in switching operations. I n the phase-shift mode of operation, smooth control can be accomplished resistively, capacitatively, inductively, and b y numerous photoelectric means. T h e y are, of course, compatible with other solid state devices and have the consequent advantages of compactness, swiftness of action, and all t h e a t t e n d a n t conveniences. A recent application describes a Simple Circuit for Nernst Glower Control [C. W. Hand, Rev. Sci. Instr. 38, N o . 7, 983 (1967)]. I n most infrared spectrophotometers, t h e source of radiation is a Nernst glower, an electrically heated ceramic rod. T h e radiation o u t p u t is monitored b y a p h o t o t u b e which controls the power input t o the glower. I n some instruments, t h e power is controlled b y means of a saturable core reactor; in others, b y a multi-contact which provides stepwise
adjustment of the series resistor in the glower circuit. I n H a n d ' s circuit, t h e phototube monitor effects phase shift control of the SCR b y controlling the firing angle of the unijunction transistor. A potentiometer (voltage-divider) sets the R C charging time of a capacitor and provides manual adjustment of the glower power, while the phototube exerts feedback control b y setting the voltage t o which the capacitor charges. I t would seem t h a t both this circuit and control by a saturable reactor would b e s u perior to any stepwise relay system in t h a t both are inherently capable of infinitely smooth control. We are still concerned, however, with the question of how thermal lag would affect such a control system. There must be a considerable lag between the time of injection of more or less energy to the glower and the attainment of a new level of brilliance in the glower, which one would expect to lead to slow oscillation about t h e desired control point. Presumably, this effect is negligible in Hand's arrangement, since he considered the new circuit to be a genuine improvement. A noncontacting voltmeter employing capacitor detector probes a n d designated as t h e Series 5050 Proximity Voltmeter is available from the Electrometer Products Division of the Victoreen I n s t r u m e n t Co. of Cleveland, Ohio. I t measures contact potentials of metals and surface potentials on plastics, paper, fabrics, semiconductors, and insulating a n d conducting materials. Selectable voltage ranges are ± 1 , 3, 10, 30, 100, 300, and 1000 volts full scale. Accuracy is better than 1%. T w o probes a r e available: one is a high resolution probe t h a t revolves circular spot charges as small as 0.075 inch in diameter a n d one, a high sensitivity probe t h a t is used for low voltages u p to 1 volt full scale with a resolution of 0.2 inch in diameter. Response is less than 10 milliseconds for a rise time of 10 to 90%. The instrument would seem to have application t o a number of chemical problems in which conventional current or potential measurements would not be satisfactory.
VOL. 39, NO. 11, SEPTEMBER 1967
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