INSTRUMENTATION by Ralph H. Müller
Developments in transducers as measuring devices, solid state particle counters, and capacitor-type moisture meters reviewed
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"iHE search for new transducers is unending. As new phenomena and new effects are being unearthed, particularly in t h e field of solidstate physics, m a n y of these turn out to offer new means of measure ment. One of t h e most promising of these phenomena is t h e piezoresistive effect. A recent discussion of this effect a n d some of its appli cations has been given by L. E . Hollander, G. L. Vick, and T . J. Diesel, [Hev. Sci. Instr. 3 1 , 323 (I960)]. Piezoresistivity, the strain-in duced modulation of t h e condition mechanism of a semiconductor, is of interest as a means of studying the carrier transport process. B e yond this, it offers t h e possibility of constructing novel transducers. Several materials have been studied in detail, largely for t h e purpose of gaining more insight a n d better understanding of t h e many-valley semiconductor transport theory. Among them a r e silicon, germa nium, indium, antimonide. rutile (TiOo), and lead telluride. Of these rutile is an interesting high-temperature transducer m a terial. T h e fully oxidized material as a single crystal is light yellow and has a resistivity of 10 1 3 ohm cm., but an oxygen-deficient n o n stoichiometric rutile is deep blue in color a n d can have a resistivity as low as 0.15 ohm cm. T h e longi tudinal piezoresistive coefficient d e pends markedly upon t h e crystal direction. I t s resistivity is verystable over a wide t e m p e r a t u r e range. Also, t h e elastic properties of rutile a r e comparable to those of the diamond, giving rise t o large elastoresistive coefficients and therefore useful gage factor values.
The authors have constructed an accelerometer consisting of a massloaded laminated cantilever, p - t y p e Si beam clamped in t h e center. I t has a sensitivity of 100 μν. per gram with 10 volts applied, and a reso n a n t frequency of 3.3 kc. I n this and similar accelerometers opera tion is far below t h e yield point. T h e y should be measurable to 1000 G a n d for t h e geometry employed, the yield point is in excess of 5000 G. By comparison, most wire strain gages have gage factors of 1 t o 2, whereas t h e piezoresistive devices have elastoresistive coefficients in the order of several hundred. Solid State Particle Counters In an entirely different field, solid-state devices a r e finding i n creasing use in nuclear physics, for the counting a n d measurement of particles. A t low temperatures some of these devices are one or two orders of magnitude more effi cient t h a n gas ionization cham bers or scintillators in converting particle energy into detected charge. A recent communication, [A. I. Yavin, Rev. Sci. Inst. 31, 351 ( I 9 6 0 ) ] has shown t h a t commer cially available transistors can de tect alpha particles a t room temper ature. I t turned out t h a t those transistors which were sensitive to light (as well as distinct phototran sistors') were good alpha detectors. Capacitor Type Moisture Meter Capacitor methods for moisture determination have been revived but in a new a n d original manner. A report by D . Γ . Leach a n d J . M . M. Neilson, [J. Sci. Inst. 37, 77 ( I 9 6 0 ) ] describes t h e design of a
single-electrode capacitor system for moisture meters. I t depends upon t h e capacitance of a single cylindrical electrode in air. T h e three variables—length, radius, and height above earth (a grounded p l a t e ) — are treated for optimum design and also as a function of surrounding t h e electrode b y di electric masses of various sizes and specific dielectric constant. T h e probe method has been used suc cessfully t o measure the moisture content of jute packages, bales, rolls, and spools of yarn. T h e novelty of t h e method would seem to lie in t h e fact t h a t one does not have to introduce a sample into a measuring capacitor. Whenever the area under a curve, such as an automatic recording, is required, a n engineer is likely t o reach for his planimeter a n d a chemist is more likely to cut out the area and weigh the paper. More recently a number of excellent balland-disk integrators have been d e signed which perform this function continuously a n d directly. T h e weighing method, however, is so simple a n d h a s t h e advantage that its precision can be checked very readily by weighing known areas of paper, t h a t it still enjoys wide acceptance. A simple aid to this technique has been described b y B . R. Russell [Rev. Sci. Instr. 3 1 , 216 ( I 9 6 0 ) ] . This is a means of continuously cut ting or perforating the chart during the recording. A small d.c. relay is fastened to the pen carriage and the tip of a sharp needle is soldered t o the moving contact arm. B y oper ating t h e relay on 30 t o 40 volts with 60-cycle a . c , t h e vibrating needle point perforates t h e paper. VOL. 32, NO. 7, JUNE 1 960
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bitals had predicted. When we saw t h a t w h a t was considered essen tial identity amounted to some 30% error, we started to get out a pencil and an old envelope. For the sake of good manners and the desire to learn something we always t r y t o p a y r a p t attention to a speaker, but when things begin to get sticky we have an unpardonable habit of day dreaming, wool gathering, a n d doodling over a few new circuits. At this stage it occurred to us t h a t an ordinary traffic signal could be used to indicate kinetic data, but a quick check showed t h a t redamber-green would involve some 37c greater error t h a n these data. We therefore redesigned a color in dicator system of red, orange, yel low, green, blue, and violet lights which would automatically repre sent the d a t a and cut the error a p proximately in half. For those not desiring an optical readout the re sults could be expressed adjectively such as abysmal, miserable, poor, good, duckv, or peachv. Before we had our circuitry com pleted, some temperature co efficients appeared on the screen and several impressive Arrhenius plots. At this moment our good wife, who is hopelessly and chronically ad dicted t o careful measurements, whispered "Where are the experi mental points?" I t did not satisfy her a t all when we ventured the guess t h a t the points could ha\ r e been placed on any one of the five parallel lines. Miiller's automatic kinetics indicator will probably never reach the market, but he is sorry to report t h a t there are still numerous researches in progress for which i t would have adequate re solution. The experience is prob ably just another way of emphasiz ing t h a t not all chemists have the interest or regard for precision which the analyst takes for granted. Whether he uses elaborate instru ments or not, the analyst has proper regard for precision. I t is one of the things which has always char acterized analytical chemistry a s an exact science.
