INSTRUMENTATION by Ralph H. Müller
"Break-Through" in Recorders Results from Use of Electromechanical Strain Gage and Design Improvements RUTHERFORD once said of P r o L ORD fessor Callender, the inventor of the first recording potentiometer, " H e seems t o be more interested in devising a new instrument t h a n in discovering a fundamental t r u t h . " Although we are living in the nuclear and spacetechnology age, in which we a r e constantly indebted t o , a n d aware of, Rutherford's discovery of m a n y fundamental t r u t h s , i t is highly debatable which of the two has more profoundly affected t h e practices of modern r e search a n d technology. T h e majority of recorders are used in industry where they record countless variables a n d control complicated processes. Their scientific and research uses a r e even more diversified. Even if we confine ourselves t o t h e field of analytical chemistry it is, a t this stage, difficult t o imagine any serious or detailed studies in x-rays, ultraviolet, visible, or infrared without automatic recording of t h e data. T h e same is t r u e in gas chromatography, nuclear magnetic resonance, and polarography. I t used to be said of the idle-rich and those born t o luxury t h a t they could "look right t h r o u g h " a servant, o r discuss the most intimate m a t t e r s in his presence as if he were non-existent. We a r e prone t o regard t h e modern recorder i n the same way and become annoyed with it only when this silent and faithful servant exhibits essentially h u m a n weaknesses of desiring a new b a t t e r y , ink in t h e pen, or a new chart. I t is i m p o r t a n t t o recognize t h a t the recording potentiometer has been i m proved constantly and i m p o r t a n t innovations are being made even a t t h e present time to accommodate or even anticip a t e more exacting research demands. Mechanical Arrangements Used I n i t i a l l y
F o r several decades, recorders used elaborate mechanical arrangements t o h u n t out t h e deflection of a galvanometer pointer a n d actuate motors t o drive the slick wire to the balance point. Industrial uses were primarily responsible for increasing dissatisfaction with galvanometers. T h e y were t o o delicate for a rugged industrial environment and, if they were made rugged • Circle Ho. 135 on Headers' Service Card
enough for the purpose, they were too insensitive. I n the early 40's superior electronic methods were achieved. B y chopping the signal a n d amplifying the a.c. signal sufficiently t o operate a two-phase motor, the slide-wire was driven in the a p p r o p r i a t e direction t o cancel t h e error-signal. This driving system also moved a pen across a moving chart. Provision was made for checking t h e working current of t h e potentiometer against a standard cell because this was the only component of the gadget which the classical chemist or physicist would trust. Automatic standardization was soon added, in which a n elaborate electromechanical mechanism periodically comjjared t h e working current with a standard cell and automatically readjusted t h e working current. This would seem t o have been the ultimate in convenience a n d certainty, b u t as every research m a n knows, this t e m p o r a r y interruption, in t h e interests of accuracy, usually occurred during t h e most interesting a n d crucial point of a recording. I n a previous column we described a n i m p o r t a n t advance m a d e b y t h e Brown Division of Minneapolis-Honeywell, in which the standard cell and all the associated re-standardizing mechanism a r e replaced b y a Zener-diode system. T h e new system is rated a t 0.025% accuracy. Several units which we tested were found t o b e constant over long periods t o within ± 0.020%.
stressed looped-wire strands enclosed in a n I-shaped frame. These four looped-wire strands form t h e variable resistance arms of t h e Wheatstone bridge measuring circuit. Resistance of these wires is proportional t o t h e tension applied t o them. As shown in Figure 1 the four loopedwire strands a r e attached t o a n i n sulated block a t the I-frame base which, in t u r n , acts as a heat sink. T h e free ends of t h e wires a r e soldered t o t h e Stranducer terminal board. T h e u p p e r section of t h e I-frame houses a horizontal torsion pivot which provides a t r u e turning radius for precise operation. T h e central portion of t h e pivot consists of a h u b over which t h e four wires a r e looped. An arm attached t o this h u b is driven b y a cable linked mechanically to the balancing motor through two stages of d r u m
" B r e a k - T h r o u g h " In Recorder Design
Once more, through t h e courtesy of Lou Schultz of Minneapolis-Honeywell's Albuquerque office, we are able t o describe another major "break-through" in recorder design. I n their new Electronik 17 recording potentiometer, a n extremely accurate, trouble-free electromechanical strain gage replaces the conventional slide-wire as the rebalancing element. I n eliminating the slide-wire, infinite resolution is achieved because the number of points a t which t h e potentiometer can come to balance is no longer limited t o the t u r n s or convolutions on a slide-wire. T h e "Stranducer," as the strain gage element is called, consists of four pre-
Figure 1 . Electromechanical gage ("Stranducer") VOL. 33, NO. ό, MAY 1961
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Miniature Assembly
Laboratory Electrode
Figure 2 . Modular construction of strip chart recorder
Plastic Flow Assembly Immersion
Assembly
To measure pH
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and sector reductions. D r u m and sector are k e p t under tension a t all times b y the torsional force of t h e pivot and an additional spring. Any change in position of the balancing m o t o r causes torsional movement of the pivot. The Stranducer also eliminates "bounce" which is characteristic of slide-wires when driven a t high speed. I n addition to the new transducer, the Electronik 17 has been almost completely redesigned a n d several import a n t improvements m a d e . Provision for rejection of s t r a y signals is such t h a t loop stray signals which have 60cycle r.m.s. values five times greater t h a n t h e total span will not impair the accuracy of the instrument. Also voltage to ground strays of 300 volts d.c. or 150 volts a.c. will not impair instrument accuracy. Thermally compensated electrical networks minimize zero shift due t o t e m p e r a t u r e changes. One reference junction device establishes t r u e reference junction compensation for all types of thermocouples. The amplifier provides critical damping and optimum instrument response for high impedance actuations. Transistor or vacuum tube amplifiers are optional. A new miniature plug-in converter is vised. F o r control purposes a maximum of eight control potentiometers provides adjustable contact action points for strip or circular chart instruments. Compact, plug-in transistor control units control the contact action of single-pole double-throw relays. One
plug-in power supply furnishes i n p u t signals to all components of the control system. A cascaded Zener diode network, which provides a constant current to the measuring circuit despite fluctuations in line voltage and changes in ambient t e m p e r a t u r e , eliminates need for periodic standardization of the potentiometer. T h e modular construction of the Electronik 17 is shown in Figure 2. T h e strip chart model, showing display, drive, and case modules, can be quickly converted to a circular chart recorder or circular scale indicator. T h e drive module encloses the Stranducer, range card and circuit board, a n d amplifier and constant current unit. T h e case module houses the display a n d drive units. The drawer-type chassis permits uninterrupted operation. Also shown are the wiring drawbridge, control units, power supply for t h e control system, a n d t h e terminal board. T h e instruments can be mounted in s t a n d a r d 19-inch relay racks, on a panel or bench, or made portable. I t is conceivable t h a t some future users of this fine instrument will grumble about further improvements which could be made. I t is encouraging t o note t h a t the chief grumblers are the manufacturers themselves. T h e y well know t h a t today's best will be obsolete tomorrow and their constant a n d u n remitting search for something b e t t e r always keeps t h e m one j u m p ahead of the exacting and impatient investigator.
