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Chemical Instrumentation feature Edited by GALEN W. WING, Seton Hall University, So. Orange, N. J. 07079
These articles ale intended to serve the reo&m O ~ T H I JOURNAL S by calliny attention to new developments in the theoy, design, or availability of chemical laboratory insbumentation, or by presenting useful insighls and ezplanations of topics that are of practical importance to those who use, or teach the use of, modern instrumentation and instrumental techniques. The editor invites correspondence from prospective contributors.
LXXXIX. Electronic LaboratoryBalances. Galen W. Ewlng "A false balance is an abomination to t h e l o r d , but a just weight is his delight.'' Proverbs 11.1.
I t is more than eight years since R. F. Hirseh reviewed laboratory halances in these columns ( I ) . Since then there have been significant advances in the application of electronics to these instruments. The first part of this article will describe the principles upon which modern electronic balances operate. This will be fallowed by a description of the products of the major manufacturers in the field. No consideration will he given to weighing machines utilizing strain-gauge load cells or other devices not generally useful for laboratory operations; same of these were discussed briefly by Hirsch ( I ) . Electronic balances can be classed as either manual or self-balancing. In both types there must he some device to sense the position of the moving element (the beam or its equivalent). In the manual instrument, the operator nulls the balance by hand, usually by turning a series of knobs until the null indicator is zeroed. The force required to equal the weight of the object on the pan can then be ascertained from the indieation of the control knobs, without electronic assistance. The balancing force can be gravitational, if the knobs control adding or removing standard masses, or it can be magnetic, as with a torque motor, to he described below. In automatic self-halancing types, the signal from the null detector is utilized by means of a feedback loop t o control the halancing force, in this ease usually electromagnetic in nature, while a t the same time actuating the digital display. This constitutes a typical servo system.
Null Detectors. The term "null detector" or its equivalent can he applied to the device which senses that the moving system is in its iquilibrium, or null, position. Its prime requirement is high sensitivitv. that is. the ahilitv t o distineuish
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t o an absolute minimum, preferably zero, which means in oractice that there must be nu mechanical runtact with the moving sy3tcm. There arc three approaches t hat can he taken to permit appropriate sensing without contact: optical, electromagnetic (inductive), and capacitive. ~
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Optical Null Detectors. The classical method of null detection, of course, is optical, namely visual comparison of the position of a pointer against a fidueial point or a scale. For electronic sensing a small flag or vane attached to the moving parts of the balance can he made to oeclude a beam of light passing from a small lamp to a photodetector (Fig. la). Several manufacturers use a light-emitting diode (LED) and phototransistor or photodiode for this purpose. The LED is preferred over an incandescent lamp because of its efficient, cool operation. Its intensity can he maintained constant indefinitelv simolv . . bv. controlline the n v r e n t The occluding vane is adjusted to intercept hali the light at the hnlnnre point, correiponaing to mid-smleon an mdrcatine meter. The d i rection of swine of the needG then indicates whether the balance I,wm I* nuny trom null in one sense or t he ~ l h e rand , how fhr. Alternat.\dy, two idenr~cnlphotocrlla ran he used with a single light source, the vane being so situated that its motion away from the equilibrium position increases the amount of light falling on one photocell while decreasine" that on the other (Fie. lbi. The photocells can be photaconductive (e.g., CdS) types, connected in a Wheatstone bridge circuit. In this arrangement variations in the intensity of the source cancel out. The voltage output from the bridge can be shown to he directly proportional to the displacement. Another null detector utilizes an optical lever and twin ohotaeells (Fie. le). A small
Figure 1. Optical null detectors. (a) Singlacell. (b) Dual-cell. (c)Optical lever wim twin photocells.
beam. The ray of light, deflected through twice the angle of motion of the mirror, changes the relative illumination on a pair of photocells in a bridge circuit. The off-balance voltage of the bridge again is a measure of the displacement of the balance beam. The optical lever can increase the sensitivity by as much as 100 times.
Inductive Null Detectors. The most widely used device within this classification is the linear variable differential transformer (LVDT). In its basic form (Fig. 2), this consists of three coils of wire arranged coaxially around a vertical tube of non-conductine material. A soft iron bob or core hangs freely wrhin t h ~ tube, s suspended trom the hnlnnce heam. In the null po
