R. G. Charles
Wes$nghouse Research Laboratories Pittsburgh, Pennsylvania 15235
A Simple Method for Recording with a Con~entioncllAnalytical Balance
Analytical balances are often adapted for such uses as thermogravimetric or magnetic s u e ceptibility work.' I n applications of this type provision for automatic and continuous recording of weight changes is often distinctly advantageous. Many methods for recording from an analytical balance have been described in the literature1 but most are rather complex and are expensive in terms of both constme tion time and equipment. We describe here a very simple and inexpensive recording device which we find to be as equally satisfactory as more complicated equipment for many applications. Our device is based on a small indium arsenide Halleffect transducer (Bell Inc., Columbus, Ohio, Model BH700. This is sold along with the required permanent bar magnets described below as part of the inexpensive "Hall-Pak Kit" offered by this company). This transducer generates an electrical potential proportional to the product of the electrical current passing through the InAs wafer and the magnetic field strength perpendicular to the wafer.= The transducing element is held, by its leads, within the inhomogeneous magnetic field generated by two small ferrite bar magnets cemented together a t an angle of about 15" as shown in Figure 1. The Hall element is held stationary with
This, in turn, results in a change in the magnetic field experienced by the element and hence in the generated Hall potential. The Hall element used requires a constant electrical current. This condition is sufficiently well met by the application of a constant potential of about 6 V to the element in series with a 30 ohm resistor. A battery suffices, but since the current consumption is appreciable, a constant voltage power supply which operates off 115 VAC is more convenient. We used a Knight KG-661 regulated power supply (Allied Electronics Corporation, Chicago, Illinois) for most of the work described here. With the balance used (an Ainsworth Type 10N single pan balance of 0.1 mg sensitivity) the electrical leads from the Hall element could be passed to the outside of the balance by closing one of the sliding glass doors against them without requiring any permanent change in the balance. The Hall potentials obtained with the arrangement shown in Figure 1are sufficiently large to be recorded, with a potentiometric millivolt recorder, without amplification. I n Figure 2, measured Hall potential is plotted as a function of weight imbalance (as measured by the built-in optical scale of the balance) with the Hall
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Front Yiw
Figure 1.
Diogrmm. of the weight-ronring orrongcment.
respect to the balance case by means of a simple metal stand resting on the floor of the balance, while the magnet bars are suspended, as shown in Figure 1, from the sample end of the balance beam. Any change in weight of the sample results in a vertical displacement of the magnet bars with respect to the Hall element.
' WENDLANDT, W. W., "Thermal Methods of Analysis," John Wiley & Sons, Inc., New York, 1964; GARN,P. D., "Thermoanalytical Methods of Investigation," Academic Press, New York, 1965. ' GILEB,A. F.,"Electronic Sensing Devices," George Newnes Ltd.,London, 1966.
I Welghl Imbalance, mg
Figure 2. Hall pokntials os o function of weight i m b d m r e ldirtaneer of the transducer fmm the apex of the magnet bars ore given in mm).
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Figure 3. rc&l.
element placed initially a t various distances from the apex of the angle formed by two magnet bars (see scale in Figure 1). (Distances in mm, in Figure 2, were measured with a cathetometer from the center of the active area of the element, with the balance beam held by its arrest mechanism.) Figure 2 shows that a linear relationship exists between weight imbalance and Hall potential a t each value of initial separation. The Hall potential characteristic of any given value of weight imbalance increases as the Hall element is brought closer to the apex of the magnet-bars (Fig. 1). To make full use of the sensitivity of our device it is necessary to "buck out" the larger portion of the generated Hall potential in order that the change due to weight imbalance can be made a large fraction of a recorder chart width. The bucking potential is conveniently supplied by means of a mercury cell and simple voltage divider. Figure 3 gives, in expanded scale, two of the plots of Figure 2, attained by using such a bucking potential, in series opposition with the Hall-potential leads. The linearity is seen to be quite adequate for most purposes. The sensitivity (from the slopes of the lines in Figures 2 and 3) of the device shown in Figure 1 is a complicated function of the distance of the initial positioning of the Hall element with respect to the apex of the angle formed by the two permanent magnets (Fig. 4). Maximum sensitivity is obtained with the Hall element near the open ends of the magnet bars but linearity of the weight imbalance-Hall potential relationship is not quite as good in this region as it is with the Hall element more displaced toward the magnet bar apex. The latter positioning, however, requires larger bucking potentials which tend to magnify the effects of any change in the power supply voltage. The device described here has good long term sta-
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open End ot Magnet
Hall potsntiols or a funcBon of weight imbalance (expanded
Journal of Chemical Education
b Figure 4.
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m so PDIition 01 Sanror With Rcrwt to Magnet Aw, mm 10
Sensitivity as a fundion of sensor position.
bility. I n a typical run (with the sensing element a t 31.0 mm on the scale of Figure 1 and using the Knight power supply) a maximum apparent weight change of 1 mg from the average was observed, with a constant weight on the balance pan, over a 22 h r period.3 However, when a power supply having superior voltage regulation (Model 5PS-2055, Plug-In Instruments, Inc., Nashville, Tennessee) was substituted for the Knight unit, the apparent drift over the same time interval was reduced to
