DATA PRECISION 938 User Manual

42-5124 MODEL 938
3.7 OPERATION
3.7.1 Operating Controls & Indicators
Table 3-1 described the Model 938 C-Meter controls and in 
dicators that are shown in Figure 3-3. Refer to paragraph 3.4.
3.7.2 Hints for Improved Operation
The ideal capacitor exhibits no electrical performance except
that of capacitance; it would convert all the “ storing ” energy into
charge, would be able to hold that charge indefinitely, and would
be able to deliver all the energy represented by its charge into a
load without loss. While many laboratory standard capacitors
approach this ideal with more or less success, all capacitors
contain elements intrinsic to their design that cause departures
from ideal in all realizable capacitor elements.
Figure 3-4 illustrates two of the more important equivalent
resitance elements that affect capacitor performance, — - and
measurement of capacitance values. These resistances exist in
a distributed fashion, but are drawn as shown in the illustration
for ease in analysis. The series resistance Rs introduces losses
in the charge and discharge actions, while the parallel resistance
Rp serves to drain the stored charge.
The Model 938 measurement technique is hardly affected by the
series resistance Rs because the Capacitance Meter measures
changes in the stored charge and changes in the charging
voltage, and does not depend upon the absolute values, except
when radically different from ideal. However, the parallel
resistance Rp parallels the range-selected precision resistor in
the Model 938 measuring circuit, and, depending upon the
relative values of the leakage resistance and that of the selected
range, may have a visible effect on the display value when
measuring leaky capacitors. In most cases, the resulting
measurement remains within the performance specification.
Measured values that exceed the capacitor specified tolerances
will, therefore, be valid indicators of degraded components.
Measurements of very low capacitances should be performed us 
ing extremely short leads and a Kelvin clip in order to avoid in 
troducing any stray capacitance.
When using the optional test leads, remember that the leads in 
troduce a measurable capacitance to the measurement. As a
first approximation, the test lead capacitance may be measured
by opening the leads at the tips, recording the open circuit value,
3-8
MODEL 938
OPERATION & RECALIBRATION
and subtracting that value value from the display results.
However, in order for this correction to be valid, the succeeding
capacitor measurements must be made with all other measure 
ment conditions exactly the same.
The existence of a leaky capacitance may be detected if the
value changes significantly as the scales are changed. Since the
shift to a lower range scale introduces more resistance in the
discharge cycle (see principles of operation), the effect of the in 
ternal leaky resistance is minimized on the lower range scales.
9 Q
d
0
Figure 3-4. Equivalent Capacitor Schematic
3-9
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DATA PRECISION 938 Specifications

General

Brand	DATA PRECISION
Model	938
Category	Measuring Instruments
Language	English