as a differential voltmeter.
It may also be used as a conventional
transistorized voltmeter for rapid voltage measurement, an accurate ac
voltmeter, or a megohmeter for measurements of resistance from 10 meghoms to
11,000 megohms.
2.
When used as a dc differential voltmeter, the instrument draws
essentially no current from the unknown voltage source up to 11 volts.
Above 11 volts the input resistance is 10 megohms.
Common-mode voltage
errors are virtually nonexistent in the Models 887A and 887B because of the
high input-circuit leakage resistance, typically several hundred thousand
megohms.
In the Model 887AB, internal batteries permit isolation from
power-line grounds with attendant reduction of ground loop errors.
3.
When used as a dc differential voltmeter, the 887A operates on the
potentiometric principle. An unknown voltage is measured by comparing it to
a known adjustable voltage with the aid of a null detector.
An accurate
standard for measurement is obtained from an 11-volt dc reference supply
derived from a pair of temperature-compensated zener diodes.
The known
adjustable reference voltage is provided by a Kelvin-Varley voltage divider
with four decades of Fluke precision wire-wound resistors and a high-
resolution interpolating vernier that are set accurately by five voltage
readout dials to give a seven-digit readout. In this way, the 11 volts can
be precisely divided into increments smaller than 10 microvolts.
The
unknown voltage is then simply read from the voltage dials.
For voltages
between 11 and 1100 volts dc, an input attenuator divides the unknown
voltage by 100 before it is measured potentiometrically.
When used as an
accurate ac voltmeter, the 887A operates essentially the same as for dc
differential measurements.
The ac input voltage is converted to a dc
voltage and this dc voltage is measured by comparing it to a known
adjustable reference voltage.
4.
Specifications.
a. As a differential voltmeter.
DC accuracy. (0.0025% of input + 0.0001% of range + 5 V) from 0
to 1100 Vdc at 23C (nominal calibration temperature), less than 70%
(0.005% of input +5 V) from 0 to 1100 Vdc within
16C to 32C (60F to 90F) temperature range, at less than 70% relative
humidity.
Derate accuracy outside this temperature range at 0.00035% to
extremes of 0C and 50C (32F and 122F).
Voltage ranges. 1, 10, 100, 1000 V ac and dc with 10% overranging
capability on each range.
Null ranges.
100 uV through 100 V end scale ac and dc, in 7 ranges.
DC input resistance.
Infinite at null from 0 to 11 Vdc. 10 megohms above
11 Vdc.
1 ppm of range (1 V maximum).
Meter resolution.
1 ppm of range (1 V maximum).
Voltage dial resolution.
b. As a conventional voltmeter.
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