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High Impedance Voltmeter and Null Detector


Manufacturer: Fluke
Model: 845AB


A sensitive high impedance DC voltmeter and null detector.


Input Range1 µV-1000V DC
Accuracy± (2%end scale + 1 µV)
Input Resistance
300 mV range and above:100 M Ohms
100 mV range and below:10 M Ohms
Maximum meter noise
1 µV range:0.20 µV
3 µV range:0.25 µV
Other ranges:0.20 µV
Meter response time (to 90% of reading)
1 µV range:5 seconds
3 µV range:3 seconds
Other ranges:1-1/2 seconds

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Operating Instructions

  1. Turn Power to LINE OPR
  2. Turn RANGE to 1000V
  3. Adjust the mechnical zero adjustment screw for zero deflection.
  4. Turn RANGE to 10 MICROVOLTS
  5. Turn the OPR switch so the line on the knob points towards ZERO.
  6. Adjust the ZERO control for an initial zero meter deflection.
  7. Place the RANGE switch to 1 MICROVOLT.
  8. Re-zero with the ZERO control.
  9. Repeat steps 2 through 8 until the instrument is zeroed.
Operation as a voltmeter
  1. Turn POWER to LINE OPR>
  2. Turn OPR to OPR.
  3. Turn RANGE to 1000 Volts.
  4. Connect voltage to be measured to the INPUT terminal, and the common point of the voltage to the COMMON terminal.
  5. Increase the sensitivity for maximum on-scale deflection.
Operation as a null detector: Consult the manufacturer's manual.


When measuring voltages in the microvolt range, use copper wire having low thermal emf's.

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keywords: voltmeter, voltage measurement device, voltage measurement device, electrical measurement device.

Document created June, 1996. Original author: David Harrison.