Doric Instruments 473 Series Portable Digital Calibrators Calibration Manual - Manual

Model 473
CALIBRATION PROCEDURE
3.1 Equipment Required:
4-1/2 digit, digital voltmeter
Precision millivolt/milliamp source:
Resolution, 1 V/ A; accuracy + .01% (+ 2 V)
Recirculating ice bath, 0.00
°
C + .05
°
C
Type K thermocouple wire
Copper hookup wire
Potentiometer adjusting tool
Refer to Figure 3-1 for locations of pot adjustments and testpoints.
3.1.1 Millivolt Calibration
a. Remove battery from instrument (refer to 2.4.1). Remove bottom case half from
printed circuit board assemblies by extracting three screws from case bottom and one
screw holding wrist strap lug. Remove top case half from printed circuit board
assemblies by extracting screw on clad side of Analog Board between output controls.
Re-connect battery to connector.
b. Measure voltage at TP-8 (+) on Display Board (top board) with DVM common (-) at
TP-2 (cathode of CR1) on Analog Board (bottom board). Voltage at TP-8 should be
between 3.680-3.72V (pin 4 of p7). If measured voltage is outside of range adjust R9
on Analog Board for DVM reading at TP-8 of 3.695 V.
c. Set instrument to indicator mode, mV input.
d. Connect precision mV/mA calibrator source to T/C input observing proper polarity.
e. Adjust source output to 0.000 mV (short).
f. Select high resolution display (.00 mV). Display should read .00 with flashing minus
sign. Turn R21 clockwise: display should read .00, .01, .02, .03. Turn R21
counterclockwise: display should read .03, .02, .01, .00, -.00, -.01, -.02, -.03. Adjust
R21 for display reading of .00 with flashing minus sign. If reading is not stable, R26
on the Analog Board may need adjusting (normally adjusted near center).
g. Adjust source output to –7.000 mV.
h. Adjust R28 on Analog Board for an exact display reading of –7.00 mV.
i. Adjust source output to 0.00 mV, check zero reading and adjust R21 on Display
board as required for correct (.00) reading.