SECTION
V
Calibration and Troubleshooting
5.1 Introduction
This section contains the instructions for calibrating and trouble-
shooting the DRC-84C instrument.
5.1
Test Equipment
A
high input impedance digital voltmeter and oscilloscope, a 25
ohm,
25
watt resistor to simulate a heater element, and a precision
resistor connected to simulate the diode wired according to Figure
2.1
(c) are normally sufficient to test and calibrate the DRC-84C instrument.
5.3
General Remarks
On installation, one of the major problems is an improperly connect-
ed temperature sensing diode.
It
is advised that other portions of the
cryogenic system be tested before the instrument is troubleshooted. Some
checks that could be made are:
1) Open or shorted sensor or heater leads (especially in an area
of frequent disassembly).
2) Leakage paths between heater and sensor leads that induce
electrical feedback in addition to thermal feedback.
If the malfunction points toward the instrument, more detailed tests
should be made.
5.4 Instrument Calibration
The DRC-84C has been factory calibrated. If a recalibration is
needed, the following procedure should be followed. Please refer to
the component layout for the DRC-84C, Figure
6.5
for the following
discussion:
5.4.1 Current Sources
a) Current Source
-
Diode Sensor
A
A
precision resistor of not less than
.01%
tolerance should be
connected across pins
A
and
B
(Figure 2.1) of the diode
A
Sensor
socket.
A
high input impedance voltmeter connected across the
precision resistor should measure a voltage equal to 10 micro
amperes times the value of the resistor. For example, a 100K
ohm
±.01%
resistor should read
1.0000
volts within 100 µvolts.
If recalibration is needed, the voltage across the 100K re-
sistor can be adjusted by varying resistor R12.
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