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TM 55-6695-220-13&P
Section III. PRINCIPLES OF OPERATION
1-8.
THERMOCOUPLE TESTING SECTION.
1-8
a.
The tester may be visualized as consisting of two sections: Thermocouple Thermometer Testing Section and the
Resistance Thermometer Testing Section. (See Figure 1-2) The millivoltage source shown is comprised of a 1.5 volt dry
cell battery, two current limiting rheostats, a precision multiple-tapped, voltage divider network, and a voltmeter. The
operator standardizes the current flowing through the network by adjusting the COARSE AND FINE RHEOSTATS and
observing the pointer deflection of the VOLTMETER. When the pointer is coincident with the red line on the dial, the
precision 27 discrete millivoltages reach their calibrated values. The proper precision millivoltage tap is selected for
output by setting the pointer of the TEMPERATURE SELECTOR SWITCH to the desired temperature, while taking note
of the thermocouple material associated with the three colored sectors. Precision resistance of 2, 8 or 22 ohms is further
added in series with the source as shown by positioning the RESISTANCE AND VOLTAGE FUNCTION SWITCH. This
resistance simulates the actual resistance characteristics of the thermocouple junction and the aircraft wiring lead
resistance.
Figure 1-2. Thermocouple Section Flow Diagram
b.
The function switch when placed in the LEAD RESISTANCE position energizes the null-balance bridge circuit, as
shown, within the tester and converts the VOLTMETER into a null-galvanometer operation. The LEAD RESISTANCE
SWITCH is positioned to the resistance shown on the dial that corresponds to the expected nominal resistance of the
aircraft's thermocouple wiring. The LEAD RESISTANCE ERROR CONTROL can be rotated to obtain up-scale
voltmeter readings on either side of the null condition. The resistance error shown on the dial, either high or low, after
the METER SENSITIVITY SWITCH has been depressed to more accurately determine the null condition represents the
difference in resistance between that of the precision resistor in the bridge leg and that of the aircraft circuit.
1-9
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