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40 Using the Relay Multiplexer Modules
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Temperature Measurements using Thermistors. This consists of 2-wire
or 4-wire Ohms measurements. See "Connecting Switchbox Channels to
Tree Terminals for Making Measurements" on page
measurements.
Measuring Temperature with the HP E1326/E1411 Multimeters. The
HP E1326/E1411 Multimeters can directly measure channels of single or
multiple multiplexer modules. The multimeter automatically calculates the
correct temperature for the specific thermistor or thermocouple type used.
For more information, see the HP E1326/E1411 Multimeter User's Manual.
Thermocouple Compensation using the Thermistor on the
HP E1344A/47A Terminal Module. The temperature inside the terminal
module can affect the temperature reading taken from a thermocouple. Use
the thermistor inside the terminal module to determine the temperature
inside the terminal module to compensate the thermocouple temperature
measurements. Do the following:
1. Measure the resistance of the thermistor on the terminal module.
Compute the temperature of the terminal module from the reading
(T
).
ref
2. For values of resistance between 92.7 to 3.685e6, use the following
equations to calculate the temperature of the terminal module:
T
= temperature in degrees C.
ref
Thermos = the resistance of channel x93, the 5K ohm
thermistor.
A = 1.28463e-3
B = .23625e-3
C = 9.2697e-8
W = LOG (Thermos)
T
= 1./(A+W*(B +C*W*W))-273.15
ref
3. Measure the voltage on the thermocouple connected to a channel (V
4. Convert the thermistor temperature (T
(V
). Use the temperature to voltage characteristics of the
ref
thermocouple measured in step 2 (different thermocouples have
different temperature-to-voltage characteristics).
5. Compute the compensated voltage (V) by the formula:
V = (V
V
).
t
-
ref
6. Convert the compensated voltage (V) calculated in step 4 to
temperature. This is the actual temperature measured by the
thermocouple.
34
for making those
) to the thermocouple voltage
ref
Chapter 3
).
t
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