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Chapter 2
Multiplexer Modules. In the scanning voltmeter configuration
described in Chapter 4, the instrument driver "VOLTMTR" controls
the Multiplexer switches.
•
Thermocouple measurements are compensated by the thermistor on
the E1476A Terminal Module. The temperature of the thermocouple
lead junctions inside the Terminal Module affects the temperature
reading taken from a thermocouple. Use the thermistor inside the
Terminal Module to determine the temperature inside the Terminal
Module and compensate the thermocouple temperature measurements.
Do the following:
1. Measure the resistance of the 5KΩ thermistor on the Terminal
Module (Thr_ohms). NOTE: A 4-wire ohms measurement should be
made to avoid measurement errors by the 100Ω protection resistors
(see "Figure 2-1. Switchbox Configuration" on page 37).
2. Compute the temperature of the terminal module (Tref) from the
reading (Thr_ohms). For values of resistance between 92.7 to
3.685e6, use the following equations to calculate the temperature of
the terminal module:
Tref = temperature in degrees C.
Thr_ohms = the resistance of the 5K ohm thermistor
A = 1.28463e-3
B = .23625e-3
C = 9.2697e-8
W = ln(Thr_ohms)
Tref ={ 1/(A + W * (B + C * W * W)) } - 273.15
3. Measure the voltage on the thermocouple connected to a channel
(Vt).
4. Compute the effective temperature at the external thermocouple
junction by doing the following two steps:
a. Compute the actual voltage across the external thermocouple
junction using the following equation:
Vtc
Texternal
where:
V
is the actual voltmeter reading and Vtc
meter
voltage across the selected thermocouple type at the terminal
module temperature (
from tabular data for the selected thermocouple.
b. Evaluate the actual thermocouple temperature using the
Vtc
value and the tabular data for the selected
Texternal
thermocouple.
5. Compute the compensated voltage (V) by the formula: V=(Vt-Vref).
6. Convert the compensated voltage (V) calculated in step 4 to
temperature. This is the actual temperature measured by the
thermocouple.
HP E1476A Switchbox Application Examples 45
= V
- Vtc
meter
Tref
). This voltage must be evaluated
Tref
is the
Tref
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