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Comments Measuring Temperature with the HP El326B/El411B Multimeters. The
HP E1326B/E1411B 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 E1326B/E1411B Multimeter User’s Manual.
Thermocouple Compensated Measurements. The HP E1353A FET
Multiplexer makes thermocouple compensated measurements. Thermocouple
compensation accounts for the temperature inside the multiplexer terminal
module, which can affect the thermocouple reading. Thermocouple
compensated measurements are made with the HP E1326B/E1411B. More
information on these types of measurements can be found in Hewlett-Packard
Application Note 290, Practical Temperature Measurements.
High-Speed Temperature Measurements. High-speed temperature
measurements can be made by measuring the thermocouple voltage,
compensating the reading, and then converting the voltage to a temperature.
The instrument configuration used for these types of measurements may
include an E1326B/E1411B Multimeter and a FET multiplexer switchbox,
or an external voltmeter used with the FET switchbox. The procedure for
these types of measurements is:
1. Measure the resistance of the thermistor (channel 93) on the
multiplexer terminal module (
CLOSe(@cc93)).
2. Measure the thermocouple voltage on the multiplexer channel.
3. Convert the thermistor resistance to a temperature using the
following equation:
t = 1.0 / (A + B
*
lnr + C
*
(lnr)
3
)calculate temperature
t = t - 273.15 convert Kelvin to Centigrade
where:
A = 0.00128463
B = 0.00023625
C = 9.2697E-8
lnr = natural log of the measured thermistor resistance
A, B, and C are the curve-fitting constants for the multiplexer’s
5K thermistor.
4. Convert the thermistor temperature (step 3) to a voltage using the
reference table for the type of thermocouple used.
5. Subtract the voltage (step 4) from the thermocouple voltage
measured in step 2. Convert the difference between the voltages to a
temperature, again using the reference table for the type of
thermocouple used.
6. Add the temperature (step 5) to the thermistor temperature computed
in step 3 to obtain the result of the thermocouple measurement.
44 Using the HP E1351A/53A FET Multiplexer Modules Chapter 3