Measurement Notes - HP 82479A Owner's Manual

Getting Started
2.12 MEASUREMENT NOTES
The programs in this pac can use a variety of sensors with the HP 3421A. The following notes provide
additional information about some of the measurements.
Current Measurement
The multiplexer assembly (option 020, 021, or 022) is capable of sensing the current in a 4-20 mA
industrial current loop. A resistor must be placed in shunt with the current path. The HP 3421A
voltmeter monitors the resulting voltage drop. The software in this pac assumes that the shunt resistor is
50, and divides the dc voltage reading by S50 to obtain the loop current, returning the result in
milliamps.
Thermocouples
The thermocouple types supported in this pac are as follows:
e T - Copper/Constantan ¢ K - Chromel-P Alumel
e E - Chromel/Constantan oe R -Pt-Pt 13% Rh
e¢ J - Iron/Constantan e S-Pt-Pt 10% Rh
Thermocouples are measured using DC Volts, 5 1/2 digits, autorange on, and autozero on. The reference
temperature is measured using the reference junction on the multiplexer card for each reading, and is used
as the reference temperature parameter in the TL OLUFI_E keyword (refer to section 3, "Keywords"). In
cases where long leads are used, the entire system should be calibrated to account for the effects of lead
resistance. If needed, a custom program can be used to make the voltage measurement, provide correction
factors, and then convert from voltage to temperature.
Thermistors & RTD's
The thermistors supported in this pac are as follows:
e 2252Q@ 25°C
o 5000Q@ 25°C
¢ RTD: 1£:0Q @ 0°C (European curve, &=.00385)
Resistance measurements are made with 5 1/2 digits, autorange on, and autozero on. In cases where long
leads are used, the entire system should be calibrated to account for the effects of lead resistance. Four
wire ohm measurements will provide additional accuracy.
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