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Temperature Measurements
A temperature transducer measurement is typically either a resistance
or voltage measurement converted to an equivalent temperature by
software conversion routines inside the instrument. The mathematical
conversion is based on specific properties of the various transducers.
The conversion routines used by the HP 34970A are compatible with the
International Temperature Scale of 1990 (ITS-90). The mathematical
conversion accuracy (not including the transducer accuracy) for each
transducer type is shown below.
Transducer Conversion Accuracy
Thermocouple
RTD
Thermistor
0.05
°C
0.02 °C
0.05 °C
Errors associated with temperature measurements include all of those
listed for dc voltage and resistance measurements elsewhere in this
chapter. The largest source of error in temperature measurements is
generally the transducer itself.
Your measurement requirements will help you to determine which
temperature transducer type to use. Each transducer type has a
particular temperature range, accuracy, and cost. The table below
summarizes some typical specifications for each transducer type.
Use this information to help select the transducer for your application.
The transducer manufacturers can provide you with exact specifications
for a particular transducer.
Parameter Thermocouple RTD Thermistor
Temperature Range
Measurement Type
Transducer Sensitivity
Probe Accuracy
Cost (U.S. Dollars)
Durability
-210
°C to 1820°C
Voltage
6 µV/°C to 60 µV/°C
0.5 °C to 5 °C
$1 / foot
Rugged
-200°C to 850°C
2- or 4-Wire Ohms
≈ R
0
x 0.004 °C
0.01 °C to 0.1 °C
$20 to $100 each
Fragile
-80°C to 150°C
2- or 4-Wire Ohms
≈ 400 Ω /°C
0.1 °C to 1 °C
$10 to $100 each
Fragile
8
Chapter 8 Tutorial
Measurement Fundamentals
345