Tutorial 7
Keysight 34970A/34972A User’s Guide 279
RTD Measurements
An RTD is constructed of a metal (typically platinum) that changes resistance with
a change in temperature in a precisely known way. The internal DMM measures
the resistance of the RTD and then calculates the equivalent temperature.
An RTD has the highest stability of the temperature transducers. The output from
an RTD is also very linear. This makes an RTD a good choice for high-accuracy,
long-term measurements. The 34970A/34972A supports RTDs with α = 0.00385
(DIN / IEC 751) using ITS-90 software conversions and α = 0.00391 using IPTS-68
software conversions. “PT100” is a special label that is sometimes used to refer to
an RTD with α = 0.00385 and R
0
= 100Ω.
The resistance of an RTD is nominal at 0 °C and is referred to as R
0
. The 34970A/
34972A can measure RTDs with R
0
values from 49Ω to 2.1 kΩ.
You can measure RTDs using a 2-wire or 4-wire measurement method. The 4-wire
method provides the most accurate way to measure small resistances. Connection
lead resistance is automatically removed using the 4-wire method.
Thermistor Measurements
A thermistor is constructed of materials that non-linearly changes resistance with
changes in temperature. The internal DMM measures the resistance of the
thermistor and then calculates the equivalent temperature.
Thermistors have a higher sensitivity than thermocouples or RTDs. This makes a
thermistor a good choice when measuring very small changes in temperature.
Thermistors are, however, very non-linear, especially at high temperatures and
function best below 100 °C.
Because of their high resistance, thermistors can be measured using a
2-wire measurement method. The internal DMM supports 2.2 kΩ (44004), 5 kΩ
(44007), and 10 kΩ (44006) thermistors. The thermistor conversion routines used
by the 34970A/34972A are compatible with the International Temperature Scale
of 1990 (ITS-90).
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