Agilent Technologies 34901A User Manual page 349

Chapter 8 Tutorial
Measurement Fundamentals
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
α
supports RTDs with = 0.00385 ( DIN / IEC 751) using ITS-90 software
α
conversions and = 0.00391 using IPTS-68 software conversions.
"PT100" is a special label that is sometimes used to refer to an RTD
α Ω
with = 0.00385 and R = 100 .
0
°
The resistance of an RTD is nominal at 0 C and is referred to as R .
0
Ω Ω
The 34970A can measure RTD s with R values from 49 to 2.1 k .
0
You can measure RTD s using a 2-wire or 4-wire measurement method.
The 4-wire method (with offset compensation) 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 RTD s.
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
Ω Ω Ω
2.2 k (44004), 5 k (44007), and 10 k (44006) thermistors.
The thermistor conversion routines used by the 34970A are compatible
with the International Temperature Scale of 1990 (ITS-90).
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