User`s guide

268
Chapter 7 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/34972A
supports RTDs with  = 0.00385 (DIN / IEC 751) using ITS-90 software
conversions and  = 0.00391 using IPTS-68 software conversions.
“PT100” is a special label that is sometimes used to refer to an RTD with
 = 0.00385 and R
0
= 100.
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 49 to 2.1 k.
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 k
(44004), 5 k (44007), and 10 k (44006) thermistors. The thermistor
conversion routines used by the 34970A/34972A are compatible with the
International Temperature Scale of 1990 (ITS-90).
34970A Refresh UG.book Page 268 Wednesday, February 17, 2010 12:34 PM