Section 4: Basic DMM operation
Series 3700A System Switch/Multimeter Reference Manual
4-38
3700AS-901-01 Rev. D/June 2018
Thermistor equation
Temperature (in Kelvin) is calculated using the Steinhart-Hart equation as follows:
Where
:
T
K
= The calculated temperature in Kelvin.
lnR
= The natural log of the measured resistance of the thermistor.
A
,
B
, and
C
=The curve fitting constants.
The constants for the three thermistor types used by the Series 3700A are listed below.
Series 3700A curve fitting constants for thermistors
Constant
2,252 Ω at 25 °C
(Series 44004)
5,000 Ω at 25 °C
(Series 44007)
10 kΩ at 25 °C
(Series 44006)
A
0.0014733
0.001288
0.0010295
B
0.0002372
0.0002356
0.0002391
C
1.074e-7
9.557e-8
1.568e-7
Selecting a thermistor
The thermistor manufacturer's specified curve fitting values (A, B, and C) may not be exactly the
same as the ones used by the Series 3700A. If they are not exactly the same, perform the following
steps to select a thermistor to use with the Series 3700A.
The specified thermistor temperature measurement accuracy of the Series 3700A is based on the
curve fitting constants listed in
(on page 4-38
). If the thermistor manufacturer’s
curve fitting constants are not exactly the same as the ones listed in
(on page 4-
38), accuracy will be affected.
1. Choose the thermistor type to be used: 2,252
Ω, 5 kΩ, or 10 kΩ.
2. Compare the A, B, and C constants from the thermistor manufacturer with those used by the
Series 3700A (see
(on page 4-38)). Series 3700A specifications are
available for download from the Keithley
https://www.tek.com/product-support
3. Select a thermistor that closely matches the A, B, and C constants in
4. Analyze the differences between
the two sets of curve fitting constants to determine the affect on
measurement accuracy.
Converting Kelvin to °C
The temperature in Kelvin can be converted to °C as follows:
T
°C
= T
K
– 273.15
Where:
•
T
°C
is the temperature in °C.
•
T
K
is the calculated Kelvin temperature.
