Appendix A Use Of The Steinhart-Hart Equation - ILX Lightwave LDT-5412 User Manual

A
A P P E N D I X
U S -H E
SE OF THE TEINHART ART QUATION

The Steinhart-Hart equation models the non-linear curve of the NTC thermistor's
R-T curve. In order to use the equation, the temperature and resistance must be
accurately determined at three or more points. Then, the constants C1, C2, and
C3 can be determined.
T = 1/(C1 + C2(Ln R) + C3(Ln R)3)
where R is the resistance in ohms; C1, C2, and C3 are the Steinhart-Hart
constants for a particular thermistor
Applying the Steinhart-Hart equation with known constants for a thermistor, the
temperature can be determined at any point along the R-T curve. However, the
accuracy beyond the endpoints of the R-T values used in determining the
constants deteriorates as the resistance value goes out of these bounds.
Therefore, it is recommended that the values of resistance and temperature used
to calculate the constants for a thermistor be at the extents of the required
temperature range. For example, if the temperature range of interest is -20 to
+30 oC, it is best to measure the resistance at these points and near the midpoint,
+5oC. The following program can be used to simultaneously solve the three
equations to determine the constants C1, C2, and C3.
Some nominal values for the Steinhart-Hart constants for various thermistors are
presented at the end of this appendix.
19
LDT-5412
