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

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C H A P T E R
by the scaling factor at a given temperature. The scaling factor is often
represented as the ratio R
For example, if the thermistor is rated as a 10 Kohm thermistor, then R25 =
10,000 ohms (the resistance at 25C). To find the resistance at 10C, find the ratio
factor at 10C as supplied by the manufacturer. If the ratio was 1.99, then the
resistance at 10C would be 10,000 x 1.99 = 19,900 ohms.
Table 2.1 shows some typical values of resistance ratios and temperatures for a
thermistor.
Table 2.1 Typical Thermistor R/T Data

Use of the Steinhart-Hart Equation

The Steinhart-Hart (S-H) equation accurately models the non-linear R-T
characteristic curve of a negative temperature coefficient (NTC) thermistor. When
the correct constants for a thermistor are known, the S-H equation can be used to
convert between resistance and temperature. This method offers the advantage of
accurately calculating R-T values at any point on the curve, not just those supplied
by the manufacturer.
There are several forms of the Steinhart-Hart equation. The form of the S-H
equation which is used by ILX Lightwave is:
T = 1/(C1 + C2(Ln R) + C3(Ln R)
where R is the resistance in ohms; C1, C2, and C3 are the Steinhart-Hart
constants for a particular thermistor
Generally, the Steinhart-Hart constants for a thermistor are not specified by the
thermistor manufacturer. These constants may be derived specifically for each
thermistor, or the nominal value for a thermistor may be used (see Appendix A).
ILX Lightwave supplies the S-H constants when a TS-510 calibrated thermistor is
purchased.
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 LDT-5412
O P E R A T I O N
Temperature from Resistance
Temperature in C
10
11
12
13
14
15
/R .
T 25
R /R
T 25
1.990
1.897
1.809
1.726
1.646
1.571
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