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Table 2.4 shows some typical values of resistance ratios and temperatures for a thermistor.
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:
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 ILX Lightwave Application Note #4 Thermistor
Calibration and the Steinhart-Hart Equation). ILX Lightwave supplies the S-H constants when a
TS-510 calibrated thermistor is purchased.
Linearized Thermistor Mode – For users wanting to display a calculated temperature without
using one of the methods above the LDT-5412B has a unique mode that when selected will
display and control to a calculated temperature. To use this mode the LDT-5412B sensor select
switch must be set to °C LINEARIZED FOR 10k. This mode can only be used with 10 kΩ
thermistors with 1% accuracy and a Beta of 3900 to 4050. In this mode, the display will show
temperature with an accuracy of ±1.0 °C. For users requiring higher accuracy or not using a 10
k thermistor one of the other methods above should be used.
Use of none 10 kΩ thermistors with accuracy greater than 1% and a Beta value outside of
the range of 3900 to 4050 can result in temperature readings exceeding ±1.0 °C of set
point. Potential damage to the load may occur due to inaccurate set points.
March 2013
Table 2.4 Typical Thermistor R/T Data
°C
Temperature in
10
11
12
13
14
15
T = 1/(C1 + C2(Ln R) + C3(Ln R)
12
R
/R
T
25
1.990
1.897
1.809
1.726
1.646
1.571
3
)
LDT-5412B
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