Temperature From Resistance; Use Of R-T Conversion Charts - ILX Lightwave LDT-5412 User Manual

Temperature from Resistance

Negative temperature coefficient (NTC) thermistors are non-linear devices. Their
typical resistance vs. temperature curve looks like that shown in Figure 2.4. The
5412 displays the resistance of the thermistor, and several methods for
determining the temperature from this resistance value are presented below.

Use of R-T Conversion Charts

Most thermistor manufacturers will supply a resistance - temperature (R-T) chart
for their thermistors. These charts provide a direct and simple conversion.
Although the temperature accuracy will not be better than the tolerance of the
thermistor, this accuracy may be more than adequate for most applications. If
temperature accuracy to within 1C is all that is required, this method is the best.
In many cases the manufacturer will supply a universal conversion table for each
particular thermistor material type that they sell. This table is used for all 25C
nominal resistance value thermistors made of that material. In these tables the
resistance is not given directly. But rather, a scaling factor is given at each
temperature. Resistance is found then by multiplying the nominal resistance value
05_11
THERMISTOR R/T CURVES
FOR VARYING ROOM TEMP. RESISTANCES
50.00
40.00
30.00
k 
20.00
2.5 k 5 k 10 k 25 k 50 k
1 k
10.00
0.00
-40
-60
-20
TEMPERATURE (DEGREES C)
Figure 2.4 Typical R-T Curves for NTC Thermistors
Temperature from Resistance
100 k
80
0 60
20 40
O P E R A T I O N
160
100 120 140
LDT-5412
2
C H A P T E R
11
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