Temperature From Resistance - ILX Lightwave LDT-5412B User Manual

information on thermistor selection, see ILX Lightwave Application Note #2, Selecting and Using
Thermistors for Temperature Control.
Figure 2.3 Thermistor Temperature Range

Temperature from Resistance

The LDT-5412B Thermoelectric Temperature Controller primarily displays resistance of the
thermistor; several methods can be used to convert the resistance to a temperature.
Resistance-Temperature Conversions 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 by the scaling factor at a given temperature. The scaling factor is often
represented as the ratio RT/R25.
For example, if the thermistor is rated as a 10 KΩ thermistor, then R25 = 10,000 Ω (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.
March 2013 11 LDT-5412B