Temperature Resolution

Thermistor Range

C H A P T E R B

You must also consider measurement resolution since the measurement resolution decreases as the thermistor temperature increases. A temperature controller (such as the LDT-5525) has a limited measurement resolution. A temperature change of one degree centigrade will be represented by a greater resistance increase at a lower temperature than at a higher temperature because of the non-linear resistance of the thermistor. Resolution figures for a typical 10 K thermistor are given in Table B.3, below.

Table B.3 10K Thermistor Voltage vs. Resolution

TEMPERATURE	VOLTAGE AT 10 µA	RESOLUTION

-20oC	56 mV / oC	0.018 oC / mV
25oC	4.4 mV / oC	0.23 oC / mV
50oC	1.4 mV / oC	0.70 oC / mV

For a typical 10 K thermistor, a temperature change from -20 oC to -19oC will be represented by a measurement change of about 56 mV (if supplied with 10 µA). The same thermistor measurement will only change about 1.4 mV from 49 to

50oC! For that case, with the LDT-5525, the temperature measurement resolution would be reduced to about 0.2oC. If the 100 µA setting were used instead, the thermistor measurement would change by 14 mV from 49 to 50*C, providing the maximum resolution of 0.1oC (with the LDT-5525).

Therefore, the sensor current you choose may impact the temperature measurement resolution as well as the set point control accuracy.

Selecting the Sensing Current

To select the current setting for a typical 10K thermistor, determine the lowest temperature you will need to sample and set the SENSOR SELECT switch according to the range limits in Table B.2. If the temperature you want to sample is below 10 oC you will probably need to set the switch to the 10 µA setting.

If you require temperatures of 10 oC to 30 oC, either SENSOR SELECT setting (100 µA or 10 µA) will work with a 10K thermistor. However, the 100 µA setting provides greater measurement resolution, and therefore better control.

Note: Generally, it is best to use the 100 µA SENSOR SELECT setting for all measurements of 10oC or greater with a typical 10 K thermistor.

Selecting and Using Thermistors

The type of thermistor you choose will depend primarily on the operating temperature range. These guidelines for selecting the range and resolution will apply to any thermistor. From Figure B.1 you can see that 10 K thermistors are

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