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Thermistor Range
06_07 LDT-5525 35
CHAPTER B
Temperature Resolution
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
For a typical 10 K thermistor, a temperature change from -20
o
C to -19
o
C 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
50
o
C! For that case, with the LDT-5525, the temperature measurement resolution
would be reduced to about 0.2
o
C. 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 0.1
o
C (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
o
C you will probably need to set the switch to the 10 µA setting.
If you require temperatures of 10
o
C to 30
o
C, 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 10
o
C 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
TEMPERATURE VOLTAGE AT 10 µA RESOLUTION
-20
o
C 56 mV /
o
C 0.018
o
C / mV
25
o
C 4.4 mV /
o
C 0.23
o
C / mV
50
o
C 1.4 mV /
o
C 0.70
o
C / mV