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62
Chapter 5
Temperature
-20 °C
For this thermistor, a temperature change from -20°C to -19°C will be represented by
737 A/D counts (if supplied with 10µA). The same thermistor will only change
about 18 A/D counts from 49°C to 50°C.
5.4.4.1.4
Selecting Thermistor Current
To select the current setting for a typical 10 K thermistor, determine the lowest
temperature you will need to sample and select the current according to the range
limits given above. If the temperature you want to sample is below -10°C you should
use the 10µA setting.
With the current set to 10µA the best resolution you will see will be a 1.0°C
temperature change. If, for example, the lower limit is 0°C you can choose either
setting, but there is a tradeoff in terms of resolution. If you need better than 0.1°C
measurement resolution you will have to change to 100µA.
If you need high resolution over a narrow range, for a very accurate measurement,
you can set the current setting for the maximum resolution. For example, at a high
temperature of 15°C, you require a measurement resolution of at least 0.05°C. This
resolution is within the range of either setting, but at the 10µA setting the resolution
is only 0.2°C while at the 100 µA setting the resolution is better than .05 °C.
Generally, it is best to use the 100µA setting for all measurements of -10°C or greater
with a 10 K thermistor.
5.4.4.1.5
Selecting 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. 10 K thermistors are generally a good choice for most laser diode
applications where high stability is required near room temperatures. Similarly, 10 K
thermistors are often a good choice for cooling applications where you want to
operate at temperatures from -40°C to room temperature.
If you require a different temperature range or the accuracy you need can't be
achieved with either current setting, select another thermistor. Thermistor
temperature curves, supplied by the manufacturer, show the resistance verses
temperature range for many other thermistors. Newport, Inc. will also offer help for
your specific application.
Voltage at 10 µA
56.0 mV/°C
25 °C
4.4 mV/°C
50 °C
1.4 mV/°C
Temperature Controller Operation
Resolution
0.018 °C/mV
0.230 °C/mV
0.700 °C/mV
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