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3-22
Temperature measurements
The Model 2790 can measure temperature using thermistors and 4-wire RTDs. When
deciding which temperature sensor to use, keep in mind that the thermistor is the most
sensitive, and the 4-wire RTD is the most stable.
Thermistors
For thermistors, the temperature measurement range is -80°C to 150°C (0.01°C resolu-
tion). Thermistor types that are supported include the 2.2kΩ, 5kΩ, and 10kΩ types.
The thermistor is a temperature sensitive resistor. Its resistance changes non-linearly with
changes in temperature. Most thermistors have a negative temperature coefficient. As tem-
perature increases, the resistance decreases. The Model 2790 measures the resistance of
the thermistor and calculates the temperature reading.
Of all the temperature transducers, the thermistor is the most sensitive. It can quickly detect
minute changes in temperature. It is a good choice when measuring very small changes in
temperature. The downside for this increased sensitivity is the loss of linearity. Since they are
especially non-linear at high temperatures, it is best to use them for measurements below
100°C.
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4-wire RTDs
For 4-wire RTDs, the temperature measurement range is -200°C to 630°C (0.01°C resolu-
tion). RTD types that are supported include D100, F100, PT385, and PT3916. A USER
type is available to modify RTD parameters, such as the resistance at 0°C. The USER type
can be enabled from the front panel, but the settings can only be changed using remote
programming.
The RTD has a metal construction (typically platinum). The resistance of the RTD changes
with changes in temperature. The Model 2790 measures the resistance and calculates the
temperature reading. When using default RTD parameters, the resistance of the RTD will be
100Ω at 0°C.
Of all the temperature transducers, the RTD exhibits the most stability and linearity. The
Model 2790 performs the 4-wire measurement using offset compensated ohms. This pro-
vides the most accurate way to measure the low resistance of the RTD.
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Model 2790 SourceMeter
Curve fitting constants are used in the equation to calculate thermistor tempera-
ture. The thermistor manufacturer's specified curve fitting constants may not be
exactly the same as the ones used by the Model 2790.
equation and the constants used by the Model 2790. It also explains how to
select a thermistor when the manufacturer's constants and the ones used by the
Model 2790 do not match.
The equation to calculate temperature vs. resistance readings is listed in the
RTD reference tables provided in
®
Swich System Reference Manual
Appendix E
Appendix
E.
provides the
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