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Series 3700 System Switch/Multimeter User's Manual
Section 5: Basic Digital Multimeter (DMM) Operation
3700S-900-01 Rev. C / July 2008
5-53
Type
Range
Resolution
T
-200°C to +400°C
0.001°C
E
-150°C to +1000°C
0.001°C
R
0°C to +1768°C
0.1°C
S
0°C to +1786°C
0.1°C
B
+350°C to +1820°C
0.1°C
When two wires made up of dissimilar metals are joined together, a voltage is
generated. The generated voltage is a function of temperature. As temperature
changes, the voltage changes. The thermocouple voltage equates to a
temperature reading. This is the basic operation principle of the thermocouple.
When you connect a thermocouple directly to the input of the Series 3700, at
least one of those connections will be a junction made up of two dissimilar
metals. Hence, another voltage is introduced and is algebraically added to the
thermocouple voltage. The result will be an erroneous temperature
measurement.
To cancel the affects of the unwanted thermal voltage, the thermocouple circuit
requires a reference junction that is at a known temperature.
Reference junctions
A reference junction is the cold junction in a thermocouple circuit that is held at a
stable, known temperature. The cold junction is where dissimilar wire
connections must be made. As long as the temperature of the cold junction is
known, the Series 3700 can factor in the reference temperature to calculate the
actual temperature reading at the thermocouple.
The standard reference temperature is the ice point (0°C). The ice point can be
precisely controlled, and the National Institute of Standards and Technology
(NIST) uses it as the fundamental reference for its voltage-to-temperature
conversion tables. However, other known temperatures can be used.
There are two ways for the Series 3700 to acquire the cold junction temperature.
It can measure the cold junction using a thermistor or 4-wire RTD, or the known
temperature value can be entered by the user.
There are two reference junction types supported by the Series 3700:
Simulated reference junction
Internal reference junction
External reference junction
These reference junctions are explained in the following paragraphs.