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Section

4: Theory of operation Model 2110 5½ Digit Multimeter

Reference Manual

4-4 2110-901-01 Rev. C/August 2013

Figure 37: Four-wire constant-current source method

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 Model 2110 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 Model 2110 to acquire the cold junction temperature:

• The instrument can measure the cold junction using internal transducers.

• The user can enter the known temperature value.

These two references are known as:

• Real (internal) reference junction

• Simulated reference junction

These reference junctions are explained in the following paragraphs.

Real (internal) reference junction

Real implies that temperature transducers are used to measure the cold junction. This value is

influenced and changed by different climate conditions and environments. The Model 2110 measures

the temperature of the cold junction, measures the input voltage, and then calculates the temperature

reading at the thermocouple.

Simulated reference junction

An example of a simulated reference junction is an ice bath. The copper wire to thermocouple wire

connections are immersed (but electrically isolated) in the ice bath, known to be 0 °C. The

temperature measured becomes an offset to the simulated reference temperature. The simulated

reference temperature for the Model 2110 can be set from -100 °C to 100 °C. The Model 2110

measures the input voltage and factors in the simulated reference temperature to calculate the

temperature reading at the thermocouple.

Keithley 2110 User Manual

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