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.
