Keithley 3720 User Manual page 110

Section 5: Basic Digital Multimeter (DMM) Operation
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:
x
x
These reference junctions are explained in the following paragraphs.
Simulated reference junction
An example of a simulated reference junction is an ice bath as shown in the paragraph titled
Thermocouple connections
are immersed (but electrically isolated) in the ice bath, and the user enters the 0°C simulated
reference temperature into the Series 3700. The simulated reference temperature for the Series
3700 can be set from 0° to 65°C.
The Series 3700 measures the input voltage and factors in the simulated reference temperature
to calculate the temperature reading at the thermocouple.
NOTE
Internal reference junction
"Internal" implies that a temperature transducer(s) is used to measure the cold junction. For
specific switching modules, the cold junction can be the switching module's screw terminals with
voltage temperature sensors strategically placed to measure the temperature of the cold
junction (see
The Series 3700 measures the temperature of the cold junction (screw terminals), measures the
input voltage, and then calculates the temperature reading at the thermocouple.
To help maintain stability and accuracy over time and changes in temperature, the Series 3700
periodically measures internal voltages corresponding to offsets (zero) and amplifier gains. For
thermocouple temperature measurements using the internal reference junction, the internal
temperature is also measured. These measurements are used in the algorithm to calculate the
reading of the input signal. This process is known as autozeroing. Note that internal temperature
references are collected regardless of whether or not autozero is enabled.
5-36
Simulated reference junction
Internal reference junction
(on page 5-39). The copper wire to thermocouple wire connections
The most accurate temperature measurements are achieved by using a simulated
reference junction using an ice point reference.
Thermocouple connections
Series 3700 System Switch/Multimeter User's Manual
(on page 5-39)).
Document Number: 3700S-900-01 Rev. A / August 2007