The reference junction types supported by the Series 3700A are:
• Simulated reference junction
• Internal reference junction
• External reference junction
These reference junctions are explained in the following paragraphs.
Simulated reference junction
The simulated reference temperature for the Series 3700A can be set from 0 °C to 65 °C. The Series
3700A measures the input voltage and factors in the simulated reference temperature to calculate the
temperature reading at the thermocouple.
An example of a simulated reference junction is an ice bath, as shown in Thermocouple connections
(on page 4-25). The input wire to thermocouple wire connections are immersed (but electrically
isolated) in the ice bath, and the user enters the 0 °C simulated reference temperature into the Series
3700A.
The most accurate temperature measurements are achieved by using a simulated reference junction
using an ice point reference.
Internal reference junction
Internal implies that temperature transducers are 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.
The Series 3700A 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 3700A
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.
Internal temperature references are collected regardless of whether or not autozero is enabled.
External reference junction
Thermocouple readings using the rear terminals can be configured to use an external reference
junction setting. The Series 3700A assumes the external reference junction is connected to
channel 1. It is recommended that this channel be configured for thermistor or RTD temperature
measurements. However, the instrument does not verify the configuration. Each time a reading is
made on the external reference junction channel, it is used as the new external reference junction
value in subsequent external reference readings. External reference readings work with dmm.close
and scanning.
For non-simulated thermocouple measurements, first make a thermistor or RTD measurement before
enabling the external reference junction.
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