Model 2701 User’s Manual Basic DMM Operation 3-35
Internal reference junction
“Internal” implies that a temperature transducer(s) is used to measure the cold junction
(CJC). For the Model 7700 switching module, the cold junction is the screw terminals
with voltage temperature sensors strategically placed to measure the temperature of the
cold junction.
The Model 2701 measures the temperature of the cold junction (screw terminals),
measures the input voltage, and then calculates the temperature reading at the
thermocouple.
External reference junction
For switching modules that do not have built-in sensors to measure temperature, each
module can use a thermistor or 4-wire RTD to acquire the reference temperature. Connect
a thermistor to channel 1 or connect a 4-wire RTD to channel 1 and its paired channel.
Position the temperature transducer near the terminals for the channel(s) being used to
measure temperature. Be sure to electrically insulate the transducer leads to keep them
from making contact with other conductors.
When you close channel 1 to measure the cold junction temperature, that temperature
reading will be used to calculate the temperature when you close a thermocouple channel.
Open thermocouple detection
Long lengths of thermocouple wire can have a large amount of capacitance that is seen at
the input of the DMM. If an intermittent open occurs in the thermocouple circuit, the
capacitance could cause an erroneous on-scale reading.
The Model 2701 has an open thermocouple detection circuit. When enabled, a 10µA pulse
of current is applied to the thermocouple before the start of each temperature
measurement. If >12kΩ is detected (open thermocouple), the OVRFLW message will be
displayed. If <12kΩ is detected, the current is turned off and a normal thermocouple
temperature measurement is performed.
NOTE Channel average cannot be used with thermocouple temperature measurements
if open thermocouple detection is enabled.
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