sure relative temperatures between its measuring junction and reference junc-
tion. In order to determine the absolute temperature, the temperature of the
reference junction must be known. With internal CJC the temperature of the
reference junction (where the thermocouple connects to the 2565) is measured
automatically with an internal thermistor temperature sensor and used in the
calculation of the absolute temperature of the thermocouple. This method is
most convenient and is often used. However, it has limitations of accuracy. This
is because it is difficult to place the sensor very close to the connectors and
maintain a low thermal gradient between the sensor and connectors. With exter-
nal CJC a reference junction is created externally and copper wires connect the
reference junction to the 2565. The reference junction is placed in an ice bath
or other temperature source that has a precisely known and stable temperature.
This technique offers improved accuracy but is less convenient because of the
more complicated connection scheme and the requirement of a precision tem-
perature source.
The CJC type is selected using the
EDIT PROBE
function in the
PROBE
menu. Set the CJC parameter to INTERNAL (this is the default) for internal
CJC. With internal CJC the CJC TEMP value is measured automatically so you
do not need to enter a value. When using external CJC with an external temper-
ature source and reference junction set CJC to EXTERNAL. You must also en-
ter the temperature of the external reference junction for the CJC TEMP
parameter. For instance, if the reference junction is placed in an ice bath set
CJC TEMP to 0.0.
1560 Thermometer Readout
User’s Guide
150
PRECISION
THERMOCOUPLE
MODULE
2565
+ –
+ –
Closed
Open
Figure 18
2565 Module Thermocouple Receptacle Operation
