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13.3.3
Selecting the CJC Type
The 1560 allows for either internal or external CJC. CJC is necessary because
thermocouples are only able to measure relative temperatures between the mea-
suring junction and the reference junction. In order to determine the absolute
temperature, the temperature of the reference junction must be known. With in-
ternal CJC the temperature of the reference junction, assumed to be the point
where the thermocouple connects to the 2566, is measured using an internal
thermistor temperature sensor. This method offers convenience and is most of-
ten 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 dif-
ferential between the sensor and connectors. With external CJC a reference
junction is created externally and copper wires connect the reference junction
to the 2566. 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 con-
nection scheme and the requirement of a precision temperature source.
The 1560 allows the use of either internal or external CJC. Most applications
will use internal CJC where the thermocouple probe is connected directly to the
2566. The CJC type is selected using the EDIT PROBE function in the
PROBE menu. Set the CJC parameter to INTERNAL (this is the default). With
internal CJC the CJC TEMP value is measured automatically and so you do not
need to enter a value. When using external CJC with an external temperature
source and reference junction set CJC to EXTERNAL. You must also enter 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.
13.3.4
Using Calibrated Thermocouples
The 1560 normally converts voltage to temperature according to the standard
equations given in NIST Monograph 175. However, the actual voltage-tempera-
ture characteristics of thermocouples may deviate from the standard equations.
For improved accuracy the 2566 can be programmed to make adjustments to
the standard equations. This requires that the thermocouple be calibrated so that
its errors are known.
You can specify the adjustments in two ways. One is to specify temperature ad-
justments at several temperatures (see Section 5.2.1.10). For instance, if the
thermocouple was calibrated and found to measure high by 0.06°C at 500°C
you would enter –0.06 as the adjustment with 500°C as the calibration tempera-
ture. You can specify adjustments for up to three calibration points. At tempera-
tures between the calibration points a polynomial equation is used to estimate
the required adjustment.
For type R and S and gold-platinum thermocouples the adjustment can be spec-
ified by coefficients of a second-order polynomial (see Section 5.2.1.10).
13 2566 Thermocouple Scanner Module
Operation
157
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