12.3.4
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 a specific thermocouple may deviate from the standard
equations. The 1560 can be programmed to adjust for this error. 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 types R, S, and gold-platinum thermocouples the adjustment can also be
specified by coefficients of a second-order polynomial (see Section 5.2.1.10).
12.3.5
Suggestions for Optimum Accuracy
The sensitivity of thermocouple measurements, especially when using internal
CJC, requires that the user take certain precautions to ensure accuracy.
12.3.5.1
Warm-up Time
Measurements may change slightly as the 2565 module warms up after being
powered on. For best accuracy allow the 2565 to warm up for one hour before
making precision measurements.
12.3.5.2
Ambient Environment
When using internal cold-junction compensation the connectors and internal
sensors are sensitive to ambient temperature disturbances. The 2565 should be
operated in a stable environment free from large and sudden ambient tempera-
ture variations. The 2565 will also be more accurate if the ambient temperature
at which it is operated is close to the ambient temperature at which it was
calibrated.
12.3.5.3
Thermal Settling
When a thermocouple is plugged into the 2565 a certain amount of time is re-
quired before heat in the connector and receptacle becomes evenly distributed.
For best results wait at least two minutes before making precision
measurements.
12.3.5.4
Ground Currents
The 2565 inputs are electrically isolated from all other components of the 1560
system and ground. This helps prevent ground loops that can produce noise and
errors in the measurements. However, the two input channels are not isolated
151
12 2565 Precision Thermocouple Module
Operation
