Measurement Computing DBK81 - Manual (Page 8)

pg. 8, DBK81, DBK82, & DBK83 989494 DBK Option Cards and Modules
Using a Temperature Calibrator
The DBK81, DBK82, and DBK83 thermocouple cards provide accurate and repeatable temperature
measurements across a wide range of operating conditions. However, all instrumentation is subject to drift
with time and with ambient temperature change. If the ambient temperature of the operating environment
is below 18
°C or above 28°C, or if the product is near or outside its one-year calibration interval, then the
absolute accuracy may be improved through the use of an external temperature calibrator.
A temperature calibrator is a temperature simulation instrument that allows selection of thermocouple type
and temperature. For proper operation, it must be connected to the DBK81, DBK82, or DBK83 with the
same type thermocouple wire and connector that is used in normal testing. The calibrator then generates
and supplies a voltage to the card. The supplied voltage corresponds to that which would be generated by
the chosen thermocouple type at the selected temperature.
The temperature selected on the calibrator will be dictated by the nature of normal testing. 0
°C is usually
the best choice. Calibrators are the most accurate at this setting, and the connecting thermocouple wire will
contribute very little error at this temperature. However, if the dynamic range of the normal testing is, for
example, 100
°C to 300°C, a selection of 200°C may give better results. In either case, the level of
adjustment is determined by comparing the unit reading to the selected calibrator temperature. For
example, if the calibrator is set to 0
°C output, and the DBK unit reads 0.3°C, then an adjustment of –0.3°C
is required. That is, the adjustment value is determined by subtracting the DBK reading from the calibrator
setting.
To implement the adjustment in DaqView:
1. Ensure that the acquisition process is turned off.
2. Click on the cell in the Units column for the channel that is connected to the calibrator. The
engineering units pull-down menu above the grid becomes active.
3. Click on the down arrow and select the “mx+b” option. This option allows post-acquisition
mathematical manipulation.
4. For the example adjustment, enter –0.3 for “b.” The channel under calibration will now
read 0
°C.
Note that this adjustment is a mathematical operation only, and in no way alters the hardware
calibration of the product. Moreover, it operates on a per channel basis, with the settings for a
given channel having no influence on any other channels.
To implement the adjustment in LogView:
1. Ensure that the acquisition process is turned off.
2. In the Analog Input Channel Configuration window, select the “User Scaling” tab.
3. Click on the “Offset” cell for the channel that is connected to the calibrator.
4. For the example adjustment, enter -0.3 for “Offset.” The channel under calibration will now
read 0
°C.
Note that this adjustment is a mathematical operation only, and in no way alters the hardware
calibration of the product. Moreover, it operates on a per channel basis, with the settings for a
given channel having no influence on any other channels.