Instructions For Use As A Calibration Furnace - VERDER CARBOLITE GERO PTC 12 Installation, Operation And Maintenance Instructions

4.0 Operation
4.4

Instructions for Use as a Calibration Furnace

The PTC 12/20/150 is designed for calibration by the comparison method, which consists
of comparing the thermocouple under test with a reference thermocouple. This
reference thermocouple can have traceability to national standards, either if it is
ordered with this requirement, or by getting it independently calibrated.
In the PTC 12/20/150 the reference thermocouple is built in and has its own indicator
marked Reference Temperature. It is located within the work tube such that its tip is
positioned close to that of the thermocouple to be tested in the part of the tube which
has the most uniform temperature.
The thermocouple to be calibrated should be passed through an end plug with a suitable
size hole, with the tip 120 to 130 mm from the step in the plug.
If thermocouples with different outer diameters are to be calibrated then end plugs with
different size holes should be used. These may be ordered from Carbolite Gero. The
maximum recommended diameter is 7.5 mm - larger diameters may experience too
much heat conduction and give an incorrect calibration.
Insert the thermocouple and end plug and support the cold end to ensure that the
assembly lines up with the axis of the furnace tube.
Switch on the instrument switch and set the desired temperature on the controller.
Allow the unit to warm up and stabilise. The reference temperature may be slightly
lower than the control temperature so adjustment can be made to the control setting to
bring the reference temperature to the exact temperature if desired; allow it to
stabilise.
Either the output of the thermocouple or an indicated temperature can be checked
against the reference temperature and any error noted. This error can now be used to
correct any readings taken by the thermocouple. The noted error is the calibration of
the thermocouple. Remember to include the reference thermocouple error in calculating
the calibration - see the reference thermocouple certificate.
The calibration should be made at temperatures at which the thermocouple is to be
used. If it is to be used over a range of temperatures then calibrate it at a number of
points over the range (for example at every 100°), and plot a graph of error against
temperature. A curve drawn through the points can then be used to find the error at
any temperature.
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