Omega Engineering CL521 Operator's Manual page 43

The following equipment is needed for calibration;
- Voltage standard, voltage 50 to 110 mV, error less than 5 uV
- Resistance standard, resistance 100 to 400 ohms, error less than 20 mohm
- K type thermocouple probe
- Ice bath, or OMEGA TRCIII Ice Point Reference Chamber
Calibration is rather different because of the software involved with the CL521. In some
cases, this means that the effect of an adjusting operation will not be seen on the display
until after approximately one minute. After turning a trimmer, always wait until you can
see the effect on the display.
Connect the voltage standard to TC.mV In/Out terminals and select mV measurement.
The reference junction compensation must be switched off (CJ is MAN and 0°C set in
Register 9). The display can be corrected to the right reading if necessary, using
trimmer R7 on the main board.
Next calibrate the resistance measurement mode. Connect the resistance standard to
the PtIOO in terminals of the Cl_521 and select ohm measurement. If necessary, adjust
the trimmer "PtIOO SPAN" at the front of the terminal panel, until the display reading is
correct.
The next calibration step is the internal reference junction compensation. Select
thermocouple measurement, type K thermocouple and internal reference junction
compensation. Connect the tj^e K thermocouple probe and put it into a 0°C ice bath or
OMEGA TRCIII. The display should show 0®C. If necessary, adjust the display reading
to the correct value using the "CJ ZERO" trimmer at the front of the terminal panel.
If there is a CL521-RS232C Cation card installed in the unit, check the calibration of the
analog output signal. The calibration can be done with a resistor of approximately 300
ohms connected to the TC.mV In/Out terminals of the CL521.
First, switch the CL521 to measure the voltage across the resistor and check that the
reading is 0 mV ±0.01 mV. Adjust the analog output to zero by using the trimmer ZERO
on the option card. Use a DVM that reads to four places.
Then, switch the CL521 to TCQ measurement and check the display reading. The
display digit representing tenths of milliohms should be less than four to make sure that
there will be no trouble with rounding. Find a proper reading, for example, by connecting
extra test leads in series with the resistor. If the resistance reading is, for example,
312.33 ohms, adjust the analog output to 3123 mV by using the trimmer SPAN on the
option card.
Repeat the ZERO and SPAN adjustments until both adjustments are within specification.
The CL521 is now recalibrated. Reassemble the unit and install the cover.
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