Calibration and adjustment sequence - GMP251 - GMP252

This section describes the calibration and adjustment sequence in more detail and describes the data the Modbus client system should exchange with the probe. Prepare a reference source such as a reference gas or reference measurement instrument (1 or 2 references depending on whether the calibration or adjustment is done in 1 or 2 points) to create suitable reference conditions. See probe documentation for more information on reference source options.

The wiring, communication settings, and Modbus implementation of GMP251 and GMP252 probes are described in GMP251 and GMP80 User Guide (M211799EN) and GMP252 User Guide (M211897EN). In addition, see Calibration-related Modbus registers and functions.

1. Write the calibration passcode 1300 in register 0x2000 of the probe to unlock the calibration mode.
2. Calibrate the first point:
   1. Insert the probe head in the reference environment for the first calibration point.
   2. Wait for the measurement to stabilize. Track the stabilization by reading the CO₂ measurement value register (0x0000). Note that the required stabilization time varies depending on starting conditions and calibration conditions.
   3. After the measurement is stable, write the reference value of the environment in register 0x2100. See register descriptions for valid ranges.
   4. When the reference value is written, the probe automatically inserts the currently measured value in the appropriate register (0x2102) for the first calibration point. Read the values written by the probe and compare with the reference to determine the calibration result for the first point.
3. Calibrate the second point:

   1. Insert the probe head in the reference environment for the second calibration point.
   2. Wait for the measurement to stabilize. Track the stabilization by reading the CO₂ measurement value.
   3. After the measurement is stable, write the reference value of the environment in register 0x2104. See register descriptions for valid ranges.
   4. The probe automatically inserts the currently measured value in the appropriate register (0x2106) for the second calibration point. Read the value written by the probe and compare with the reference to determine the calibration result for the second point.

   Note that you must write an empty (NaN) value in all fields for any unused adjustment points. This prevents any possible earlier values left in those points from affecting the current adjustment.

4. When you have calibrated all intended points, activate the adjustment by writing 1 in the function register 0x2108.
   • If the calibration results show that no adjustment is needed, you can also exit without activating the adjustment. To exit without activating the adjustment, move directly to step 7 and close the calibration mode.
5. To verify that the adjustment was successfully activated, read the return code of the adjustment function from the same register address. See Adjustment function return codes.
6. Write the new calibration date and information text in the appropriate registers.
7. Write -32768 (0x8000) in register 0x2000 to close the calibration mode.