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HPM-2002-OBEVacuum Gauge Page 24 of 39
3.5. Calibration of the HPM-2002-OBE
3.5.1. Zero Coefficient Adjustment
The zero coefficient corrects for the constant power level which is present over the entire pressure range.
Typically, this adjustment corrects for low pressure errors. The instrument will need to be re-zeroed often if
measurements are being made in the 10
-4
Torr range, especially if the ambient temperature changes. The transducer
may have a temperature coefficient of up to 2x10
-4
Torr/
o
C. The instrument remote zero input will allow an external
gauge such as an ion gauge to automatically re-zero the HPM-2002-OBE whenever the pressure drops below the desired
pressure level (if it has a TTL output).
NOTE: Do not attempt to zero the Model 2002 in pressures above 10
-2
Torr; the microprocessor will not accept a
zero above this pressure.
To manually adjust the zero use the following procedure:
1. If possible, evacuate the vacuum system into the low 10
-6
Torr (1.33x10
-6
mbar) range or as low as possible
below 10
-4
Torr.
2. Allow the sensor to operate in this condition for a minimum of 15 minutes.
3. Place the instrument in the CAL mode, using the “SELECT” button, then turn the “ADJUST” rotary encoder.
The CAL light will start to flash indicating that the calibration mode has been activated.
4. Use ADJUST until the display reads 0.0 (or 0.0 with an occasional -0.0). OR using the RS232/485 send
“/0<CR>”.
5. The unit is now fully zeroed. Place the HPM-2002-OBE back in the RUN mode to store the zero in permanent
memory.
3.5.2. Midrange Coefficient Adjustment
The midrange coefficient corrects for errors in the slope of the power curve of the thin film Pirani.
Typically, this is due to the geometry of a particular sensor and will only need to be performed once in the
lifetime of the sensor. This adjustment might be needed if a full calibration is being performed in a gas other than
nitrogen. The zero adjustment will need to be performed before making this adjustment.
To adjust the midrange coefficient use the following procedure:
1. Evacuate the vacuum chamber and refill with the desired gas to a pressure of 7 Torr, as indicated by a
reference vacuum gauge.
2. Place the instrument in the Cal mode using the SELECT button.
3. Turn the ADJUST rotary encoder until the HPM-2002-OBE display matches the reading on the reference
gauge OR using the RS232/485 send “CM={m.d}E{e}<CR>”.
4. Place the HPM-2002-OBE back in the Run mode.
3.5.3. Atmosphere Coefficient Adjustment
If a reference high pressure gauge is not available, the ambient barometric pressure acquired from the
weather channel or other weather service can be used to adjust the proper reading.
To adjust the atmosphere coefficient, use the following procedure:
1. Backfill with the desired gas to a pressure between 700 and 800 Torr.
2. Place the instrument in the Cal mode using the SELECT button.
3. Turn the ADJUST rotary encoder until the HPM-2002-OBE display matches the reading on the reference
gauge OR using the RS232/485 send “CM={m.d}E{e}<CR>”.Place the HPM-2002-OBE back in the Run
mode.