CALIBRATING GAS SENSORS
Part. No. 2400M2501_6 Touchpoint Pro
59 Technical Handbook
8 Calibrating Gas Sensors
All gas sensors must be calibrated on commissioning, on replacement, at regular intervals specified in their user manuals, or
when dictated by usage. (Touchpoint Pro can be programmed to generate periodic calibration reminders. See Ch. 9.2.16 &
9.3.16 for information on Calibration Reminder Settings for AIM-mA and AIM-mV settings respectively.)
Note: Calibration data is not retained following a restore operation and calibration reminders may require updating following
recalibration.
The basic calibration procedure is to first clear the sensor of any target gas and then introduce a known concentration of a
test gas followed by a rest period to ensure that the sensor goes back to normal.
Note: Oxygen (O
2
)
calibration is done at the sensor while Carbon Monoxide (CO) calibration should follow the user’s local
procedures.
8.1.1 Calibration Definitions
Clean Air describes ambient air that has a nominal zero concentration of the target gas being sensed.
First Span is the name given to the first gas span reading of a new mV sensor, which is used to monitor the sensor’s
sensitivity over time.
Over Range Warning is when a mV sensor exceeds 100 % of Full Scale Deflection (100 %FSD). A gas over range
condition can cause catalytic sensors to operate outside of their working range and incorrectly indicate low gas
concentrations when hazardous concentrations may be present.
Span gas is a compressed gas containing a specified concentration of the target gas being sensed.
Span is the gas concentration reading at which calibration is carried out, typically at 50% of full scale.
Zero Gas describes a test gas that has a zero concentration of the target gas being sensed, and it may be a different
gas for different sensors.
Ensure that executive actions/outputs are inhibited and alternative safety arrangements are made before starting
any TPPR testing or calibration.
It is expected that mA sensors will be calibrated at the sensor or field device. The TPPR mA calibration is not
intended to calibrate the sensor, but to align the TPPR 4-20mA current loop with the sensor current loop zero and
span settings.
The TPPR mA calibration should not be used to calibrate the sensor zero and span, or to compensate for
deficiencies in the sensor or transmitter calibration. Failure to observe this can result in significant accuracy errors.
DANGER
An ‘Over Range Warning’ may indicate a rare but potentially catastrophic event that must be fully investigated.
When Over Range Warning is programmed, TPPR displays a warning on the Touchscreen that requires an
additional Reset operation before the alarm may be cleared.
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