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12.
Calibration
12.1. Why Calibrate ?
All electro-chemical gas sensing cells have a finite service life. Through reactions with ambient atmosphere that may
contain contaminants, and in some cases, to target gas, the internal electrodes slowly become less active and
efficient. In operating conditions where the cell is not exposed to its target gas (other than for calibration purposes),
and the surrounding environment does not contain volatile organic compounds (VOC), the cell should operate reliably
for around 18 to 24 months.
Regular exposure to non-clean air, or VOC's will shorten the cells lifetime. The reduced lifetime will depend upon the
frequency and nature of exposure to ideal 'clean-air' conditions. Once a cell has reached the limit of its lifetime, it
must be replaced (see maintenance section).
For Infra-Red type sensors, the process of performance degradation is much slower and is generally related to changes
in the characteristics of the optical components used to measure the gas concentration. Occasionally, detector
operation in a harsh environment can cause stress to the optical components causing a shift in its performance.
For all sensor types, as a result of its aging mechanism, gas proportional electrical outputs will degrade from the levels
they were when new. This signal degradation can be observed as a baseline measurement shift, either positive or
negative. Similarly, the cell's sensitivity to gas will reduce with a corresponding reduction in output signal for a given
gas concentration.
If left uncorrected, eventually the detector will no longer operate reliably in detecting its intended gas.
In order to maintain accurate and reliable operation, the detector must be calibrated at regular intervals. Calibration
will return the detector to a correct zero gas baseline and re-adjust operation to compensate for the reduced cell
sensitivity.
Some environmental standards mandate that detectors must be calibrated regularly if the monitoring system is to
remain compliant with its rules. Gastech recommends calibration at 3 monthly intervals for a detector operating under
normal conditions.
12.2. Preparing for calibration
The method for calibrating the detector has similarities to bump testing. See sections 9.3 to 9.5.
To ready the detector for calibration, conduct the operations covered in sections 9.3 to 9.5.
Unlike bump testing however, a minimum target gas level of 25% of full scale must be used for calibration to ensure an
accurate final calibration and for the detector to achieve its published performance specifications.
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prevents zero calibrations from being performed if the apparent gas level is greater than 10% of full scale.
Span calibrations are prevented if apparent gas concentration is less than 20% of full scale .
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