Quite often our clients asked "how often should we calibrate the gas detectors?". The question can be at time trivial and tricky. To avoid giving an ambiguity answer, the generic answers given away are "one year for combustible gas" and "six months for toxic gas".
Thinking deep about these questions, ones need to understand the inherent risk faces by the operators or technician during the day to day operation. Gas detector is a risk mitigation device to mitigate explosion, intoxication and asphyxiation risk. It is important for the plant owner to conduct risk assessment and Hazop to understand the implication from the process hazards to the assets and the people.
Calibration service is the operational/maintenance cost to the production facilities. There is always a tendency to strive for cost saving years after years but it is utmost important not to compromise safety.
The six months or one year calibration service interval are very much subjective to the condition of the environment. Ones need to understand there are quite a number of gas sensor variants in dealing with different types of gases. The operating principles are different for different type of sensors. The common denominator is most of the sensors are consumable and subject to deterioration from the continuous operation.
The deterioration rate will increase if the gas sensors are in contact with the targeted gases or interference gases (crossed sensitivity). It is important to re-assess whether that particular group of gas detectors need to be calibrated slightly more frequent because the sensors are likely to suffer accuracy drift or "saturation".
There is a common misconception about regular bump test once every few months in substitution for calibration too often like once a year. The objective of bump test is to check the responsiveness of the gas detectors to the known concentration of calibration gases. There is no specific standard written on how the bump test should be carried out but the followings need to be considered:
i. The type or concentration of the gas use.
ii. Up to how many % of full scale of the sensor.
iii. The response time to benchmark.
Bump test is good for providing an indication of the sensor responsiveness to gases. However, it does not help to enhance or recondition the gas sensor performance. For a typical calibration procedure the sensor requires "Zero" and "Span" adjustment.
During the "Zeroing process", the gas sensor is conditioned to recognize the ambient air is clean and free from interference gases. The "Span adjustment" is to condition the gas sensor to "recognize" the known concentration of the calibration gas and program into the sensor memory as a benchmark. It sounds very simple but quite often it has been taken for granted.
It is important to engage the authorized distributor, service company or product manufacturer to conduct the calibration services because their personnel are trained to follow the procedure stipulated in the service manual. Calibration service is much more than engaging a service personnel to press the "Zero" button and subsequently injecting the span gases. There are some fine details or delicate procedures that the authorized personnel are aware of. It is important not to compromising the performance of the gas detectors by performing a substandard calibration.
In the event of gas leakage, the properly maintained gas detectors by the authorized party can safe life and assets. It is an assurance to the plant owners that the gas detectors are properly calibrated and traceable to the valid calibration report.
While the unauthorized party can still produce and submit a calibration report, the report could not help to justify the gas detector has been properly calibrated according to the manufacturer standard and procedure.
Gas detector is a life and asset saving device. It requires regular calibration and maintenance by qualified and authorized personnel. Bump test is only an indication of the gas detector is still responding to the gases but it does not help to maintain the accuracy of the gas detector.