How Frequently Should You Test Your Gas Monitors?

Gas monitors and gas detectors, are used in many kinds of manufacturing plants around the world. In a typical chemical plant, petrochemical complex, oil & gas production platform or a similar facility that handles chemicals in large quantities, you may come across gas monitors in the hundreds. These gas monitors and gas detectors, are installed to warn the operators and technicians that run these facilities, about gas or vapor leaks, fugitive emissions and for area monitoring. Most gas detectors get added, after either during the design phase of the plant, or later on, after a HAZOP study or a safety audit. In some cases, they may be added after some undesirable events occur.

This is all very well, but how do the plant managers and facility owners ensure that these noble intentions are met? They have to have a system in place, that makes these various kinds of gas detectors and gas monitors continue to work, even years after they have been installed. This requires these instruments to be part of the calibration schedule of the instrumentation maintenance crew, which is typically entrusted with calibrating all the other instruments as well (like those for flow, pressure, temperature,etc). The only problem is that most plants do these calibrations and testing for their process instruments, only once a year, whereas gas monitors require more frequent testing. Why do these gas monitors require frequent testing? Because they may fail without revealing their failure. Seems complicated? Not really. Let me explain. An instrument like a pressure transmitter is continuously monitoring pressure and sending it's signal to the central control room. If it stops doing this, it can be considered to have failed and this failure is revealed immediately , when an operator notices that it is no longer working.

On the other hand, a gas monitor that has been installed to monitor the leak of a hazardous gas may give out a 0% reading in either of two situations:

1. If there has been no gas leak (as in normal operation) or
2. If it has failed!

How does one distinguish between these two conditions? I know you may be thinking “Oh but there are supposed to be diagnostics and all”, but hang on, think for a moment “Do diagnostics cover every possible fault?” The simple answer is "No! They do not cover all possible faults". Therefore there is no alternative, but to test these gas monitors more frequently than other “normal” process instruments.

OK now that you may be convinced that these gas monitors need more frequent testing and calibration than the regular instruments. But what should be the actual frequency? Should you you calibrate them or just "bump test" them with a known gas mixture in a gas bottle? How often should you check, whether your installed gas monitors & gas detectors are working OK? In order words, what should be an ideal calibration frequency for these instruments? Nobody seems to have a universal answer.

Some experts suggest every year is fine, others recommend it should be done every half year and some others insist, it should be done no less than every quarter. So who is right? Some may feel the more the frequency, the better. However, the problem is, that in most electrochemical type gas detectors, every time you test it, a small amount of the electrolyte is depleted due to reaction with the test gas. This means that the more you test it, the detector’s useful life will reduce. It may so happen, that on a particular test (say the fourth one on the same sensor since its installation), almost all the electrolyte will get depleted. However because sufficient electrolyte was present during the test, the detector will pass out with flying colors. BUT, suppose the next day there is a gas leak AND the electrolyte is now completely depleted, the instrument that was just declared healthy yesterday, will fail in the actual emergency !

Does this bother you? It should. Probably some manufacturer will start indicating the level of useful electrolyte left (I think that already there are some models that have this, but I am not sure), or there is some kind of other sensor diagnostic available, but in the vast majority of these detectors, it does not seem to be present.
What about the catalytic combustion type gas detectors? Well, frequently exposing them to %LEL gas mixtures may cause damage of the bead (repeated explosions taking place on the bead) and could render them ineffective in an actual gas leak.

The only types that can escape this "destruction by calibration" seem to be the semiconductor and the optical/infrared types, that may be unaffected by frequent exposures to gases to which they are sensitive. Does it mean that we should replace all of the electrochemical types and the catalytic combustion types with IR sensors and semiconductors? This won't be really practical to implement, as they are much more expensive than the electrochemical type and the catalytic combustion type.

Do we simply replace all the electrochemical sensors and catalytic combustion sensors every two years? This would be a similar approach to the concept of preventive maintenance that seeks to replace parts, irrespective whether they have failed or not, in order to increase the reliability of the system. But doing this in such cases involving gas monitors, may be much costlier, since the sensors form a large percentage cost of the total cost of the gas monitor.

Hence, each plant or facility management should think over this in more depth and decide the frequency of the testing or calibration, depending on their particular installation, the type of the gas monitors and the risks of the gas monitors failing when least desired.