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Safeguard OSH Solutions - Thomson Reuters

Safeguard OSH Solutions - Thomson Reuters



Safeguard Magazine

Look & learn—A matter of interpretation

CAROL McSWEENEY says gas monitors are your friend, but there are some traps for young players when it comes to interpreting the results.

Gas testing is very common. Lots of you out there will buy, hire or borrow gas monitors and find that they are “easy” to use. However, even when they are maintained, calibrated and regularly bump tested, there are still a few traps to be aware of.

One such trap is that we may not be measuring what we think we are due to issues of sensor cross-sensitivities. Every sensor has some cross-sensitivity, where the sensor responds to other gases that are not filtered out and can react on the electrode. It is important to be aware of potentially cross-sensitive compounds when interpreting data. These cross-sensitivities can be your enemy but they can also be your friend!

An example of a situation where cross sensitivity can be a trap is when you are trying to measure chlorine gas in the presence of hydrogen sulphide. The presence of hydrogen sulphide has the effect of creating a negative reading on the monitor so that the chlorine levels look lower than they really are. This can be a problem.

Another example is measuring hydrogen sulphide in the presence of nitrogen dioxide or some particular organic compounds, which once again cause the sensor to produce a lower reading than the true concentration. This is a big problem, as low concentrations of hydrogen sulphide can be very toxic. So beware! Things aren’t always what they seem.

There are some good stories as well. If you use a carbon monoxide sensor in the presence of hydrogen gas, the reading will be higher than the actual carbon monoxide concentration present, because you are actually measuring hydrogen. This effect might be a nuisance when obtaining data on concentrations in a mixed gas environment, and you will have to carefully consider what your monitor readout means. But if you are measuring in an area where it is very unlikely that there is any carbon monoxide, and very likely that there is hydrogen gas, you can use that to your advantage.

One of my colleagues was recently testing in a battery charging plant and was looking for any build-up of hydrogen and stibine from lead antimony and lead calcium batteries. She had a specific method for testing for hydrogen and then used the gas monitor with a carbon monoxide cell to look for any hot spots. There was no combustion source in the plant so this was a good indicator of hydrogen.

A couple of months later she was measuring combustion gases at a site where there was restricted ventilation and several large diesel machines were being used. Our client was well aware of the possible exposure hazard to these combustion gases and these were being continually monitored. Due to the remoteness of the site, a caravan had been set up so personnel could have a break, a hot drink and some food. It was also a good place to charge the lead acid battery required for the next shift!

On a break from site monitoring, Helen came into the caravan with the combustion monitor still running and noticed the carbon monoxide levels suddenly increase. Being an observant occupational hygienist, she made the connection between the charging battery and the build-up of hydrogen gas. Although the levels weren’t at an explosion level, it is not a gas you want building up in a caravan. The levels soon dropped when the window was slightly opened. The client was very happy to observe this control, even though there was some snow on the ground outside!

Battery charging is such a common operation in many workplaces that the hydrogen gas hazard is sometimes overlooked. The operators present asked good questions about how much hydrogen is produced from a charging battery and what levels of hydrogen gas are explosive, so it was a timely reminder about the hazards of battery charging and overcharging.

In summary, your gas monitor is your friend. Learn to understand what it is measuring and what it may be measuring, and be open to interpreting different situations, as things are not always what they seem.

An alert occupational hygienist realised that an unexpected high reading for carbon monoxide in this caravan was really a sign of hydrogen gas generated by the recharging battery.

Carol McSweeney is director of occupational hygiene and environmental science consultancy Air Matters.

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