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02 October 2014
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Getting yourself out of a hole

A common question we encounter at Crowcon is when to use a pump or aspirator with a portable gas detection device. I’d like to share some thoughts about the use of personal detectors with pumps or aspirators as part of an effective confined space pre-entry check.

In day-to-day use, most portable gas detectors are used in diffusion mode; gas reaches the detector by normal air movement. The detector should be worn near the head, so that it samples the same environment as you are breathing. If entering a confined space, a pre-entry check is recommended, to test the environment is safe.

It would be disastrous to be overcome by gas while halfway down a ladder, so in this case, the environment can be actively sampled from above, using an aspirator or motorised pump and tubing to draw gases to the sensors.

Don’t be Passive about Gas Detection

So, what are the pros and cons of the aspirator against the motorised pump? The short answer is accuracy and convenience versus cost.

An aspirator is a manual system for drawing the air through a tube to the sensor. You may have to pump by hand for several minutes, maintaining a consistent flow. When you begin, the initial pressure pulse routinely causes the sensor to trigger an alarm. It can take around a minute for the sensor to normalise and produce a stable reading.

The rate at which air is pumped over the sensor is important, as well. Pumping too fast or at an irregular pace may cause a pressure pulse and trigger another false alarm. But don’t pump too slowly either, as gas takes longer to reach the sensor, delaying any alarm.

After normalisation, you must continue pumping long enough to ensure that sufficient air has been sampled to alert to any immediate danger present. If the space you are entering is deep, you may want to sample at different depths to ensure you have tested a representative sample. This can easily take 15 minutes.

Using a motorised pump is much simpler. It draws air over the sensor at a suitable rate automatically, avoiding false and late alarms, not to mention the tired hand.

Don’t Get into Deep Water

Another problem arises if you accidentally pump water into the sensor. If the end of the tubing is in darkness, it could become submerged in water. A motorised pump can sense the increased effort of drawing up water. Its flow-failsafe function would shut the pump down. Using a water trap can also avoid this problem. It catches the water, allowing you to see it before it reaches the sensor. Alternatively, a ball float holds the end of the tubing clear of the water. If the detector does get wet it may need testing and recalibrated before being used again.

Be Proactive about Reactive Gases

With reactive gases, sampling remotely, whether using an aspirator or motorised pump, can not be relied on. The reactive gases (chlorine, ozone, hydrochloride, nitrous oxide) stick to standard tubing. As a result, the sensor will produce a low or no readout. In this case, a detector should be lowered into the space. Once lowered, most gas detectors rely on diffusion so will only test air in the immediate vicinity.

Confined spaces can be deadly! Using a personal gas detector with a motorised pump or aspirator enables you to sample a space before you step into it, giving you reassurance that you are not lowering yourself into an immediately hazardous environment. As part of confined space best practice, it will help ensure that, at the end of your shift, you can climb back up the ladder and go home safely.

If you are interested in more information on working safely in confined spaces, many government bodies provide free information, for example the Health and Safety Executive in the UK and Occupational & Safety Health Administration in the USA.

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