Good confined space entry procedure requires the use of a pumped portable detector to check the space is safety to enter. But some detectors don’t have pumps built in, in which case, an aspirator should be used. An aspirator is a manual system for drawing the air through a tube to the sensor, and it can work well. However, it is something that requires practice to give confidence that you are doing it right.
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.
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 should sample at different depths to ensure you have tested a representative sample. This can easily take 15 minutes.
Maintaining the pressure
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.
Staying above 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. Using a water trap can also avoid this. 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 gets wet, it may need testing and recalibrated before being used again
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, etc) 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.
Tried and trusted
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 a tired hand. A motorised pump can sense the increased effort of drawing up water. Its flow-failsafe function would shut the pump down. Nevertheless, using an aspirator is a tried and trusted alternative in the hands of an experienced operator.
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.