The problem with “sticky” gases

Reactive or “sticky” gases pose additional issues when it comes to gas detection. As their name suggests, they react with many of the surfaces with which they make contact. This becomes a particular problem if the atmosphere to be tested is being “sampled” rather than tested directly.

Here the air is drawn into the testing apparatus through a tube. Tubing is narrow by its nature, with a high internal surface to volume ratio. Therefore, as an air sample passes up the tubing, a high proportion of the air comes into contact with its surface. The result is dilution of the gas in the air sample, leading to a significant increase in time for an alarm to be raised or the genuine level of gas to be measured.

The time required to take an effective sample is always affected by the length of the tubing, as the air being sampled has further to travel to reach the sensor.  However, for a sticky gas, the type of gas and the material the tubing is made from also have to be considered.

Examples of sticky gases include ozone, chlorine, nitric oxide and nitrogen dioxide, but there are many more. Use of sampling systems rather than fixed detector heads in areas where these gases are a risk is not advisable. The length of tubes between sampling heads and the detector unit (sometime 100m or more) can make delays of 30 minutes in raising the alarm a real possibility.

Care needs also to be taken if using a portable detector with pump or aspirator to sample an environment, for example, before entering a confined space. While this would often be considered best practice, the risk of sticky gases makes the situation much more complex.

According to a Health and Safety Executive research report , “Reactive gases are defined as gases which, because of their high chemical activity, are easily sorbed (adsorbed and/or absorbed) by the exposed surfaces of gas detection systems including detector housings, calibration adapters and remote sample draw accessories (tubing). Because of their greater tendency to be depleted from a gas sample by the exposed surfaces of gas detection systems, special care must be taken to ensure accurate monitoring results. The principle danger is that failure to use compatible materials and proper calibration procedures can result in dangerously inaccurate (low) readings and increased response times.”

Where feasible direct testing of an environment is the best option if sticky gases are any kind of risk.