Castle Group Ltd

One major concern about health and safety in the work place is that of gases hazardous to health. These are controlled under the control of substances hazardous to health (COSHH) regulations 2002.

You have a duty to protect your employees from any dangerous gases they might come into contact with such as toxic gases like carbon monoxide (CO) or explosive gases like methane (MH₄) and any gas that might lead to the depletion or displacement of oxygen in the atmosphere thus causing asphyxiation.

Many of these are odourless and colourless thus it is important we have a device that can measure the presence of these gases. You can find safe levels of gases in the EH40 document which sets out occupational exposure limits issued by the HSE.

The main 4 gases that people measure for are explosive gases (LEL), Carbon Monoxide (CO), Oxygen (O₂) and Hydrogen Sulphide (H₂S) other gases people may want to measure for are:

By far the best way to measure gases is to use a personal gas monitor which will alert the user that there is a potential hazard so that he/she will be able to act upon it before it becomes a hazardous to health.

There are a range of detection devices available on the market from portable to fixed meters, single gas to multigas chargeable units capable of detecting Volatile Organic Compounds (VOCs). There are also disposable meters that will last from one to three years which require no maintenance or calibration and are thrown away once their time is up. You may also decide to choose a pumped gas detection system which can be very useful when it comes to confined space entry. You may even want to use simple gas detection tubes. Most gases detection meters use specific sensors for different gases and the majority should be re-calibrated every 6 months.

Gas detection tubes are small glass tubes that contain chemical that will react specifically with a target gas, once placed onto a hand operated hand pump you draw a sample of air through the tube and if there is any target gas in the air it will react with the chemical in the tube causing a colour change which can be read to give you your concentration of gas. This method can be used on a wide variety of gas BUT it is only a spot check of the concentration thus if there is a sudden change you will not detect it. This method is not always appropriate, especially for personal exposure monitoring.

Catalytic Bead Sensor are used for detecting flammable gases, the work by burning the gas against a heated catalyst coated bead. Although the sensor will react to any flammable gas the response will be different for different gases.

Electrochemical Sensors are used for detecting oxygen and certain toxic gases such as carbon monoxide and hydrogen sulphide. They work in a similar way to a battery in that chemical react with one another to produce and electric current but in the sensor it is a chemical and the target gas the react together. The gas diffuses into the sensor through a small hole and the current output of the sensor is proportional to the amount of gas diffusing in thus the concentration of gas. Oxygen is measure as a % volume where as toxic gases are measured in parts per million (ppm).

Infra Red Sensors are used to measure carbon dioxide. This sensor relies on the fact that different gases absorb infra red light at different frequencies. These sensor are usually very reliable however they are often much more costly.

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