Compressed Air Purity for divers

Standard BSEN12021

Compressed air used by divers should conform to BS EN12021. This standard gives the basic parameters for the composition of the air and the limitfor for contaminants that are often found in breathing air. These limits are based on a value that is 10% of the 8-hour time weighted average in the workplace in the UK. (Carbon Monoxide is 8%).


Oxygen (O2) should make 21% (±1%)by volume in dry air.


Lubricants such as oil droplets/mist must not exceed 0.5mg/m³

Carbon Dioxide

Carbon Dioxide (CO2) must no exceed 500ml/m³ (or 500ppm)

Carbon Monoxide

Carbon Monoxide (CO) shall be as low as possible and must not exseed 3.5mg/m³ (or 3ppm)

Water content

for a cylinder pressure greater than 200bar measured at the outlet must not exceed 35mg/m³


The air must be without significant odour or taste.

Air Purity Testing

BSEN12021 specifies that the air purity should be at least tested every 3 months. For clubs operating their own compressor who need not strictly comply with this standard should have the air tested every year as an absolute minimum and again after any work or maintenance that may affect the air purity.

In addition to these parameters the compressor manufacturer may also specify that tests for specific chemicals be carried out. The compressor operator should also test for contaminents in the vicinity that may find their way into the system. A good example may be a test for chlorine if the compressor is located near a swimming pool.

Air Purity Test

Firstly you will need to fill a cylinder from the compressor. The cylinder should be clean, a freshly oxygen cleaned cylinder is ideal, this will minimise any contaminants that may have built up inside a frequently used cylinder. The compressor should be run up to it’s normal working temperature and in normal conditions. The cylinder should be empty to start with and filled to at least 100Bar. The air cylinder should then be emptied again and then filled for testing. Doing this will reduce the contaminants that may have been in the existing air in the cylinder down to a minimal level.

There are many ways to test for air purity. The following is a simple method using gas detection tubes. Most detection tubes require a fixed volume of air to pass through them. This can be done using a fixed volume syringe type pump that will be available from the manufacturer. The following example uses a constant flow valve.

Step 1 – fit flow regulator

Firstly a regulator is fitted to the cylinder that will provide a fixed flow of gas from the cylinder under test.

Flow regulating valve fitted to cylinder

Flow regulating valve fitted to cylinder

Step 2 – Fit gas detector tube

With the test tube in place the regulator is turned on to provide a fixed flow rate that is specified for the tube. The tube will also have a time that the test flow should be allowed to run through the tube. It is very important to allow free flow of gas through the tube and not to restrict the outlet.

Gas detector tube fitted to flow regulator

Gas detector tube fitted to flow regulator

 Step 3 – Take the reading

During the test the reagent in the tube will change colour in proportion tot he amount of contaminent in the sample. At the end of the test the measurement can be read of the scale where the reagent has changed colour.

Colour change in tube during test

Colour change in tube during test

Step 4 – Repeat for other contaminents

Each tube is designed to detect a specific contaminant in a specific range. So the test is repeated with all the relevant tubes necessary.

The instructions with the tube will give the color change as well as the time and flow (or volume) of gas that should be used during the test. |Other parameters such as corrections for temperature and which other contaminants may give rise to a false reading are detailed. Lastly instructions for safe disposal information and first aid for the chemicals in the tubes are included.

Example of test tube

Example of test tube

Further guidance can be found from the Health and Safety Exective Divers Breathing Air Standard

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