Breathing Gas Quality

by Peter Fear, The Scuba Doctor,
first published in Dive Log Australasia, February 2006

How do you know what's going into your cylinder and what you are breathing meets the Australian Standard?

The average diver has probably never thought about it other than during their Open Water Course, when they may have learnt that the compressor inlet should be away from all exhaust fumes and your cylinders are filled with compressed air delivered via a filtration system and fill hose to your cylinder.

When you take your steel cylinder for its annual test and are told there was rust inside that had to be brushed and rumbled to remove at additional cost, did you wonder how it got rusty?

There are a several ways your cylinder can rust all of which, with simple care, can be prevented. I have found that the main cause of rust in cylinders is from moist air delivered into your cylinder from a Fill Station that has a poorly designed system, inadequate or poorly maintained filtration or a faulty compressor. Any of these can deliver moist air or other contaminants into your cylinder considerably decreasing its life expectancy.

Australian Standard 2299.1, 3.13 Breathing Gas Quality states in part:

Breathing air used in diving operations shall

  1. have no objectionable or nauseous odor;
  2. contain not more than (10 p.p.m. by volume) of carbon monoxide;
  3. contain not more than (480 p.p.m. by volume) of carbon dioxide;
  4. contain not more than (130 p.p.m. by volume) of water;
  5. contain not more than 1mg/m3 of oil;
  6. where supplied from a compressor, not be used for diving unless the compressor has, within the last six month period and every six months during operation, undergone a test to ensure that the compressed air satisfies the above requirements.

After operating an approved, Hydrostatic Cylinder Test Station for many years, I have seen cylinders contaminated with oil, water and other substances, some with excessive rust found during the cylinder's first annual test. Cylinders so rusted internally that removing the valves became difficult or when opening the valve to drain the cylinder, the odor of the air inside makes you nauseous. Some unaware divers are breathing contaminated air.

So how do you know what's in your cylinder? And for that matter how does your Fill Station know that the air they are filling your cylinder with meets the above standard?


AS 3848.2. Section 2.2.1 states in part:

Testing of the purity of compressed SCUBA air shall take place at least every three months or more often as appropriate to verify the performance of filters and driers.

A Breathing Air Test Kit can be used to quantitatively determine the content of the following in respirable air from compressors, filling systems and gas cylinders:

  1. Carbon Monoxide (CO)
  2. Carbon Dioxide (C02)
  3. Water Vapor (H20)
  4. Oil

The test kits are reasonably expensive and most Dive Shops do not own one, however Air Purity Tests are readily available for as little as $150.

When the tests are completed a Gas Purity Test Report is issued indicating the levels of contaminants, if they have passed or failed the standard and the date of test.

This report should be displayed at the Fill Panel for your satisfaction and verification.


Breathing contaminated air underwater under pressure, is not what we divers should have to think about. It is your right to demand as a minimum, air quality to the Australian Standard prescribed and the responsibility of all Filling Stations including University's and other Dive Clubs to supply and verify it. Accept nothing less?

Helpful Hints

If rust or other contaminants are found in your cylinder, question your Filling Stations. A chat with the Hydrostatic Test Station could help determine the cause.

The smell test will usually reveal odors from oil and some other gasses. This is done by holding your hand loosely over the valve, cracking the valve gently and smelling the expelled air.

Finishing a dive with an oily taste in your mouth is a dead give away for bad air.

Before filling your cylinder, aluminum or steel, crack the valve a couple of times to blow out any water that may have accumulated in the valve or it will enter the cylinder with the air fill. All experienced operators do this naturally.

Never bleed all the air from your cylinder.

Make sure you use a reputable Fill Station that proudly displays their current Gas Purity Test Report.

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