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There's more to air than meets the eye.
Clean Air
The current UK Health and safety standard for breathing compressed air for use by sports divers is BS 4001 and BS4275 the latter being the norm as it is the higher specification. Within these air standards are maximum levels of "contaminants", e.g., oil, water vapour, carbon monoxide gaseous hydrocarbons and other impurities. In BS4257 these are as follows:
- Water Vapour: 39 ppm
- Oil mist: 0.5mg/m3
- Carbon Monoxide: 5ppm
- Gaseous hydrocarbons: Not specified
Standards for oxygen enriched air the standards are higher.
Compressors
In a standard oil lubricated compressor water, oil and carbon monoxide are standard by-products of the compression cycle and these have to be filtered out. This requires rigorous compressor maintenance and specific filtration. Alternatively you can use oil free compressors, these are more expensive but do not require extensive filtration regimes.
Conventional compressors suffer from another disadvantage, the oil content of the warmed compressed air can ignite with devastating effect, you have the three ingredients of the "fire triangle", i.e., fuel (oil mist), heat and oxygen. On no account must gas mixes with an oxygen content greater than 21% be compressed through an ordinary oil lubricated compressor.
Conversely you can increase the oxygen content of the gas prior to compression with an oil free compressor This can be done by sprinkling oxygen into the air inlet of the compressor.
Filtration
Oil contamination creeping past piston rings and inadequate filtration are the main causes of impurities, these have to be filtered through charcoal or a molecular sieve. These can themselves offer problems, if the charcoal becomes damp or wet it becomes useless, so air drying and regular water purging is required. Clearly, inadequate maintenance will exacerbate the impurities. Bear in mind that these oil and contaminants build up over time and as previously mentioned can cause micro-fires.
The only way to be certain about the degree of impurities within the air is to have a sample professionally analysed.
Oils
Compressor oils come in two main types, Mineral oils and Synthetic oils, both meet the lubrication and temperature stability criteria and have low levels of detectable odour. generally synthetic oils are preferred as mineral oils are more readily combustible.
Mixing Gases
There are four ways to mix gas:
- by partial pressure
- by weigh
- by continuous blending
- by molecular filtration
Partial Pressure
This is by far the commonest method of mixing gasses. Essentially you place a measured pressure of 100% oxygen into a cylinder and top off to the working pressure with air. The danger here is that the cylinder will come in contact with pure oxygen, a recipe for disaster unless the cylinder has been "oxygen cleaned", this means all the cylinder has been cleaned and all combustible materials have been removed or replaced, e.g., 'O' rings. Once a cylinder has been oxygen cleaned it must remain contaminant free or in "oxygen service"
Weight
This is the most accurate method although it is expensive and rarely used. Essentially this method consists of calculating the mix using the following equation:
Continuous Blending
This mixes low pressure oxygen with air prior to compression, this is then passed through an oil free compressor. By controlling the amount of oxygen entering the air inlet you can practically "dial a mix".
Molecular Filtration
By squeezing air through a membrane or molecular sieve it is possible to separate the oxygen from the nitrogen. This method is rarely used.
Other Mixes
Apart from air, divers can use a number of other mixes, these provide their own benefits and include: Nitrox or EANx, Trimix and Heliair
Trimix
In trimix, helium is added to the oxygen and nitrogen mix.
Heliair
Heliair is a simple form of Trimix which is created by mixing helium and Air.
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