Almost empty tank...

[giladshay]

There's a developing argument among some friends of mine...

As well known, tank accumulates dirt via time.
Suppose talking on average rental tank from general diving facility.
Suppose diver insist using compressed air from tank almost up to the end.

The question is:
Does the diver risk his health by breathing the air from the tank almost up to the end or a regular base ?
In other words- Is there a possibility for this dirt to be mixed with the compressed air, so it might breath together with the air ?

 

[mikerault]

Not really likely unless you are really going to a sleezy LDS or dive facility. The air is filtered as it goes into the tank.

Mike

 

[DeepSeaExplorer]

I think that it's just as likely with the first breath as the last. On occassion, I breathe my tanks low and I've never noticed a difference. It's either safe to breathe or it isn't...

Most regs have a filter in the inlet of the first stage and the dip tube keeps the water out. So even if there was some 'dirt' in the tanks, as long as it doesn't affect the gas quality, it shouldn't be able to get into your lungs.

 

[drew52]

I wouldn't worry about air quality at the end of a tank. Should be the same as at the beggining.

I'd be worrying about the safety issues of breathing a tank so low that you are worring about such things. ie having enough air to bring you and your buddy safely back to the surface should he/she run out of air.

 

[spectrum]

I agree with the above. First it will be the same through the dive and second the buddy support here is sub par.

Since air is homogeneous and since it is not gravity fed or in a situation to stratify none of the typical sediment analogies apply. Furtermore it is highly filtered in the first place.

Pete

 

[divenutny]

Giladshay - I doubt you will have an issue with the air from a LDS compressor. The air is filtered by very good filters and they are usually maintained. However, you may pick up some particles from the tank. There is a filter on your first stage that will catch any particles before the regulator gets the air. I doubt you will breath any particles. This filter is one of the reasons to have your equipment serviced annually. If you are very concerned, check the filter periodically. If it is discolored, your will know which tank(s) might have a corrosion issue. - Tom

 

[giladshay]

Maybe I should modify the question, since I see digress:

What we're not talking about: Not talking about particles which went through filters, or buddy will not be able to help me, etc...

What we're talking about is on the posibility that some of the dirt will have an interaction with the air, and become to gas too. Maybe it doesn't have any taste, so can't feel it. While tank is full- no problems, since its presency is neglectible, but when the tank is getting empty- the gas starts to be more & more significant.

 

[captndale]

What we're talking about is on the posibility that some of the dirt will have an interaction with the air, and become to gas too. Maybe it doesn't have any taste, so can't feel it. While tank is full- no problems, since its presency is neglectible, but when the tank is getting empty- the gas starts to be more & more significant.

With the exception of some particles of rust and carbon (which won't hurt you) the amount of particulate matter (dirt) in a scuba tank is so small as to be non-existent. You will breath more dirt, by orders of magnitude, in fresh, outdoor air.

The contaminants that are of concern are water, oil, volatile hydrocarbons and carbon monoxide. Water is a concern because it leads to rust or corrosion in the cylinder. Oil can burn in an enriched oxygen environment and can pose a health risk if it is present in very high concentrations. Volatile hydrocarbons are rarely present in scuba cylinders in appreciable concentrations. We all know the risks of carbon monoxide.

The concentration of any gasses in the breathing mix will not be affected by the pressure in the tank unless it is also present as a liquid in the tank.

The small sintered metal filter at the inlet of a scuba regulator is not designed to filter out contaminants that you might breath. It is there to protect the high pressure seat or the regulator from particles which might scratch or score the seat and cause it to start leaking. The filter is quite coarse and only stops rather large particles, such as rust.

The bottom line is that the gas you breath from a tank is virtually the same from full to completely empty.

 

[drbill]

I'd also worry about breathing a tank down to such a low internal pressure that one risks taking in water through the reg.

 

[RonFrank]

I would worry less about what happens to a tank during a dive that you breath down (assuming you live! :11: ), and more about what happens to the tank that has little or no pressure in it once it sits about in between fills for a while.

What will happen is that the tank will oxidize. For a Steel tank this means rust. That's a good way to retire an expensive tank before it's time. For Alum it results in a powder like corrosion. At least two divers with different OPS in Coz had their first stages shut down with plenty of pressure in the tank in 2007 because of tank Oxidation.

For the OP, if you are not planning your dives with a SAC rate, and expected return pressure in mind, you NEED to start. There should be no reason to dive a tank down to below 500psi on a regular basis.