Compresser requirements

[BLT]

Hi,
Since my DA has a Hookah port that I haven't used, I have been considering, well, umm using it! Or at least designing a system that the port could be used on. If I actually get the thing working I might also see about getting the old Kirby-Morgan (KMB-8 / mk 1 mod 1 if I remember my numbers right) out of mothballs and seeing how that works.

What initiated all this is that I had been thinking about getting a small compresser to have around the house so I was wondering what kind of air requirements would be necessary to run the DA as a hookah?
I know that I will need an oilless compresser, but are the air purity requirements as strict as you need for filling scuba tanks? (yes I do know that I will be BREATHING whatever comes thru the system so obviously I don't want too much crud)
I am also wondering about the capacity required to do this. I figure this would be a shallow water exercise (~40 feet or so max).

So far, the VERY rough system I have imagined starts with a pony tank and reg as a bailout, then the regular tank with the Aquamaster DA and hookah. I figure a short hose from the hookah port to a one way valve which would be secured to the tank/reg as strain relief, then a section of air hose up to the surface. On the surface would be a small volume tank, like an old steel 72, then the compresser.

Any obvious or not so obvious flaws in any of this logic?

[cstmwrks]

Eons ago I had a DA with the hookah port on it. I had set it up one time and used it. Mind you this was 25 years ago so memory is fuzzy. Your question is answered in a formula.. it is all math. anyhow if you know your max dive depth of say 40 feet then as I recall the compressor needs to deliver about 80 to 100 psi over the givein max depth. The other detail is the compressor has to deliver the needed CFM at that pressure to give you some thing to breath.
As a starting point a 3cfm at 60 psi diaphram compressor rated to 100psi would be a great place to start.

[gr8otis]

Any of the old surface-supply manuals (USN, Admiralty, etc) will give you the requirements for air flow and the reasoning for them.

A large part of the question is the amount of dead air space being rebreathed - a lot in heavy dress and a helmet, somewhat less in a mask, very little in an open circuit second stage.

Rather over-simplified, the flow rate must be high enough to keep the CO2 partial pressure below X (some number below 3%, depending on yer neighborhood physiologist) for a hard working diver. The result is that the cfm requirements can be a lot higher than you would surmise just using surface air consumption adjusted for depth.

m

[cstmwrks]

A surface supply manual would give CFM requirments that are overkill. Venting a markV helmet is a sizable demand next to feeding a 2 hose regulator at the end of a 100 foot hose. Another factor is the size of the reserve tank.. the bigger it is, to some extent the compressor can be a little smaller. The tank is a surge chamber, a short period of hi demand or excursion to a greater depth can be handled by the tanks volume. When you accend the demand goes down and lets the compressor catch up.
Bill