BajaJim writes:

Am wondering if anyone has set up a small air compressor rig, 110V,
similar to the ones used by divers for hull cleaning services. I assume
just a small oil-less compressor and couple it directly to a stock dive
regulator?

Yes, we do this in our swimming pool. I run an old ScubaPro regulator 2nd
stage ($25 on eBay) off 90 psig shop air from a standard piston/oil type
compressor. Despite all the dire warnings (from people just reciting
paranoia they've heard, who've never tested for oil or tried it), the air
should be OK if your compressor doesn't consume oil such as from bad piston
rings. And the pressure is fine for feeding a 2nd stage.

The fear of "any oil in air will harm you" is silly. First, it is mineral
oil, which is non-toxic. Second, if were that bad, a whiff of 2-cycle
exhaust would kill you. I run this compressor for hours and hours,
thousands of cubic feet, and it loses only tiny amounts of oil, most of
which condenses in the tank.

I suppose an oilless compressor would eliminate even the possibility of
entrained oil. Although they're hideously noisy. You would want at least
a 2 cfm compressor rating, since you typically breathe about 1 cfm. But 2
cfm at 90 psi is only about a 1/2 (true) horsepower unit.
But see my essay http://www.truetex.com/aircompressors.htm on horsepower
ratings.

You need an adapter from the SAE scuba fitting to NPT pipe thread and
thence to a shop air quick-connect, which all then goes on your regulator
second stage hose where it would have connected to a first stage. You now
have a scuba regulator that terminates in an air tool quick-connect. I
made my own threaded adapter on my machine shop lathe. The only source for
such a part otherwise that I can think of is the adapters made for buoyancy
compensator connectors to shop air for filling tires or running tools from
scuba tanks (although this is the opposite direction of adaptation, it
might have the critical SAE-fitting-thread to NPT-pipe-thread conversion).
You could do it by simply splicing hoses with a hose barb if you were
willing to cut up the regulator feed hose.

Here is my drawing of the adapter essentials:

http://www.truetex.com/scuba_lp.pdf

Shallow water hookah diving is easier than most open water scuba diving.
The dive gear industry makes everything but this year's model sound like
it's going to kill you. The old 2nd stage regulators were very simple and
easy to repair yourself. They too were once the thing you had to have and
all the old models would kill you.

If anyone wonders if this is unreliable and therefore hazardous, I would
only use it for shallow diving, like working or practicing diving in a
swimming pool, or working on a boat hull, and I would insist on having a
trustworthy person for a topside tender. Being suddenly out of air is not
a problem with some simple training; we as tenders routinely disconnect the
air unexpectedly to our diving family members to help them practice the
drill (a habit I got from my first scuba instructor in the 1970s, who liked
to turn off your main valve when you weren't expecting it).

To anyone suggesting you just go full scuba instead of improvising a
topside compressor, I would say that full scuba is absurdly cumbersome and
expensive compared to hookah for shallow diving on a fixed location, and in
my opinion, no improvement in risk.

Of course this is diving, which takes training and recent diving experience
to minimize risks.