Smallest compressor for diving

[transmitterdan]

Since this question comes up quite often I'll put in this plug for steel 95 cubic foot tanks. These run at a lower pressure (2450psi) and have higher volume. Thus they fill faster and take less top end pressure to finish off. They also allow divers to get rid of about 5 pounds (2 kg) of lead weights.

The dive shops use Al 80's because they are lighter to lug around and arguably last longer. Water in the tank isn't a disaster. And I think they are cheaper at initial purchase. All good things for the dive shop budget. But filling them is more expensive which customers pay for so that isn't a big concern of the dive shops.

If you want to have your own compressor you will quickly appreciate the benefits of steel 95 tanks. Just keep them dry inside and inspect twice annually or any time the pressure drops below 400psi.

[skipmac]

Originally Posted by transmitterdan

Since this question comes up quite often I'll put in this plug for steel 95 cubic foot tanks. These run at a lower pressure (2450psi) and have higher volume. Thus they fill faster and take less top end pressure to finish off. They also allow divers to get rid of about 5 pounds (2 kg) of lead weights.

The dive shops use Al 80's because they are lighter to lug around and arguably last longer. Water in the tank isn't a disaster. And I think they are cheaper at initial purchase. All good things for the dive shop budget. But filling them is more expensive which customers pay for so that isn't a big concern of the dive shops.

If you want to have your own compressor you will quickly appreciate the benefits of steel 95 tanks. Just keep them dry inside and inspect twice annually or any time the pressure drops below 400psi.

Low pressure steel tanks can be a good option but a couple of cautions.

Most important, make sure you will be able to be positively buoyant by ditching weight if you need to make an emergency accent to the surface.

Also the steel tanks are heavier and can be awkward climbing in and out of the boat, especially if the seas pick up a bit.

[transmitterdan]

Originally Posted by skipmac

Low pressure steel tanks can be a good option but a couple of cautions.

Most important, make sure you will be able to be positively buoyant by ditching weight if you need to make an emergency accent to the surface.

May I gently disagree with this? The absolute last thing you should consider is an uncontrolled emergency buoyant ascent (short of drowning). I think most (all?) training agencies recommend this as the last thing on the list of ways to self rescue.

A good diver is always neutrally buoyant and never needs to ditch weights in an emergency. Once on the surface then ditching weights is a good way to make life on the surface easier. But without weights a diver will very likely not survive the ascent or will die on the surface from pulmonary barotrauma.

Even if you can't breathe from the regulator usually the BCD low pressure inflator will still work.

[skipmac]

Hi Dan,

Guess I should clarify and elaborate.

Originally Posted by transmitterdan

May I gently disagree with this? The absolute last thing you should consider is an uncontrolled emergency buoyant ascent (short of drowning).

I assumed anyone dive trained would know this without saying but guess it's always good to be very clear. Yes, short of drowning or other life threatening event, never do an uncontrolled ascent.

I think most (all?) training agencies recommend this as the last thing on the list of ways to self rescue.

Yes. It is pretty much the last thing on the list BUT it still needs to be a last ditch option. Something happens, something really terrible and you can't achieve positive buoyancy then that could be drowning.


A good diver is always neutrally buoyant and never needs to ditch weights in an emergency.

Neutral as close as possible but even with a single Al80 there is a several lb difference in buoyancy from full to empty. If I recall a Luxfer 80 full is -3, empty +2 or something like that. You have to weight for neutral with empty tanks so you will be a bit negative when you first go in the water.

Once on the surface then ditching weights is a good way to make life on the surface easier. But without weights a diver will very likely not survive the ascent or will die on the surface from pulmonary barotrauma.

Bent you can fix. You even have a reasonable chance to survive barotrauma or pneumothorax (which are unlikely if you remember to exhale on the way up). However, it's 100% you don't survive drowning.

Even if you can't breathe from the regulator usually the BCD low pressure inflator will still work.

Yes but how many amateur divers are going to think about that if they run out of air and panic.

So no argument from me at all on this point. A bail out to the surface is pretty much at the bottom of the options list when you have a problem. But it does need to be an option and you should always have some quickly ditchable weight for bailout or, as you mention, on the surface if you're lost from the mother ship and need to float or swim. Of course in that case your tanks and regs are also disposable.

[ski69sail]

Some compressors can be belted of the main engine, could be a slick set-up, charge the batteries and fill the tanks at the same time.
Planned to do just that on my CSY 33, but not enough room as the engine box was part of the galley. Too much work and too little room to extend it.


I have been wondering about this option with my Bauer Junior 11. It currently has a 6.5hp petrol engine. I was told that to run the dive compressor off the front of the Yanmar 75hp engine,the most load that is recommend to run a device off the front of the engine is 4hp. Would 4hp be enough to run the compressor ??

I also have plenty of room and options for mounting the compressor
Thanks Ray

[Charlie]

Can you add a PTO Power take off to an engine? I have one on my tractor and wonder if I could do that to my boat engine it would be great as a switch for water maker, dive compressor, generator etc.

[a64pilot]

Often PTO's are part of a transmission, so with the right transmission, yes.
PTO would be a nice thing to have.

[Bulawayo]

Originally Posted by Time2Go

Long Time Advanced Open Water diver (PADI Certification)
and the idea of having a gas motor running the compressor
gives me the chills (all the spots that I have dived have used shore based
electric powerwith plenty of extra bottles on the boat)
You would need really good seperation and ventilation
as not to compress the exhaust of the gas engine into the tanks.
This set uo would definitly get my attention.
If you want to live long and dive be Paranoid

There is zero issue with using petrol or diesel powered compressors as long as you ensure separation of the intake and exhaust. On a boat if you are using an electric unit there is still a generator running somewhere..........hmmmmmm, doesn't that mean an exhaust somewhere? Pretty obvious I would have thought. I am a BSAC qualified instructor - it does not make me an expert on compressors - that comes with experience of filling bottles, following the manufacturers instructions and using certified spare parts especially the filters.

[rcontrera]

WOW ... this thread sure took a lot of tangents!

To get back on subject, small compressors capable of pumping scuba grade breathing air start in the $3000 price range and go up way past $12,000. To hook up safely to 230 VAC you should have at least a 7KW genset.

As has already been noted, if you don't have the power to run one of these, you should look at gasoline or diesel. Bauer Junior II gas - about $5100, Bauer Junior II Diesel - about $7900, Coltri MCH6 Gas - about $3300

One thing that HASN'T been mentioned is that painted compressor frames are notorious "rusters" so you may want to look at going with a stainless steel frame. The Bauer stainless steel "Yachting" model is only available in electric. Coltri stainless steel adds about $500

As for the other compressors mentioned, Utilus hasn't been made in quite a few years. The shoebox takes about 12 hours to fill a scuba tank and requires a low pressure compressor to feed it.

As for best bang for the buck, the Coltri MCH6 is it. However, as has already been mentioned, you HAVE to do your maintenance. If you don't change your oil regularly, you WILL have valve problems. If you are bad at service, then the Bauer is a little better for you.

HOOKA is NOT scuba diving so I won't go into those.

Hope that helps.

[monte]

I see the shoebox isn't to be used for scuba tanks btw. We also have amps to spare and no rush too fill tanks so a slow filling compressor that works off 12Vdc or 220Vac with a 1500W inverter would be the solution we are looking for. Probably less maintainance and no worrying about carrying extra fuel and quieter operation. Is there a reason this can't be built? I'm not familiar with the mechanics but I would have thought less output would mean less power needed. Or is it simply because the demand is t there for a manufacturer to market them.

[hoppy]

Originally Posted by monte

I see the shoebox isn't to be used for scuba tanks btw. We also have amps to spare and no rush too fill tanks so a slow filling compressor that works off 12Vdc or 220Vac with a 1500W inverter would be the solution we are looking for. Probably less maintainance and no worrying about carrying extra fuel and quieter operation. Is there a reason this can't be built? I'm not familiar with the mechanics but I would have thought less output would mean less power needed. Or is it simply because the demand is t there for a manufacturer to market them.

It's a solution you will never find.

Time or air volume is not an issue with filling scuba tanks, it's the pressure needed. If you want to fill a tank to 200+bar, you need a compressor that can pump air at that pressure and to achieve high pressure, you need a lot of power.

[rcontrera]

Originally Posted by hoppy

It's a solution you will never find.

Time or air volume is not an issue with filling scuba tanks, it's the pressure needed. If you want to fill a tank to 200+bar, you need a compressor that can pump air at that pressure and to achieve high pressure, you need a lot of power.

You are dead on!

Hi pressures mean high stresses on parts and, more importantly, on whatever is going to turn it. There IS, however, a hand pump that works like a bicycle pump. It can pump up to scuba tank pressures and I have actually seen guys buy these for that purpose. Of course, you are going to end up with Popeye arms before you get the tank full!

[monte]

Sorry I'm still not getting it. Take the hand pump as an example. Ok it's hard to push! But given the correct gear ratio of an electric motor, it could be pushed slowly by a very small electric motor. Or imagine the shaft is 1/4 the diameter. Easier but slower. I do t know but I imagine a compressor works by forcing air into a chamber that allows the air to flow one way with some kind of non return valve, so isn't it just a case of decreasing the chamber size or the gearing to allow a smaller motor to do the work?

[skipmac]

Originally Posted by monte

Sorry I'm still not getting it. Take the hand pump as an example. Ok it's hard to push! But given the correct gear ratio of an electric motor, it could be pushed slowly by a very small electric motor. Or imagine the shaft is 1/4 the diameter. Easier but slower. I do t know but I imagine a compressor works by forcing air into a chamber that allows the air to flow one way with some kind of non return valve, so isn't it just a case of decreasing the chamber size or the gearing to allow a smaller motor to do the work?

You are correct that you could use a small motor, as long as it will achieve the pressures needed, and take a long time to fill a tank. However at the end of the day you will still use the same total energy. A lot of work for a short time or a little bit of work for a long time still adds up to the same total energy requirements.

[monte]

Thanks skip, but that's the point. We and a lot of other yachts have the power resource with solar, but not the generator or inverter capacity. I just checked the power required to fill a scuba tank and it's around 600Ah. Ok that's a lot! I guess if it was buildable I would want something that consumes 60A for 10 hrs and I could fill one tank over two sunny days. 4 days between dives...
The gas powered compressor is looking good!
Actually our solar will put out about 60A so it would be possible to fill in a day without really taxing the house bank too much. Definitely an option I'd consider if it was available...