Home Tech Tips Tech Tips: Portable Air Compressors Part I

Tech Tips: Portable Air Compressors Part I

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Tech Tips - Compressor

Hi everyone, it’s Alec Peirce Scuba with tech tips again. This a neat tech tip for a couple of reasons. I haven’t spent a lot of time talking about compressors – I’ve covered a couple fill stations and so on – but quite a few divers (though not proportionately) own their own compressor. This is a tech tip about compressors, specifically your own. Everybody talks about it, “I’ll get my own compressor. I’m not going to pay that dive store owner five dollars for air when I could for three.” That’s not the way it is – you get an air fill for five to 10 dollars and you’re stealing it. A lot of divers ask about having their own compressor.

By Alec Peirce

This is not a typical diver-owned compressor. This is a special and sophisticated, and maybe a bit of overkill. If you are a diver that dives a lot, maybe with a club or friends, then this system will take care of it. There aren’t many disadvantages with a system this big other than the sheer bulk of it. This is a small compressor by dive store standards but is a true commercial dive store compressor. Quite frankly, the brand doesn’t matter a great deal; the basic principle of compressors doesn’t change a bit whether it’s a Mako, a BAUER or an Ingersoll Rand. This particular one, a Mako, is fairly popular.

Roger, one of our most active members, brings this compressor in a trailer plus a full cascade system. The reason why we have this – why you might want your own compressor – is because we need to get air fills. We have eight to 10 divers in the water every half hour exactly, and we can’t run off to dive stores. Roger brings his compressor system and sets it up: it’s electric so it plugs into a typical R/V outlet.

This is an eight CFM (cubic feet per minute), which means when the compressor is still and the tank is empty, you connect it and start the compressor, and that point it is technically producing eight cubic feet per minute – if you have an 80 cubic-foot tank, you should be able to fill it from zero PSI to 3,000 PSI in eight minutes. Simple, but trust me, it doesn’t work that way. A variety of things interfere with that. First of all, there may be pressure in the tank when it begins. Is the compressor too hot or cold? What is the humidity like? Plus, if it’s eight CFM when it’s brand new, it’s not eight CFM two or three years later since they tend to wear. Eight CFM is quite big… a more typical one is around three CFM, so it takes about 30 minutes to fill the same tank. This is quite a system: a three-stage compressor, which simply means it’s three cylinders.

Anybody can hook up a compressor – we’re used to air pump. Roger has a big hose, which hooks onto the inlet filter, and he runs it about 10 feet away. And if you have the hose too long… there’s so many technical things about running a compressor: it’s not a matter of going to your dive store, buying the compressor, and start filling your tanks. You want to have a long hose that’s away from the heat, oil, and smell down here: you want to eliminate any pollution and want the best air possible going in. If you take good clean air in, then the filters last longer and the air is safer to use. An eight to 10-foot hose is usually common and is enough so the inlet air can be up fairly high and not right adjacent to the compressor.

After that, you just start it. It has an all pressure gauge, a temperature gauge, and an hour meter so you can know how long it’s been running and know when to change the filters.

You may have noticed this big barrel right beside it. What’s it for? Corn oil for your cupcakes? These compressors are oil-based: they’re an engine and they have engine oil in them, be it synthetic or real. As it runs, the oil gets air into the oil, and sometimes it travels through the compressors and the airline to your scuba tank. We can’t have that so there are traps in here that stop the oil, which is usually mixed with water, because when the air is sucked in and compressed, it gets hot; as it expands, it gets cold and you get a lot of condensation. You’ll get this white goopy mess – an oil and air mixture – and you’ve got to get rid of it because it builds up: once the trap separator gets full it could carry on. If you have a normal, smaller compressor, manually open it up and blow it out every 15 minutes. Roger’s Mako is very sophisticated and has an automatic dump with a drum, and he’ll take it to his local environmental disposal place to get rid of it.

There are two downfalls to having a large compressor like this: one, it’s a bit noisy, and two, it’s fairly heavy. Roger has mounted this on wheels so he can wheel it around; I’d suggest it’s the better of 200 pounds. The motor is the heaviest part (about 100 pounds). Now there is a another downside to this particular compressor, and that’s the cost: roughly $1,000 per CFM, plus another $1000 for extras. Using that simple formula, this one would be close to $10,000.

Not bad, huh? It’s like standing beside a truck and the running isn’t too bad though there’s a rattling sound. There’s some very special pistons that take air and puts it up to 3,000 PSI.

I’m going to stop so you can see the fill system. That’s pretty slick, too.

Join Alec in concluding this special on portable air compressors in next month’s edition of Scuba H2O Adventures Magazine.