$500 Compressor Kit

[joebob24]

How many people would be interested in a $500 kit that you could assemble with basic tools that would produce a compressor capable of something like 3-4 SCFM at 3500 PSI? Motor, filtration and mounting not included. Just trying to gauge interest at this point.

 

[Gone for diving]

Thos thread probably isn’t a good place to debate this, but plenty of people have become ill or even died from bad scuba air that was pumped by electric compressors. There have been specific examples where a truck unknowingly idled outside of the intake while the compressor is running.

I have had it myself once, when my wife was mowing the lawn while I was running the compressor. She was dozens of feet away, and the mower wasn’t exactly sitting still underneath the intake. Still has enough carbon monoxide that I was able to detect it with my sensor. I have since added triplex filters and a carbon monoxide sensor on the intake, not just the output.

In addition, lubrication pyrolization is a thing.

Does it happen often? Not at all. The problem is, the consequences are severe: 10 ppm carbon monoxide might not even be noticeable at 1 atm. At 5 atm? now you have a problem.

So your examples all but one have an engine around and running...

And keeping temps down is important.


If you're running you own compressor anywhere, you need to know what you're doing, and need to pay attention,
that just makes sense,

Ugh, I just knew it would be that one…

The MCH6 cartridge is pretty much universally considered by everyone – except maybe those who actually sell MCH6 compressors – as being too small to be useful. As a duplex cartridge, meaning without CO protection, they last maybe a dozen tanks. As a triplex cartridge, even less.

Even the Bauer P0 filter/coalescer system are uncomfortably small, and they are significantly larger than the MCH6 filter. And much more expensive.

I have been following this thread with interest. But that is my main problem with your project. While a $500 compressor that compresses 3 ft.³ a minute is impressive, it’s far from complete. The big elephant in the room is filtration. And unless you want to make a homemade grenade, you either better know very much what the heck you’re doing before you create your own pressure vessel holding 4500 psi of pressure, or you by a premade one. And I haven’t seen a premade one actually worth owning for less than about $800. Add on other extras like fill whips, a gauge or two to know what’s going on, etc. and you’re approaching $2000.

For something put together out of spare parts and Harbor Freight.

I give you all the props in the world. What are you doing so far looks amazing. But by the time you’re done, I think you’re going to understand exactly why compressors cost what they cost. This reminds me very much of someone who builds a custom roadster, or their own RV camper or some such thing. For a talented builder, they can achieve absolutely amazing results. But if you try to aim for reproducibility (which usually precludes extensive use of found or repurposed materials) and factor in the value of your time (which usually precludes extensive use of hand – manufactured parts), the costs absolutely skyrocket.

But I wish you much success: I hope you prove the naysayers – including me! – wrong. And I will continue to follow with interest.

You can always buy 2 for 620$

Be like a dual scrubber

 

[joebob24]

The fact that you don't realize that air, even at 150psi, is completely saturated is a big red flag for me.

I don't think you understand where the inlet air I am using is coming from. I agree that if you take ambient air and compress it to 150 psi it would be saturated and water would drop out of it as soon as it cools. I am using air that goes through a refrigerated drier and has a typical dewpoint less than 36f at 120psi. At that dewpoint, it only has 0.6 ml of water total in every cubic meter of air. That means that when I raise the pressure to 700psi, the dewpoint is now 89F, which is only a few degrees above the temperature of the air coming into the final stage. Yes, the air is saturated at that point, but no appreciable amount of water is going to drop out.

 

[rjack321]

I don't think you understand where the inlet air I am using is coming from. I agree that if you take ambient air and compress it to 150 psi it would be saturated and water would drop out of it as soon as it cools. I am using air that goes through a refrigerated drier and has a typical dewpoint less than 36f at 120psi. At that dewpoint, it only has 0.6 ml of water total in every cubic meter of air. That means that when I raise the pressure to 700psi, the dewpoint is now 89F, which is only a few degrees above the temperature of the air coming into the final stage. Yes, the air is saturated at that point, but no appreciable amount of water is going to drop out.

Why? This is a completely unrealistic test condition. And the point still stands, the air leaving the 3rd stage is fully saturated.

Have you even bothered to do an air test?

 

[joebob24]

The MCH6 cartridge is pretty much universally considered by everyone

I came to the same conclusion myself, but I thought for someone just filling a few AL80s it might still work OK. I think the ones that are in the $800 range seem like they would be the lowest cost option if you were going to put some hours on it. Unless you are willing to pack your own cartridges.

Add on other extras like fill whips, a gauge or two to know what’s going on, etc. and you’re approaching $2000.

I use really cheap fill whips with the gauge already on it. I have no idea how they make a gauge for that cheap, let alone the whole setup. Nowhere near the $700 you tacked on for that.

I would say all in the whole thing should cost around $1500 to build if you want a large filter. If you go with the tiny one, you could probably keep it under 1k.

There really isn't that much labor involved. I had all of the parts that I machined myself quoted from a machine shop and provided that pricing. The only labor would be bolting it together, bending some tubing and drilling a few holes and the theory is that everything would just come in a box to be assembled by the user.

 

[Gone for diving]

I maybe wrong,
but my understanding is that rix compressors use the moisture as lubricant because they are oiless,
And drop out the water after the last stage.... so I really don't see why a oil compressor can't do the same thing...

 

[berndo]

I love a good diy project but IMHO something like a compressor is too complicated/has too many parts to beat a commercial unit like a small coltri model.
If you don't really know what you're doing, you can really hurt yourself.

 

[joebob24]

Why? This is a completely unrealistic test condition.

I wasn't going to respond to this because I don't really see any value in arguing on scuba board, but then I realized that the title of the thread and some of the early questions might be a little misleading as to what my intentions with this project are. If my only goal were to mass produce these things, this test condition would make no sense.

My primary goal with this is for me to have a compressor that I can use to fill my tanks without shelling out $2500 or more. While I was designing it, I had the thought that there may be others that could utilize the same design. By putting in a little extra effort up front, I might be able to design something that could be replicated instead of a one off. I really didn't know if there would be any interest in something like this and I wasn't going to spend extra time on it if nobody was interested.

I also realized from the start that there is a real possibility that I would run into a problem where something would not work the way I thought it was going to and the whole thing would fail catastrophically. I would be out a lot of time and some money while being no further toward having a compressor I could use.

The reason I am testing with dry air is because it is what I have available and was the least amount of effort required to get something operational. This is the same reason I am using compressed air cooling, it is available and is the least amount of effort to get something working. The only reason I would have to use regular air and adjust the cooling would be to test if it would work for someone else, which I am not quite to that point yet. I am thinking if I hit the 100 hour mark, I will consider switching to more realistic conditions and add the extra hardware that would be required.

 

[joebob24]

I love a good diy project but IMHO something like a compressor is too complicated/has too many parts to beat a commercial unit like a small coltri model.

The definition of DIY typically involves a non professional doing work that a professional would do. I am a mechanical design engineer doing design work, not really sure the term DIY really fits. The stuff I work on for my day job is at least as dangerous, if not more, to make design mistakes on. I just wanted to point this out in case someone does get the idea that anyone could do this and decides to make a filter housing out of a soda bottle or something.

As far as the small Coltri model, they cost close to 3k all in, can't even run continuous and didn't someone just point out that the filter housing isn't even large enough to be useful? Mine cost <$350, runs continuous and has a way bigger filter housing. Seems like a pretty clear cut choice to me. Although, it just crossed the 20 hour mark, so only time will tell.

 

[berndo]

I am a mechanical design engineer doing design work, not really sure the term DIY really fits.

Sure it fits. It's a diy project you're doing in your garage. Working mechanical engineer usually specialise in a field... I assume you don't specialise in designing compressors.

Have you talked to a compressor tech about this design? Or someone else that has experience with compressors for breathing gas? I think you really should.

 

[joebob24]

Have you even bothered to do an air test?

I am looking to get an air test done, which testing place do you recommend? I am considering either trace analytics or compressed air supplies and equipment.

Funny you should ask…

Forensic Detectors Carbon Monoxide (CO) Detector Review

Like everything I write, this is long and detailed, and you may not need much of this detail. I use a CO monitor both for continuous CO monitoring of my compressor, as well as ad-hoc checking of tank fills I get elsewhere. This is divided into sections so that you can skip the sections you...

scubaboard.com

I got the forensic detectors CO meter have been checking the CO level coming out of the separator. So far I haven't seen any CO, but I will keep checking it regularly. Thanks for the suggestion, the meter was exactly what I was looking for. Also works really well for checking for low levels of CO at home.


I don't remember if I mentioned it before, but I added a seal to the final stage to reduce the blow by. The compressor is now running around 2.75 CFM and has 27 hours on it with no issues. (other than a few small oil leaks that I need to address next time I tear it down)