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SurfLung
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Compressor Refresher

Tue Jul 20, 2021 9:58 am

Compressor Refresher
Reviewing My Knowledge of Scuba Compressor Basics

1. Avoiding Carbon Monoxide - CO has two sources: It can come from the surrounding atmosphere OR it can come from dieseling in an oil lubricated compressor. To avoid CO in surrounding atmosphere, use a remote air intake hose, located upwind from all sources of exhaust including that from a gas operated compressor. Operating in a fresh breeze is ideal. To avoid CO from dieseling, use a modern synthetic compressor lubricant, and don't overheat your compressor. For example, the Coltri MCH-6 is recommended to allow cooling between each tank fill.

2. Avoiding Moisture - This is accomplished with condensation towers and filtration media.

3. Maximize Moisture Extraction - Use Pressure Maintenance Valve (PMV) or Back Pressure Regulator (BPR) to keep the Condensors and Filters above 1800 psi. This will squeeze something like 99.7% of moisture and vaporized lubricant out, to be drained via pitcock, and the rest will be absorbed by filtration.

Modernizations

1. My 1982 Coltri MCH-6 has a sticker that recommends SAE-30 oil for the compressor lubricant. Modern synthetic compressor oils are far less prone to dieseling. The 2020 Coltri MCH-6 manual now recommends a modern synthetic oil called "Coltri 157".
2. My 1982 Coltri MCH-6 had a filter that was nothing but charcoal. The 2020 Coltri MCH-6 manual recommends pre-packed filters with 13X Sieve for moisture absorption, activated charcoal for smells and tastes, and hopcalite for CO conversion (converts CO to safer CO2).
3. My 1982 Coltri MCH-6 is gas operated with a Briggs and Stratton engine. The air intake has a filter but it was right on top of the compressor, no more than 15" away from the Briggs and Stratton exhaust pipe. It must have been safe enough in a stiff breeze. But nobody does it that way now-a-days. I have a 10 foot remote intake hose to keep the intake well away from the exhaust and upwind.
4. I have a "Tornado" Kidde compressor that used to operate in a fire station. It's filtration system was a condensation tower filled about 30% with aquarium rocks and a filter full of charcoal. the filter media was apparently never changed in all of its history. I had to upgrade this.
5. Oil-Less Compressors - I have a RIX SA-3E compressor. The upside is that it has teflon rings and needs no oil-based lubricant in the compression cylinders. So, there can be NO Carbon Monoxide generated by overheating the compressor. The downside is dealing with the delicate nature of the teflon rings. AND, if you're running a RIX with a gas engine, You have to guard against CO at the intake just like oil lubricated compressors.
6. Neither the 1982 Coltri MCH-6 nor the Tornado/Kidde had a PMV or BPR. The Coltri didn't even have a check valve. The RIX had a BPR. Now I operate all of them with BPRs.

Observations

1. The Tornado/Kidde has the highly desirable 4 stage Kidde compressor unit that pumps a true 4 cfm at 3700 rpm. My motor is 3400 rpm so the fill rate is more like 3.5 cfm. These were originally made for military use. They were/are extremely durable and reliable. Compressor Guru Jim Shelden had two of them running side by side for years to keep his scuba shop cascade system filled. However, without Jim Shelden I would never have got mine up and running and now Jim has retired. So unless its a $200 bargain, I would not recommend the Kidde.
2. Gas vs. Electric - I love my little Coltri AeroSub MCH-6 with it's Briggs and Stratton engine. It is the smoothest running and rattle free of all three compressors. AND it is so cool to know I can take it on a trip and fill tanks if I want. BUT I can't fill tanks with it unless I have a shady, breezy place to run it... OUTSIDE... Where the noise can bother neighbors. My RIX and my Tornado/Kidde run on electric motors so I can fill tanks just about any time in my garage or my basement.
3. Oil-less vs. Oil lubricated - It strikes me in general that the oil-lubricated compressors are less NEEDY to operate. You just check and maintain the oil level and filter media before each use and they run fine for years. The teflon rings on my RIX seem to get scummy and sticky and I need to pull the pistons and scrub the rings and cylinder walls to restore their flexion and sealing capabilities. AND instead of a crank shaft bathed in oil, the RIX has external turn buckles, swash plate, and bearing races that have to be greased regularly. And finally... Considering the new synthetic compressor oils that are less prone to dieseling (and CO), I don't think the RIX "oil-less" feature offers as much advantage as it once did.
4. Cooling - My 1982 Coltri MCH-6 is recommended to position the fan directly into the wind... So that the wind airflow enhances the fan airflow. I have found that a good wind with shade and not too hot of a day will keep the compressor cool very well and I can fill tanks one after the other without stopping. I can check the heat by touching a cylinder head with my bare hand in these conditions

Recommendations
- I'm kind of tired of fixing up used compressors. It has been fun learning how they work and satisfying to get them up and working properly. But lately I have just wanted them to work properly and fill my tanks when I want to fill tanks. Both the RIX and the Kidde fill tanks just fine but they make rattles and clanks and other noises that I have not been able to solve. The Kidde recently developed a leak so I set it aside until I feel like working on it. The little 1982 Coltri on the other hand is solid as a rock. It had some rattles and clanks... But they were findable and easily solved. I had to fabricate a fix for the condenser and I had to clean up and re-build the Briggs and Stratton carburator. Oh yeah I fixed several other things too... Remote air intake, new pitcocks, repaired the belt guard, etc. But everything I fixed stayed fixed and that thing runs smooth and solid now... I think it runs the best of all three compressors. The only drawback is the gas engine (see above).
- So I think if I was to buy a brand new scuba compressor today... I would go with an electric motor powered Coltri MCH-6.
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SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

bakodiver391
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Joined: Sat Mar 20, 2021 8:22 pm
First Name: Brian

Re: Compressor Refresher

Tue Jul 20, 2021 5:03 pm

One factor for readers to bear in mind is that your old Coltri will have spring actuated disc valves in the first stage head, like Bauers do. The new Coltris have plastic reed valves in the first stage. These are to be changed at the 500 hour service but in my experience they may not last 500 hours, and the 500 hour kit is nearly $300. This seems to be a cheapening down of the compressor and, given the frequent service intervals required and the price of service kits, a small Bauer, while initially more expensive, may be cheaper and more reliable to run in the long term, and certainly have more resale value. I was given a Coltri and did not have good luck with it, however i was able to retro-fit the old style first stage head with the disc valves onto a friend's newer Coltri, so it was of some use. Some people have good luck with the Coltri but many do not. Their main advantages are price and portability. i also have a Rix Sa6 but do not run it much. As you say, they are "needy". My go-to compressors are both Bauer, a K-14 for the shop and a Capitano for portable. Both have been great.

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SurfLung
Master Diver
Posts: 1653
Joined: Sat Jan 01, 2011 2:03 pm
First Name: Eben
Location: Alexandria, MN
Contact: Website

Re: Compressor Refresher

Wed Jul 21, 2021 2:52 pm

Thanks BakoDiver for that info on the Coltri 1st stage head valves. I haven't had to take any heads apart on my '82 Coltri. So I went to the Coltri site and watched a disassembly video to see what you are referring to. Yes I see they have reed valves and they are similar to the reed valves in the heads of my RIX SA3... Which I HAVE worked on. The RIX reed valves are metal and needed to be cleaned to restore their ability to seal. I see the Coltri maintenance schedule says to inspect and clean the reed valves at 250 hours. If they're anything like my RIX valves, the simple cleanup might restore their function to like-new. I wonder about having to buy an entire 500 hour kit if all you need are new reed valves? Or maybe just another cleaning?

Nothing to fault in the Bauer compressors. I just haven't seen very many of them nor do I know anyone who has one. But I have four Coltri MCH-6 compressors in my circle of diving friends plus mine makes five. And all of us are very happy with our machines. Only one (1) problem reported and that was just a leaky fitting that was cracked. I'd be interested to hear from other MCH-6 owners.

Another compressor that is well spoken of and reasonably priced is the Alkin W31. Much lower RPM than MCH-6 and run all day durability is what I hear.
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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