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.
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.
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
- 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.