Back Pressure Video

Home compressor/fill stations. Post photos of your rig, questions, help,maintenance tips.

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SurfLung
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Back Pressure Video

Fri Mar 31, 2017 9:17 am

Back Pressure Regulator or PMV Video
- This is a demonstration of how two back pressure regulators work on my RIX SA-3E Scuba compressor. It's my understanding that back pressure regulators and/or PMV valves are needed to settle and condense moisture so it can be drained from the system before filtration.
- I have two black colored moisture collector towers that are standard for the RIX. I have installed a temporary pressure gauge on the line after the 2nd moisture tower. (This is what the RIX manual instructs to do so that you can check and or adjust the back pressure regulator if needed). After that is a gold colored over pressure valve. Then after that is the gold colored rectangular shaped RIX "Back Pressure Regulator" which has a pressure gauge on top. This device is factory set to block air flow until the pressure reaches 1800 psi. The gauge on top registers pressure in the line AFTER the Back Pressure Regulator so, you won't see the needle move until the first gauge reaches 1800 psi of pressure.
- With the help of my compressor guru AntiqueDiver, I have added a filter array that has yet another Back Pressure Valve just before the gauge that registers the pressure in the tank I'm filling. The needle in this gauge does not move until the back pressure in the filter array reaches 1500 psi.
- This crudely edited video gives an unscripted, unsophisticated oral explanation and then you can watch the needles as the compressor runs. Please excuse the background scenery... I have the compressor set up in our utility room for the winter. (In the summertime, it will have a different mess in the background.)
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SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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ScubaLawyer
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Re: Back Pressure Video

Fri Mar 31, 2017 11:25 am

Eben, very nice. Although I've filled my fair share of tanks over the years using other folks' compressors/banks I've never really thought about how they actually work. Your video peaked my interest. Just the other night my wife asked me why I didn't have my own compressor given the fact I seem to have every other kind of dive-related gear in the garage, closets, basement storage...... Being the dimwitted guy I am I launched into an explanation of excuses, -- no room, air fills at LDS cheaper in long run, maintenance costs, too noisy for the neighbors, etc... What I should have done is set down my third scotch of the evening and say you know honey, your right, I do need a compressor. That way I can spend more time at home with the family rather than waiting in line for an airfill at the dive shop..... Oh well.... Mark

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Bryan
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Re: Back Pressure Video

Fri Mar 31, 2017 11:53 am

I appreciate you sharing your compressor exploits with the group. Folks are often intimidated when asking questions on subjects they are not familiar with and videos like yours answer a lot of questions.

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Doing it right should include some common sense, not just blindly following specs and instructions. .Gary D, AWAP on SB

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couv
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Re: Back Pressure Video

Fri Mar 31, 2017 12:35 pm

Thanks for the demo video. I'm a former tank monkey, but never really understood the finer points of compressor operation and maintenance. So please, talk to me like I'm stupid. What is the purpose of the second back pressure regulator?

Probably more question to follow, but unfortunately my neighbor who just got his compressor up and running is trying to sell it......now where did I put that bottle of single malt?
A sincere THANK YOU to all at VDH who make this wonderful resource available and to all the thoughtful contributors.

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captain
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Re: Back Pressure Video

Fri Mar 31, 2017 1:28 pm

A little clarification. To me it appears you have one back pressure valve, it is down stream of the black compressor mounted filters. I believe what you refer to as the second back pressure valve just before the fill whip pressure gauge is a check valve (back flow preventer). Its purpose is to prevent pressure in the tank being filled from flowing back into the filters and compressor.
If it weren't there you would need to wait until the compressor / filter pressure was equal to or greater than the tank pressure before opening the tank valve.
Captain

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antique diver
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Re: Back Pressure Video

Fri Mar 31, 2017 5:21 pm

Hi Eben, it has been fun watching your interest and knowledge of compressors continue to grow since you first acquired your Rix. You are doing great with it, and helping to spread your knowledge to others.

I would like add a little info about the two back pressure valves (also sometimes called "PMV" for "pressure maintaining valve", so I will use PMV for brevity here). As Eben said, the first PMV does help wring the moisture from the air and let it condense in the separators. That's because the greater the pressure, the less moisture the compressed air can hold in the vapor state. The pressure builds quickly at that point of installation due to the PMV, dropping a good percentage of the total moisture out as liquid, with is collected in the separator for periodic manual draining.

There is actually an additional reason that the first PMV is in place right after the third stage (final stage on this compressor). The third stage piston is called a floating piston since it is not physically attached to the crankshaft like conventional pistons. It is basically too small to attach in the conventional manner that we think of with piston rods, etc. It depends upon high pressure air to hold it firmly against the driving gear or drive piston and prevent pounding damage there. The quicker the pressure builds in that small air space between the piston head and the PMV, the sooner the drive piston and the actual floating piston begin to act as one, with no pounding together of the two.

Now, to answer Captain's concern about the identity of the gold anodized aluminum object at the outlet end of the filter array. That item looks virtually identical to the Aqua Environment Check Valve, but note the tiny vent hole in the housing. This is actually a PMV, and for all practical purposes also acts as a check valve as well... which is good because of the reason Captain stated. (I really haven't thought about why the little vent hole is there, but it does not bleed air during the fill process) It is also made by Aqua Environment, and appears to use the exact same body as their popular check valves.
In fact, a PMV is for all practical purposes just a check valve with a really strong spring holding the check ball in place.

The importance of this particular PMV is that the filter media also works more efficiently at higher pressures. Without one located after the filter towers, all the air pressure in the towers would equalize with the pressure in the cylinder being filled. So if you started filling a cylinder from empty or low pressure, it would take a lot longer for higher pressure to build within the filter media, and technically the air quality may be somewhat less in the early period of the fill process.

This final PMV is smaller, but it does serve the same purpose as that of the aforementioned larger PMV. Just has smaller passages, but it is entirely adequate for small compressor volumes around 5 cfm. The larger one (btw, also made by Aqua Environment) is commonly used on all compressor sizes up to at least 40 cfm.

Hmm, I am half asleep here, so if anyone sees some corrections needed please chime in!
Keep up the good work!
BT

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Re: Back Pressure Video

Fri Mar 31, 2017 6:52 pm

Enjoyed the video Eben. Nice set up! I've had my compressor for three or four years now. I could not imagine not having one! Investment? Sure. But I'd rather spend my time in my shop filling tanks and doing other things while the compressor runs. To loud? my neighbors roll their eyes and walk away. I guess I'm like the guy in the movie, "The Worlds Fastest Indian". If you haven't seen it, you need to. :wink:

Jim

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captain
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Re: Back Pressure Video

Sat Apr 01, 2017 11:21 am

http://valvesandregulators.aquaenvironm ... lator/1310

Learn something every day. I had never seen one like that. I still can't figure out what would be the purpose of the vent hole.
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luis
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Re: Back Pressure Video

Sat Apr 01, 2017 10:17 pm

In the diagram the hole just looks like an ambient pressure reference port for the backside of a poppet. It is design to hold around 1800 psi above ambient.

Here is the manual with a diagram.
http://valvesandregulators.aquaenvironm ... 21127R.pdf
Luis

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captain
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Re: Back Pressure Video

Sun Apr 02, 2017 10:55 am

Thanks Luis, It seems to vent the back end of the poppet to the atmosphere, otherwise air would be trapped behind the poppet restricting length of travel. makes sense now.
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SurfLung
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Re: Back Pressure Video

Mon Apr 03, 2017 1:01 pm

- This compressor has been quite a satisfying journey outside of my comfort zone. I am always learning something new. Sometimes I learn something new from just re-reading what I thought I already knew. I was just re-reading the "101" article Bryan posted and caught the statement that a properly set up PMV will remove 99% of the moisture in breathing air. That's huge!
- This concept that air under pressure forces moisture to condense is yet another one that I didn't understand quite right until reading Bill's post above. And the Check Valve/PMV... I didn't think of it that way until I read Captain's post.
- Capturing and draining moisture from RIX compressors has been kind of a neat topic for me. The RIX itself has only the two moisture condensation and drainage towers. Each of these has an internal spiral cyclone generator to help settle condensed moisture with centrifugal force. It has no additional filtration nor drying equipment and I've heard some people say the military divers liked it that way because the air still had a little moisture and was more comfortable to breath. That sounds wrong to me but they say the Back Pressure Regulators can condense up to 99% of the moisture. Since the RIX SA-3 is "Oil-less" there is no need for a carbon or hopcalite filter... "SA" stands for Sweet Air and with 99% of the moisture pulled out in the towers, maybe its not so crazy to think of filling tanks straight off the Back Pressure Regulator.
- Early in my RIX project, I discovered that the 2nd RIX tower had a crack in it. So, to save money, Bill recommended I bypass it and use the moisture condensing capability of my filter array instead. This worked perfectly until I was able to replace the cracked tower.
- So now, I think I have EXTRA moisture removing capability. I've got the two RIX towers PLUS the filter array tower, PLUS two 13X Sieve filters... Plus one more final filter with 13x sieve, hopcalite, and carbon filtering... It may actually be overkill.
- "Sweet Air" - So, when I'm cruising along the ridge at 70 feet deep in Fortune Pond... And I'm getting a dry mouth and throat from breathing my own compressor air... I say "All- RIGHTY-Then... Dry as a bone. THAT's my idea of Sweet Air!" :)
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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