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SurfLung
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Fill Rate Challenge

Fri Apr 27, 2018 4:23 pm

Compressor Fill Rate Challenge...
- I have been on a 4 year quest to bring the fill rate of my RIX SA3-E up to the 3.0 cfm that it is supposed to be achieving. The first big jump came when AntiqueDiver Bill helped me discover that it had the wrong size pulley/sheave and was running the compressor at about half of its specified 2300 rpm. A bigger pulley increased the fill rate noticeably. Later I rebuilt the cylinder heads and re-ringed the pistons. Then I got an RPM meter and discovered I still wasn't running at 2300 rpm. So I got an adjustable sheave and was able to dial the rpms up to 2250.
- At this point, it was filling tanks pretty fast and since most tanks still have something left, I was estimating and thought I was filling at about 2.9 cfm.
- Well, somewhere in the winter boredom of waiting for the lakes to unfreeze, I stumbled on the AireTex compressor site and read a method to measure the true fill rate and not an estimate. Apparently it has become acceptable for compressor companies to quote an "SCFM" fill rate that is based on filling an 80 cf tank from 500-3000 psi and counting it as a full 80 cf fill. This is not accurate.
- With the AireTex method, you have to fill a tank from completely empty, ZERO pressure. And you put a stop watch on how long it takes to reach a known pressure/volume. You hook up an empty, 0 psi tank, open the valve but keep the fill whip valve shut. Then you turn on the compressor and pump everything up until the fill whip reads the same pressure as you're going to fill the tank. (You don't want to count how long it takes to pressurize the compressor, moisture separators, filters, and fill whip). Then you open the fill whip valve and start the stop watch.
- Long story short, my RIX SA3-E filled that tank from 0-2200 psi at a rate of 2.7 cfm. I emptied the tank and tried it again to get the same result. So, that method is consistent and true. And I may not be getting the full 3.0 cfm rate, but 2.7 ain't bad and my RIX is running perfectly.
- BUT, I wasn't this sensible about it right away. I thought about adjusting the rpms higher to achieve the elusive 3.0... I could get that thing screaming just to save a few minutes of filling time. I talked to Sea Hunt Jerry about it and I think he got the bug and tried measuring the rate on his Bauer compressor... It was supposed to be 4.9 and I think he said he got 3.3 cfm. "It took FOREVER to fill that HOG TANK up to 500 psi!" We were BOTH upset about our less than expected fill rates... But we're okay now. The truth has set us free! :roll:
- TAKE THE CHALLENGE! Have you measured the true fill rate of your compressor from 0-full psi? Do you DARE? After all it may be disappointing. It's almost guaranteed to be a lower fill rate than you THINK you're getting. Good luck and please post results here! :D
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

swimjim
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Re: Fill Rate Challenge

Sat Apr 28, 2018 6:46 am

My compressor is a MaxAir 35 which is rated for 3.5 CFM @ 2800 rpm. When I bought it, there was a gas engine on it. I repowered with a 220 single phase electric motor. That motor turns the compressor at 2400 rpm. I'm thinking that's okay as if the compressor isn't "working" as hard, it will last longer. Yesterday, I filled two 72 double sets from empty to 2800 psi. Each set took one hour to fill. Math and I don't get along, but if I figured out correctly that is a fill rate of 2.7 cfm. Those tanks are now true 80's with the, eh hem, slight over fill. Lol
I keep a log that covers hours on the unit and what I did every time I use the compressor. That way, I know when to change oil and filters. This also keeps track of the compressors "health". It's very consistent and that's a good thing! 2.7 cfm is close enough for me.

Jim

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captain
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Re: Fill Rate Challenge

Sat Apr 28, 2018 10:26 am

Over the years I have routinely use the Airetex method but it was of my own conception. To be honest we have no idea how the manufactures determine the fill rate. Maybe they use the cubic inch capacity of the first stage and RPM to calculate the theoretical CFM. Perhaps they use the 500 psi starting point or do an actual test at one of the recognized standard atmospheric pressure and temperature.

https://en.wikipedia.org/wiki/Standard_ ... d_pressure

My compressor is rated at 3.5 cfm at 2400 PSI, I don't recall it ever coming close to that, at best 3.0 CFM on a cold low dew point day.
Captain

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SurfLung
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Re: Fill Rate Challenge

Fri Jun 01, 2018 10:37 am

Yet Another RIX Adventure... And Maybe Improved Fill Rate
- I was filling tanks for the Wazee trip and sensing that it was taking a long time. Checked for leaks at all of the connections, drains, and bleeders but found nothing. Then on the last tank that was supposed to fill to 3000 psi, I couldn't get the pressure to go up past 2700. The soapy spray wasn't catching it. Then, as I was running my fingfers along the plumbing, I felt a bunch of air coming out of the elbow... Right in line with where the air filter put-puts its little back puffs... Those back puffs were camouflaging the leak. I assumed it was just a bad seal.
- So, I turned off the compressor and depressurized everything. Unscrewed, re-taped and screwed the elbow back in. Turned on the compressor and the leak was just as bad. Actually kind of a torrent. So I took it all apart again and looked for cracks. Nothing visible. So I took the elbow out and screwed back in... No leaks. It was the elbow for sure. Has to be a crack in the thread but I still can't see it.
- I ordered a new high pressure elbow, re-assembled everything last night and filled three units: One AL80 and two Twin 38s. Filling from 2/3rds empty so I can only estimate the cubic feet. BUT... I filled all three in only 56 minutes, about 165 cf. The AL80 got warm and the Twin 38s got hot... Signifying a fast fill rate. I estimate 2.95 cfm compared to my earlier AERETEX method measurement of 2.7 cfm. I'm thinking I may have had that leak going on a smaller scale back then since I had re-plumbed my fittings after re-building the Back Pressure Regulator.
RIX060118.jpg
- You can see the shiney NEW steel elbow in the lower right of the photo. Note the inlet for the air filter pointing in the direction of the fittings.
- I'd like to re-do my Aeretex method fill rate measurement to see if this really made a difference. I think I'm also going to give the intake filter another turn to point away from everything. :roll:
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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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SurfLung
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Re: Fill Rate Challenge

Mon Jun 04, 2018 9:23 am

Cripes!
- I think I should quit looking... I did the Fill-From-Zero test the next day when it was hot and humid out and came up with 2.6 cfm... It was 2.7 when I measured in the cool dry basement this winter.
- One thing worth noting here is the TIME difference to just fill a tank is really pretty minor between 2.6 and 2.7 cfm... It's about 1 minute. So, trying to eek that extra .3 cfm out of the compressor doesn't really gain all that much. :oops:
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

tbone1004
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Re: Fill Rate Challenge

Mon Jun 04, 2018 4:27 pm

remember that compressors are rated in SCFM which is normalized to standard atmospheric conditions. You can think of that as a mass flow rate. Since gas is compressible, and that compressibility becomes nonlinear at high pressures, and the gas density changes as a function of temperature and humidity, while the compressor is rated at 3scfm you will likely never see 3scfm. If the humidity is high or temperatures are high it will go down, if the pressures are above 3000psi you'll be able to notice the charge rate go down, etc etc.

I.e. filling LP72's in the basement will charge noticeably faster than filling HP120's in the summer when you're outside, and that difference can easily be 10-15%

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SurfLung
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Re: Fill Rate Challenge

Tue Jun 05, 2018 10:28 am

-Thanks T-Bone. I've seen the formal "definition" of SCFM and it is as you say. But we discovered that some compressor companies are using that same term to specify a fill rate of cylinders starting at 500 psi rather than starting out empty. This exaggerates things significantly.
SurfLung
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Vintage Diving Technique are Why I Got Back Into Diving.

tbone1004
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Re: Fill Rate Challenge

Wed Jun 06, 2018 2:24 pm

SCFM shouldn't be used for cylinder filling times. It should be the flow rate at whatever the back pressure valve is set to, with "standard" atmospheric air on the intake. Everything is obviously rounded, but I do think that you are pretty bloody close to "ideal" running conditions with this thing

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antique diver
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Re: Fill Rate Challenge

Fri Jun 15, 2018 5:20 pm

A good way to determine the CFM is to use a flow meter with calibrated scale appropriate for your compressor. I happen to use one calibrated in Liters per Minute that is part of the kit used when taking air samples to be lab tested at Trace Analytics. You can multiply the LPM reading by 0.03515 to convert to CFM.

A simple test gives a pretty good average, and is done by opening your fill line valve fully and letting the PMV (Pressure Maintaining Valve) do its thing. Letting air flow unrestricted out the fill hose end for while will let the pressure stabilize at the PMV setting, and is a pretty good average on my rig. I have the PMV set to 1600 psi, and in the photo below you can see my air flow is 70-71 LPM. (about 2.47 to 2.49 CFM on my Bauer Utilus 10). If you test the air flow with no PMV in line, and with line valve fully open, you will most likely see a greater flow volume. Also, if you crank up your PMV to 2250 or whatever you normally fill to, you will most likely see a drop in flow volume as the greater pressures on the piston rings can cause an increase in air getting past the pistons. That air (Blowby) goes into crankcase and is either introduced back into the first stage intake area or vented to atmosphere.

Basically compressors will pump more volume of air at low pressure output than at high pressure output. When you time how long it takes to fill a given tank from 0 to 2250 or whatever psi you are getting a meaningful average flow rate. I get about 2.5 CFM when I test by timing a fill as accurately as I can. These numbers can vary with atmospheric conditions, as noted in previous posts.
2. flow meter test.JPG
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SurfLung
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Re: Fill Rate Challenge

Tue Jun 19, 2018 11:14 am

Thanks for the explanation and photo. At first I didn't understand what you were getting at but now I get it: The Flow Rate meter confirms the accuracy of simply measuring the time it takes to fill a tank from 0-2250 psi. :!:
SurfLung
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Re: Fill Rate Challenge

Tue Jun 19, 2018 5:28 pm

SurfLung wrote:
Tue Jun 19, 2018 11:14 am
Thanks for the explanation and photo. At first I didn't understand what you were getting at but now I get it: The Flow Rate meter confirms the accuracy of simply measuring the time it takes to fill a tank from 0-2250 psi. :!:
It cuts out the tedious part about watching the clock.
"I get plenty of excercise just pushing my luck!"

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antique diver
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Re: Fill Rate Challenge

Wed Jul 11, 2018 2:02 pm

I just can't leave this subject alone about compressor manufacturers misleading consumers about the CFM output, so here's another way of looking at it.

Here's an example of a Bauer label back when they bothered to put the actual CFM up for all to see. You'll notice that the "Charging Rate" is 20% higher than the real output.
Bauer BP10.JPG
A little misleading is the Model No. that calls this system a BP 10, implying that it is a 10 CFM compressor (I guess the BP stands for Bauer Propaganda)
On more recent compressors they don't bother to list the actual CFM on the ID plate, leaving most consumers to actually believe they are buying a 10 CFM system. I'm not just picking on Bauer, since they probably are not the ones that started this Caca del Toro, but they just followed along with the crowd of not so truthful compressor vendors.

Now to put this in perspective, this compressor costs as much or more than 10 Ounces of GOLD, at least $13,000+. So, how would you feel about paying for 10 Ounces of Gold, but only receiving 8.5 Ounces from the vendor? :cry:
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tbone1004
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Re: Fill Rate Challenge

Wed Jul 11, 2018 3:07 pm

Bill,
It would be nice if they called them out by their actual capacity vs. the "charge rate" crap, but as long as they put the actual capacity out there, then so be it. It's the true Free Air Delivery of the compressor, and I bet that the actual "charge rate" when you keep the compressor at or below 3000psi is much closer to 10 due to the compressibility factor

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SurfLung
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Re: Fill Rate Challenge

Tue Sep 25, 2018 1:42 pm

Good Enough...
- Last night I was filling tanks for the Fortune Pond trip, nice cool weather in the garage. So, I thought I'd try the fill it from zero Fill Rate test again. This time I filled an empty 38. Fill rate worked out to 2.1 cfm. Then I filled a 72 from a starting pressure of 600 psi. Figuring the actual remaining capacity, this time the fill rate was 2.5 cfm. Neither is as good as the 2.7 cfm rate I calculated downstairs last winter.
- Maybe something is leaking but I checked everywhere with soapy water. Checked the RPMs and they're right on 2260 (2300 is spec). The compressor is running about as smooth as it ever has. It's filling my tanks just fine and giving me clean, dry air. There are no leaks when I turn it off.
- So... I've decided that what I've got is Good Enough! (at least for now).
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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Re: Fill Rate Challenge

Tue Sep 25, 2018 1:48 pm

Compressors can drive a good man to drink :lol:
"I get plenty of excercise just pushing my luck!"

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