User avatar
SurfLung
Master Diver
Posts: 1463
Joined: Sat Jan 01, 2011 2:03 pm
First Name: Eben
Location: Alexandria, MN
Contact: Website

Fill Rate and Pressure

Tue Apr 04, 2017 1:09 pm

Compressor Fill Rate and Pressure
- Before re-ringing my RIX SA-3E compressor, I had thought it was filling tanks at around 2.5 cf/min. Then AFTER installing new rings, I tested it by filling my twin 38s from zero to 2000 psi. That's 84 cf and it filled them in 29 minutes. That's 2.9 cf/min.
- Then last weekend I filled 3 AL80s to 3000 psi, 1 steel HP80 to 3000 (65 cf), and 1 steel 72 to 2500 psi. My calculations figured the fill rate at 2.4 cf/min. 377 cf filled in 2:36... 156 min. 3 of the tanks were at or near empty.
- So here are my questions:
1. Does the fill rate slow down at higher pressures?
2. Or, does the fill rate go slower when tanks are nearly empty?
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

User avatar
antique diver
Master Diver
Posts: 1877
Joined: Sun Dec 10, 2006 12:50 pm
First Name: Bill
Location: North-Central Texas

Re: Fill Rate and Pressure

Wed Apr 05, 2017 9:59 am

SurfLung wrote:Compressor Fill Rate and Pressure
- Before re-ringing my RIX SA-3E compressor, I had thought it was filling tanks at around 2.5 cf/min. Then AFTER installing new rings, I tested it by filling my twin 38s from zero to 2000 psi. That's 84 cf and it filled them in 29 minutes. That's 2.9 cf/min.
- Then last weekend I filled 3 AL80s to 3000 psi, 1 steel HP80 to 3000 (65 cf), and 1 steel 72 to 2500 psi. My calculations figured the fill rate at 2.4 cf/min. 377 cf filled in 2:36... 156 min. 3 of the tanks were at or near empty.
- So here are my questions:
1. Does the fill rate slow down at higher pressures?
2. Or, does the fill rate go slower when tanks are nearly empty?
1. it would not be unusual to lose a little volume at higher pressures, especially before the new rings have fully broken in... but not necessarily just during that period. Also it is entirely possible that the motor speed is dropping a little at the higher pressure load. If you have or can borrow a mechanical tachometer that pushes against the center of the motor shaft you can detect that.
Without a tach, you could shoot video of the pulley or fan as it progresses from low pressure to highest, and the stroboscopic effect of the "shutter" will show a change in speed. That won't tell you how much, but it might give you a hint that the speed is changing.

2. I can't think of any reason for the blowby to increase at lower pressures. Blowby can actually increase with lower speeds, but that shouldn't be happening during low pressure operation. Do keep in mind that if your beginning separator and filter pressures are lower than your starting cylinder pressure it will take more time to fill all that up to final pressure. (See #4)

3. Here is a question that I am not sure of the answer, but should also be considered. Are the 38's rated that at the 10% overfill to 1980 psi, as were most steel cylinders. If that is the case, you would only have about 76 cubic ft at your 2000 psi fill instead of your calculated 84. Try running your calculations for that cyl volume.

While on that subject, most aluminum 80's are really only about 77 at 3K.

4. Also to consider when filling is air volume in the filters. To take that factor out of the calculations, I start with filters and separators filled to the desired ending cylinder pressure, but keeping the compressor running. Only then opening the valve to let air flow to cylinders as I simultaneously start my timer. Time then runs from full separators and filter towers to full everything, giving closer approximation of your fill rate.

There may be other factors that I have failed to consider, and welcome some input on varying voltages and atmospheric density, etc. from day to day, and the potential effect on how discharge volumes may vary.... but probably not very much. (Also how many adult beverages have been consumed prior to operation of the equipment and timer)
The older I get the better I was.

User avatar
captain
Plank Owner
Posts: 1367
Joined: Mon Jul 04, 2005 11:32 am
Location: LaPlace, LA

Re: Fill Rate and Pressure

Wed Apr 05, 2017 11:25 am

One thing to consider is atmospheric condition. Filling when it is 60 degrees and 40% humidity will be faster than at 80 degrees and 80% humidity.
Compressor output is rated at a specific atmospheric temperature, pressure and moisture content so fill rate will be either faster or slower depending on the deviation from conditions it was rated at.

Also I only fill 72's so it is easy to note any changes in fill rate. My check is to time a 500 PSI change (500 to 1000, 1000 to 1500, etc. ). My compressor is rated at 3.5 CFM which turns out to be about 1 minute per 100 PSI or 5 minutes per 500 PSI. When ever those times become significantly longer I start looking for problems, usually a leak. It doesn't take a large leak to effect such a small compressor and you can't hear a leak when it is running so you have to use a soapy leak detector solution to look for bubbles.

Pick one size tank, best a 72 and do a 0 to 2500 PSI fill and time it at each 500 PSI increment. That should give you a good base line to judge performance. If you really want to get anal keep a log of times and the temperature and humidity at each fill..
Captain

swimjim
Master Diver
Posts: 1529
Joined: Thu Mar 23, 2006 12:28 am
First Name: Jim
Location: Belgium WI

Re: Fill Rate and Pressure

Wed Apr 05, 2017 5:03 pm

I keep a log myself. That way I know when to change filters and the compressor's oil in my case. I always write down what I did as far as what was filled. I'm relatively new to the compressor game, but I can tell by looking at the log that everything is copacetic. My fill rate is the same as Captains. About one minute per 100 psi. A pretty easy guide.

User avatar
SurfLung
Master Diver
Posts: 1463
Joined: Sat Jan 01, 2011 2:03 pm
First Name: Eben
Location: Alexandria, MN
Contact: Website

Re: Fill Rate and Pressure

Thu Apr 06, 2017 10:39 am

- Thanks Bill, Captain, and SwimJim. You've given me a lot of good information and advice.
- I DO fill a pretty wide variety of tank sizes and pressures... Which makes it hard to nail down an exact fill rate. For example, when I fill the little 18 cf tanks I made for my grandsons, they fill up to 1800 psi very fast and get hot to the touch. The "38s" that I fill are 1800 psi service pressure and I'm referencing the Basic Scuba book to figure they are 42 cf when filled 10% over. These too, fill pretty fast as singles and get warm to the touch. When I double them as Twin 38s, they still fill pretty darn fast as my last fill went from 0-2000 psi at nearly 3 cf/min. It seems to fill slower mainly when I'm filling the big AL80s to the 3000+ psi pressures. Which goes to Bill's point #1 above.
- Bill, you've been helping me with this compressor since the beginning... Remember when you told me to check the motor RPMs and we discovered two things: First it is a 3400 RPM motor and not 3600 RPM as recommended in the manual. And second, it had too small a sheave (pully) to run the compressor at optimal RPM. So the motor is not original factory spec equipment and possibly a little under powered. (At Northern Tool, 3600 RPM motors are about twice the price of 3450 rpm motors... Something to think about).
- With the larger sheave/pulley, it is pumping my 38s to 2000 psi at almost 3 cf/min which is what the factory rates it at. For it to slow down at higher pressures is further evidence to me again that it is slightly underpowered.
- I didn't think about how much time it takes to pressurize everything up to the PMV and check valve pressures. Next time I will wait for that before timing the actual fill rate.

Now for my Next Experiment...
- I looked up the actual cubic feet of a Luxfer 80 at 3000 psi and it is 77.4 cf. If I pump that same tank from 0-2000 psi, it should be 51 cf of air. If it fills to that level at 2.9 cf/min but fills to 3000 at 2.5 cf/min, I think that should prove that the motor is just a little underpowered at the high pressures... Assuming no leaks elsewhere of course.

- It probably looks like I'm making too much of this fill rate topic. But this compressor was used when I got it and had some issues that I've fixed one by one... In each instance restoring things to like-new condition. At this point, the whole thing should be as good as new if not better than new. And a new one is suppose to fill at 3.0 cf/min. So that's gotten to be kind of a benchmark to attain. Otherwise, the darned thing fills tanks just great... And if the motor is a little underpowered at high pressures, it is only hard on the motor and not the compressor. I can surely run it like this and if the motor ever dies, I'll replace it with a better one.
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

User avatar
antique diver
Master Diver
Posts: 1877
Joined: Sun Dec 10, 2006 12:50 pm
First Name: Bill
Location: North-Central Texas

Re: Fill Rate and Pressure

Thu Apr 06, 2017 10:59 am

Eben, something else to consider is that manufacturers are prone to round the cfm's up when stating their specs, so I wouldn't worry too much about getting a full 3. I would also guess that you could be losing some rpm at the heavier load of high pressures. Not only that, but I am not convinced that Roberts was correct about the volume calculation on the 38, since most every other steel cylinder was rated with the 10% overfill. Also don't forget Captain's remarks about atmospheric conditions.

Slower fill just give you more time to relax and enjoy the process.
The older I get the better I was.

User avatar
luis
VDH Moderator
Posts: 1593
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Fill Rate and Pressure

Thu Apr 06, 2017 12:29 pm

You can get a non-contact laser optical tachometer for as little as $12. With one of this, you don’t have to guess at RPM or RPM change as a function of pressure/ motor load. You can also get contact tachometers for similar price, but for this application I would get a non-contact.

https://www.amazon.com/Generic-Digital- ... X0AZ3Y1E6S

https://www.amazon.com/Pyle-PST43-Preci ... X0AZ3Y1E6S

That been said, I haven’t bought one yet, but I will soon. I will probably splurge and get the $23 model. :wink:


When I checked my fill rate (in my used compressor), I did a video of the gauge and a watch side by side. That way I could stop the video at any moment to get corresponding time and pressure. That way I could calculate average fill rate and specific fill rate at any point in time.

I will try to post some videos tonight.

Keep in mind that tank volumes can vary from one cylinder to the next of the same exact type.
In a different project, during hydro test, I have also measure actual tank water volume and have found it to vary more than what it is published. The aluminum cylinder I used for my testing has an actual volume of about 80.7 cu ft at 3000 psi.
Luis

Buceador con escafandra autónoma clásica.

User avatar
SurfLung
Master Diver
Posts: 1463
Joined: Sat Jan 01, 2011 2:03 pm
First Name: Eben
Location: Alexandria, MN
Contact: Website

Re: Fill Rate and Pressure

Thu Apr 06, 2017 2:02 pm

luis wrote:You can get a non-contact laser optical tachometer for as little as $12. With one of this, you don’t have to guess at RPM or RPM change as a function of pressure/ motor load. You can also get contact tachometers for similar price, but for this application I would get a non-contact.

https://www.amazon.com/Pyle-PST43-Preci ... X0AZ3Y1E6S
Good idea... I just ordered one. :)
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

swimjim
Master Diver
Posts: 1529
Joined: Thu Mar 23, 2006 12:28 am
First Name: Jim
Location: Belgium WI

Re: Fill Rate and Pressure

Thu Apr 06, 2017 4:42 pm

I bought one of those laser tachometers when I repowered my compressor from gas to single phase 220. They work pretty sweet. I'd have to look up the spec, but if I remember correctly my compressor is rated for 2800 rpm max. According to the tach it spins around 2400 with the current motor. The fill rate is right up there and all is well, so I look at the motor being a little slower as a plus. It's probably a little easier on the compressor.
Now Eben, if you want to fill something that takes a while, try my LP 108 double set. That one takes awhile! It's the equivalent of three 72's. :shock:

User avatar
antique diver
Master Diver
Posts: 1877
Joined: Sun Dec 10, 2006 12:50 pm
First Name: Bill
Location: North-Central Texas

Re: Fill Rate and Pressure

Thu Apr 06, 2017 7:30 pm

Thanks for the info on the laser tach, Luis. I didn't even know they existed.

In the interest of safety and convenience I will be retiring my old fashioned contact tach while I still have all my fingers. I sure learn a lot from this forum!
The older I get the better I was.

User avatar
luis
VDH Moderator
Posts: 1593
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Fill Rate and Pressure

Thu Apr 06, 2017 10:15 pm

antique diver wrote:Thanks for the info on the laser tach, Luis. I didn't even know they existed.

In the interest of safety and convenience I will be retiring my old fashioned contact tach while I still have all my fingers. I sure learn a lot from this forum!
Yeah... that is kind of how I feel about contact tachometers. I have used them, but I have a lot of respect to moving machinery. I like my fingers...
Luis

Buceador con escafandra autónoma clásica.

User avatar
luis
VDH Moderator
Posts: 1593
Joined: Thu Jun 30, 2005 9:28 pm
First Name: Luis
Location: Maine

Re: Fill Rate and Pressure

Thu Apr 06, 2017 10:19 pm

Here is the video filling an AL80. It is going a bit faster than real life, like 8x faster. The regular speed video was too big to upload, plus way too boring.


[video]https://vimeo.com/212187436[/video]


I was taking temperature readings and sound pressure levels.


Here is the data I took at the time. I did this three time with the same cylinder to check the compressor. I have not replace the filter so I just filled the cylinder and then just emptied it. I replace the oil and intake filter, but I don’t know how old is the main filter. I have a knew cartridge that I will replace before I start filling for real...


Image


The aluminum cylinder I used for my testing has an actual volume of about 80.7 cu ft at 3000 psi.
Luis

Buceador con escafandra autónoma clásica.

User avatar
SurfLung
Master Diver
Posts: 1463
Joined: Sat Jan 01, 2011 2:03 pm
First Name: Eben
Location: Alexandria, MN
Contact: Website

Re: Fill Rate and Pressure

Fri Apr 07, 2017 9:39 am

Are We Having Fun Yet?
- Anytime you find yourself emptying a perfectly full SCUBA tank just so that you can fill it up again with your compressor, you have to admit you are just "playing" with your compressor. So it was with me last night... I came home from work with a plan of action. And I ended up with fill rate data very similar (I think) but much cruder than Luis' data above.
- I emptied a Luxfer AL80 that the Luxfer chart said is 77.3 cf at 3000 psi. I hooked it up to my compressor. It took 6-7 minutes for the compressor to pressurize all of the condensation towers and filter array. I confirmed no leaks with soapy water. And, I started timing the moment the tank pressure needle moved from zero. My calculations were crude but sufficient: I used 77.3 cf at 3000 for my constant to calculate cubic feet at each pressure level. And I saw SOME reduction in fill rate as pressure went up but not really a significant amount. Here are my results.
Image
- The wine was flowing like the "Swallows of Capistrano" by the time I finished so I used Core Curriculum estimating instead of calculating an exact average fill rate. It looks like about 2.5 cf/min which is just fine. I then called up SwimJim to plan some better ways to empty our tanks! :)

Later... I calculated the average fill rate and it is 2.55 cfm
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

User avatar
Bryan
Plank Owner
Posts: 5233
Joined: Fri Jun 24, 2005 11:40 am
First Name: Bryan
Location: Wesley Chapel Florida
Contact: Website

Re: Fill Rate and Pressure

Sat Apr 08, 2017 10:20 am

Great stuff gang :) Threads like this are what make a forum interesting and fun...If your not careful you just might learn something as well.

Sent from my SM-T810 using Tapatalk
Doing it right should include some common sense, not just blindly following specs and instructions. .Gary D, AWAP on SB

User avatar
SurfLung
Master Diver
Posts: 1463
Joined: Sat Jan 01, 2011 2:03 pm
First Name: Eben
Location: Alexandria, MN
Contact: Website

Re: Fill Rate and Pressure

Tue Apr 11, 2017 8:00 pm

I got the laser tachometer and measured my RPMs easily....
- The motor is running 3534 rpm
- The compressor is running 1974 rpm
- This is measured at 3,000 psi tank pressure

Factory spec for the RIX SA-3E compressor is 3.0 cfm at 2300 rpm. I used my high school algebra for the actual measured speed of 1974 rpm and came up with 2.57 cfm as what it SHOULD be pumping at that rpm if it is working properly:
3.0 cfm/2300 rpm = X cfm/1974 rpm... X = 2.57 cfm

The average of my running test (above) was actually 2.55 cfm. So, we have a fill rate confirmed by two different methods! But more importantly to me is that my compressor is sealing and pumping air as good as new... Limited only by the speed it is being turned at.

Thanks everyone for the tips and suggestions.
SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

Return to “Compressor Talk”

Who is online

Users browsing this forum: No registered users and 6 guests