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Old 23-02-2019, 22:02   #1
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How many W of solar to fill an AL30 dive tank

I am tossing up between hookah and tanks, I don't like the restrictions of hookah but I don't want to run a fossil engine running in the background. I'd rather spend more for solar panels if they can fill a 30cf bottle daily. I don't use much air and I think 2 x 30cf tanks will do the job, I expect high use will be 1 tank per day and I may go weeks without using them. I will have a 2200 Honda generator as back up but the objective is to never to start it.

If I need bigger bottles at less pressure for the same dive times to make it work, that's ok.

On top of my current use, how much more solar/storage will I need or is it not feasible.
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Old 23-02-2019, 22:16   #2
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Re: How many W of solar to fill an AL30 dive tank

The correct question is how many kilowatt-hours of energy does it take to fill a scuba tank. I donít know exactly but it is probably about 4 kilo-watt hours to fill 2X30 cubic feet tanks to 3,000 PSI. It depends on the compressor.

It might be less KWH to use 1 steel 110 tank and fill it to 1,500-2,000 PSI. You will need less lead too.

I suspect you will find it impractical to use solar to make compressed scuba air.
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Old 23-02-2019, 22:19   #3
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Re: How many W of solar to fill an AL30 dive tank

Work = Pressure x Volume


Power = Work / time


1 Watt = 1 Newton Meter / sec



Just use consistant units Meters/Seconds. This is with no losses (heat, friction, etc)
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Old 23-02-2019, 22:22   #4
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Re: How many W of solar to fill an AL30 dive tank

Small compressors are really inefficient.
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Old 23-02-2019, 22:30   #5
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Re: How many W of solar to fill an AL30 dive tank

It's very difficult to not use a ICE when dive tanks need filling. I have a bauer Jr and except that the noise filling is part of my diving experience.

I remember reading somewhere that a guy had designed a compressor that functioned via batteries/solar using the appropriate gearing.

Internal combustion engines are still a real part of cruising, it's just the how much that varies.
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Old 24-02-2019, 07:31   #6
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Re: How many W of solar to fill an AL30 dive tank

Dale has a good point. Small compressors run off single phase motors of 1-2 HP. It takes a lot of current to start these without a variable speed drive.

There just is no simple way I have found to fill tanks on board. There are air filters and desiccant canisters to maintain. The compressors are bulky and noisy. Itís all a compromise.

For hull cleaning, a hookah system with the long hose is about as simple as it gets. And they make these for 12VDC. Quite often one will appear for sale on the classified section on CF.
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Old 24-02-2019, 11:06   #7
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Re: How many W of solar to fill an AL30 dive tank

Like people wanting to run aircon off their batteries.

Sure it **can** be done, but so expensive and usually impractical, most should just fire up the genset.
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Old 24-02-2019, 11:38   #8
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Re: How many W of solar to fill an AL30 dive tank

You can work out the number of Watt hours it would take by looking at the specs of some compressors. Motor draw and how long it takes to fill a tank.

Then add another 5 or 10% if you're planning to run it through an inverter. I suspect you'd need a pretty big inverter, and a lot of solar panels though.
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Old 24-02-2019, 11:46   #9
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Re: How many W of solar to fill an AL30 dive tank

Have you seen the new Brownie Sea Lion?

3 Hour + Battery Powered Diving System
2 divers to 65 ft and 3 divers to 35 ft* and 4 dives to 15-20 ft*

https://www.browniedive.com/sea-lion.html

It's pricey, but we're planning on going this route rather than storing tanks and a compressor.
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Old 24-02-2019, 12:30   #10
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Re: How many W of solar to fill an AL30 dive tank

The last time i was in the Sea of cortez, a fellow cruiser anchored nearby would run the SCUBA compressor every afternoon after diving, the compressor being powered by a gas motor, was very noisy, this would go on for maybe 2 hour's, i had to change anchorages, i have a small bower unit, and am going to power it through my inverter, while my small deisel generator is charging batts, and making water, having to listen to all that noise in the still of the evening is just not having any regard for ones neighbors, my small deisel generator is inside the engine room, and the exaust is very quiet.
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Old 24-02-2019, 13:10   #11
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Re: How many W of solar to fill an AL30 dive tank

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Originally Posted by 44'cruisingcat View Post
You can work out the number of Watt hours it would take by looking at the specs of some compressors. Motor draw and how long it takes to fill a tank.

Then add another 5 or 10% if you're planning to run it through an inverter. I suspect you'd need a pretty big inverter, and a lot of solar panels though.
Funny you should say that.

I saw one, it had a 3500W motor and filled tanks in under 20min.

I am guestimating on the high side that it will draw it all on start up, then a half of that on average to run once started. If I empty the tanks before filling and maybe a clutch or capacitor start. It should be able run drawing 1750W. If I only fill to 2000psi as transmittidan suggests, that should halve the power requirements from 3000psi again to 875W.

If it takes 1/3 of an hour to fill that should be 282Wh, that's not too bad at all.

Please feel free to correct me if I have got it wrong.
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Old 24-02-2019, 13:16   #12
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How many W of solar to fill an AL30 dive tank

I have a friend that is trying to run his Watermaker and his I believe Coltri Scuba compressor off of one motor that he will connect to either one but not both by a belt and run it off of a 4000W generator.
He can get it started etc, but about the time the tank gets to 2000 PSI the generator kicks out, I assume meaning blows the breaker.
So itís not just start up load that is the problem, itís simply it takes a tremendous amount of energy to compress air to that high pressure.

I would assume in his case that a smaller pulley that would slow the compressor some what would work, it wouldnít reduce the total amount of power required in watt hours or amp hours, but it would spread it over a longer time, meaning that I think his 4KW generator could do it, just would take maybe a half hour per 80 cu Ft tank as opposed to 20 min.
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Old 24-02-2019, 13:47   #13
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Re: How many W of solar to fill an AL30 dive tank

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Originally Posted by a64pilot View Post
I have a friend that is trying to run his Watermaker and his I believe Coltri Scuba compressor off of one motor that he will connect to either one but not both by a belt and run it off of a 4000W generator.
He can get it started etc, but about the time the tank gets to 2000 PSI the generator kicks out, I assume meaning blows the breaker.
So itís not just start up load that is the problem, itís simply it takes a tremendous amount of energy to compress air to that high pressure.

I would assume in his case that a smaller pulley that would slow the compressor some what would work, it wouldnít reduce the total amount of power required in watt hours or amp hours, but it would spread it over a longer time, meaning that I think his 4KW generator could do it, just would take maybe a half hour per 80 cu Ft tank as opposed to 20 min.
That doesn't sound to good for my maths. :-)
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Old 24-02-2019, 14:42   #14
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Re: How many W of solar to fill an AL30 dive tank

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Funny you should say that.

I saw one, it had a 3500W motor and filled tanks in under 20min.

I am guestimating on the high side that it will draw it all on start up, then a half of that on average to run once started. If I empty the tanks before filling and maybe a clutch or capacitor start. It should be able run drawing 1750W. If I only fill to 2000psi as transmittidan suggests, that should halve the power requirements from 3000psi again to 875W.

If it takes 1/3 of an hour to fill that should be 282Wh, that's not too bad at all.

Please feel free to correct me if I have got it wrong.
I'd be very surprised if a 3500W motor (about 4HP output) could really fill an air cooled 80cu-ft tank in 20 minutes, but let's accept that as true...

A couple things wrong with the analysis. First, the power rating of such a motor is NOT the startup draw, but the fully loaded draw for the motor. As the tank fills it will draw 3500 Watts. It might draw a bit lower at the beginning, (that first cubic foot of air goes in really easy!) but as the tank comes close to full it will need the full 3500 Watts to push against the pressure in the tank. It will likely take 45 to 60 Amps at 120 Volts to start that motor running.

You'll need at least a 6kW inverter to start such a large motor. And you'll need to multiply what ever power use you calculate for the motor by 1.1 or 1.15 for losses in the DC conversion.

You should assume that the motor draws 3500 W (30 amps at 120 Volts, 300+ Amps at 12 volts!) for the entire cycle. Your estimate of 283 W-hr to fill a tank to 2000psi is almost an order of magnitude low.

And, last but not least, is the inefficiencies of drawing power out of a battery at such a high rate thanks to Peukert's law.

It is, of course, theoretically POSSIBLE to fill a scuba tank with a battery and solar, but not really practical in the real world.
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Old 24-02-2019, 15:20   #15
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Re: How many W of solar to fill an AL30 dive tank

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Originally Posted by billknny View Post
I'd be very surprised if a 3500W motor (about 4HP output) could really fill an air cooled 80cu-ft tank in 20 minutes, but let's accept that as true...

A couple things wrong with the analysis. First, the power rating of such a motor is NOT the startup draw, but the fully loaded draw for the motor. As the tank fills it will draw 3500 Watts. It might draw a bit lower at the beginning, (that first cubic foot of air goes in really easy!) but as the tank comes close to full it will need the full 3500 Watts to push against the pressure in the tank. It will likely take 45 to 60 Amps at 120 Volts to start that motor running.

You'll need at least a 6kW inverter to start such a large motor. And you'll need to multiply what ever power use you calculate for the motor by 1.1 or 1.15 for losses in the DC conversion.



You should assume that the motor draws 3500 W (30 amps at 120 Volts, 300+ Amps at 12 volts!) for the entire cycle. Your estimate of 283 W-hr to fill a tank to 2000psi is almost an order of magnitude low.

And, last but not least, is the inefficiencies of drawing power out of a battery at such a high rate thanks to Peukert's law.

It is, of course, theoretically POSSIBLE to fill a scuba tank with a battery and solar, but not really practical in the real world.
+1. My thoughts exactly
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