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Old 30-03-2015, 06:33   #46
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Re: Smallest compressor for diving

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Originally Posted by clownfishsydney View Post
A compressor that can only pump to 600 psi will only fill a tank to 600 psi, no matter how long you run it for. As 600 psi is essentially an empty scuba tank, it is of no use.
How is it of no use?

If you can dive 10 minutes, there is a lot you can do in this time compared to holding your breath which is only good for 2-3 minutes.

I could say that 3000psi is of no use because there are some carbon fiber tanks that can hold 9000psi of pure oxygen in a rebreather giving me a few days dive time. What a waste of time to have a compressor at all on board, since using this system you only need to fill once a year to do all your diving.

Consider your energy consumption requirements to achieve longer dive time. What I am getting at is to go from 0 to 600 psi requires little energy, higher pressures, specialized gasses etc requires exponentially higher energy to achieve a linear increase in dive time.

So, for efficiency reasons, when filling the tank off of solar power it makes sense to target lower pressures, and use a 3 stage piston compressor that runs slowly.

My idea is to have a very small quiet efficient compressor that only draws 2-3 amps at 12 volts and is cheap to build, and can fill a tank in 2-3 hours, but it is conceivable that someone else might prefer a compressor that uses 10 amps and achieves 1200psi after 2-3 hours if they have enough power for this.
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Old 30-03-2015, 07:42   #47
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Re: Smallest compressor for diving

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Originally Posted by boat_alexandra View Post
How is it of no use?

If you can dive 10 minutes, there is a lot you can do in this time compared to holding your breath which is only good for 2-3 minutes.

I could say that 3000psi is of no use because there are some carbon fiber tanks that can hold 9000psi of pure oxygen in a rebreather giving me a few days dive time. What a waste of time to have a compressor at all on board, since using this system you only need to fill once a year to do all your diving.

Consider your energy consumption requirements to achieve longer dive time. What I am getting at is to go from 0 to 600 psi requires little energy, higher pressures, specialized gasses etc requires exponentially higher energy to achieve a linear increase in dive time.

So, for efficiency reasons, when filling the tank off of solar power it makes sense to target lower pressures, and use a 3 stage piston compressor that runs slowly.

My idea is to have a very small quiet efficient compressor that only draws 2-3 amps at 12 volts and is cheap to build, and can fill a tank in 2-3 hours, but it is conceivable that someone else might prefer a compressor that uses 10 amps and achieves 1200psi after 2-3 hours if they have enough power for this.
Yeah, I'm gonna spend all that time gearing up so I can dive to only a few feet for a few minutes. 600psi is only 100 lbs above the amount you can safely dive to. There aren't any SCUBA cops out there giving tickets for folks who surface with less than 500psi in their tanks but there is a reason this is safe accepted practice. I'm sure you know why that is. So, feel free to take unnecessary risks every time you don your gear and do your safe entry just so you can make your point.

I ain't gonna spend all that time donning my gear so I can dive with only 100psi of useable air in my tanks. That's just plain silly.
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Old 30-03-2015, 08:04   #48
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Re: Smallest compressor for diving

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Originally Posted by boat_alexandra View Post
How is it of no use?

If you can dive 10 minutes, there is a lot you can do in this time compared to holding your breath which is only good for 2-3 minutes.
Why would you do this when you could run a 110-volt hookah that is less complex, less expensive, has a smaller footprint and much longer run time?
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Old 30-03-2015, 08:32   #49
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Re: Smallest compressor for diving

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Why would you do this when you could run a 110-volt hookah that is less complex, less expensive, has a smaller footprint and much longer run time?
maybe a hookah is a good option but you are connected to a tether and can't go very far from it. You wouldn't have the option of swiming on the surface for a longer distance then diving etc..

Quote:
Originally Posted by crazyoldboatguy View Post
600psi is only 100 lbs above the amount you can safely dive to.
Ambient pressure is 14.7 psi.

Starting at 600 psi leaves 585 psi above the amount where you no longer have air. This gives 10-12 minutes of dive time. There is no safety risk because you can surface when you run out of air.

Did you read in some book that you need 500 psi so you follow it without further thought?

People with this attitude lead predictable lives. Indeed living your life would be entirely pointless because you can already know everything that will happen to you just by reading about other people.
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Old 30-03-2015, 09:03   #50
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Re: Smallest compressor for diving

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Originally Posted by crazyoldboatguy View Post
Yeah, I'm gonna spend all that time gearing up so I can dive to only a few feet for a few minutes. 600psi is only 100 lbs above the amount you can safely dive to. There aren't any SCUBA cops out there giving tickets for folks who surface with less than 500psi in their tanks but there is a reason this is safe accepted practice. I'm sure you know why that is. So, feel free to take unnecessary risks every time you don your gear and do your safe entry just so you can make your point.

I ain't gonna spend all that time donning my gear so I can dive with only 100psi of useable air in my tanks. That's just plain silly.
In general I agree with your logic and also the fact that a hookah would be cheaper, easier, and work better.

However I don't really subscribe to the policy of it's not safe to be in the water with only 500 psi in your tanks. If you're diving shallow and have no deco obligation then why exit with 500 psi? If you are diving deeper and want/need to do a safety stop then why not use that 500 psi for the safety stop.

I know it's a good policy for newbies and inexperienced divers who are not good at monitoring their gas supply but there is no technical reason why one cannot use that last 500 psi.

When I was tech diving and regularly carried several tanks on a dive I would always breath my stage tanks all the way empty before switching to the next tank and never had a problem.
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Old 30-03-2015, 09:18   #51
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Re: Smallest compressor for diving

Alexandra,
Hard to tell if you're being serious or just pulling a good troll.

Assuming you're actually serious about actually going scuba diving with a total of 600 psi...there are a few very serious consequences and concerns:
1. Depth: Air compresses with depth...the deeper you go, the more compressed the air becomes, effectively reducing your usable PSI. At 6' you might actually get ~5-10 minutes from 600 PSI (exertion/breathing rate plays huge into real #'s) but at 30'? Half that. Any deeper? Forget it. Which brings up
2. Expanding gas embolism: Given your example, that you dive until your tank is empty and simply surface. Sure. Cleaning the hull at 6'...byt at any actual depth...you'd better be very well trained in boyles law or you'll blow out a lung or worse holding your breath to make the surface.

Please...if you haven't gotten real training in the physics of scuba do so before trying any of the scenarios you're discussing.


To the OP: Look at the Airetex 45. Cheap, compact, can opt for gas or electric and will fill an AL 80 (to +3000psi) in 20ish minutes.
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Old 30-03-2015, 09:29   #52
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Re: Smallest compressor for diving

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Originally Posted by boat_alexandra View Post
Did you read in some book that you need 500 psi so you follow it without further thought?
This is actually taught in many basic diving classes and is a policy on many dive boats. In some it is seriously enforced, as in banning you from further dives on the boat if you surface with less than 500 psi in your tank.

Since dive instructions in the US in some cases have been very lax, graduating divers that are not well prepared to go out into the real world a lot of operators have gotten strict on this rule to protect themselves from litigation.
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Old 30-03-2015, 09:53   #53
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Re: Smallest compressor for diving

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Originally Posted by boat_alexandra View Post
maybe a hookah is a good option but you are connected to a tether and can't go very far from it. You wouldn't have the option of swiming on the surface for a longer distance then diving etc..



Ambient pressure is 14.7 psi.

Starting at 600 psi leaves 585 psi above the amount where you no longer have air. This gives 10-12 minutes of dive time. There is no safety risk because you can surface when you run out of air.

Did you read in some book that you need 500 psi so you follow it without further thought?

People with this attitude lead predictable lives. Indeed living your life would be entirely pointless because you can already know everything that will happen to you just by reading about other people.

Clearly you are more about the argument than your are about reality.

YOU WIN!

Now, for the realists - most reef diving occurs within a few feet of the surface - even skin divers can see most of what is there. A hookah may tether you but - see above. Yes, your 600 psi compressor will get you that 100 psi (or go to zero if you wish, oh you rapscallion risk taker you!!!) which will give you 10 minutes of dive time. BFD - the hookah can last much longer without anywhere near the hassle.

Your turn.
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Old 30-03-2015, 11:32   #54
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Re: Smallest compressor for diving

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Originally Posted by boat_alexandra View Post
maybe a hookah is a good option but you are connected to a tether and can't go very far from it. You wouldn't have the option of swiming on the surface for a longer distance then diving etc.
Have only used a hookah a couple of times but did not find the tether restricting at all. This was a gas powered unit however so not limited by the cord.. We set it in the dinghy and towed it all over the dive site with no problem at all and it was a bit windy that day.
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Old 30-03-2015, 11:48   #55
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Re: Smallest compressor for diving

All the SCUBA organizations used to, and probably still do, teach that you always want to keep a couple of hundred PSI in the tank just to make sure you don't empty it. At which point moist outside air might get in through an open valve, etc. and start moisture problems in the tank. But even ignoring that, and rashly assuming you splurged on a dive computer since a plain old fashioned pressure gauge doesn't read "32" pounds or anything that low...


Why screw around with four hour long fills at all, to get 600# in a tank? Less time, money, weight, and space to just a couple of pony tanks, or a couple of 50 cf shorty tanks, and have a shop fill them to 3300+PSI, so you've got six or seven times as much air, sitting in the same space, with no compressor nonsense to deal with.


Or you could buy one of those nifty hand compressors they used to feed the hardhat divers, and have your buddy stand on deck cranking it. Go real low tech! Works when the battery's dead too.
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Old 30-03-2015, 12:43   #56
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Re: Smallest compressor for diving

The point of having your own compressor is not to need a dive shop.
The available small compressors don't have a small price. and still consume a lot of power.

Seems to me that for personal use, refill time is not a big issue.
If it takes 48 hours no problem, as long as it is quiet.
I have the same opinion about watermakers, where some crank up a loud high-rate one for a short time, but me thinks a quiet one can run for hours is no problem.

Slow is of no use to a dive shop, and they are the majority of the market/customers, or big boats that have engine rooms with generators and the power draw and noise is no problem.
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Old 31-03-2015, 04:40   #57
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Re: Smallest compressor for diving

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Originally Posted by boat_alexandra View Post
maybe a hookah is a good option but you are connected to a tether and can't go very far from it. You wouldn't have the option of swiming on the surface for a longer distance then diving etc..
yes you do

Power Dive units | Powerdive



and there are several other 12v floating hookahs on the market.
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Old 31-03-2015, 06:43   #58
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Re: Smallest compressor for diving

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Originally Posted by boat_alexandra View Post

Did you read in some book that you need 500 psi so you follow it without further thought?

People with this attitude lead predictable lives. Indeed living your life would be entirely pointless because you can already know everything that will happen to you just by reading about other people.
You need 250psi relative to ambient pressure for the scuba first stage to work. In the "old" days submersible pressure gauges were not that accurate so they wanted divers to never get close to 250psi. The "rule" still exists today. The reality is that 600psi starting fill in an aluminum 80 gives 9cfm of usable air after you take away the 250psi minimum for the reg. Most rec divers consume between 0.5 and 0.8 cfm at the surface. At 15 feet that would give between 6 and 9 minutes of air. Most safety conscious divers would like to be on that surface before that happens just in case something goes wrong.

Hookah units don't use the same first stage as scuba.
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Old 31-03-2015, 07:20   #59
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Re: Smallest compressor for diving

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Hookah units don't use the same first stage as scuba.
Hooka units I assume don't have a first stage? I plan on putting one together myself for bottom cleaning and I plan on just using a 50' LP SCUBA hose and one of my old second stages I have laying around. Intermediate pressure I think is usually around 150 PSI, and most little compressors are working real hard to achieve that.
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Old 31-03-2015, 07:56   #60
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Re: Smallest compressor for diving

Some compressors can be belted of the main engine, could be a slick set-up, charge the batteries and fill the tanks at the same time.
Planned to do just that on my CSY 33, but not enough room as the engine box was part of the galley. Too much work and too little room to extend it.
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