SCUBA Tank as input for Shoebox - Theory Busted...

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ChuckInOhio

ChuckInOhio

Member
Apr 16, 2017
104
6
Ohio, United States
Thanks to the members of the forum who have, in a round about way, reminded me that when I’m diving I don’t have a cylinder with 80cf of volume (at ambient pressure of course) strapped to my back, therefore Boyle’s Law will not grant me the mythical amount of air required to run the Shoebox for 100+ hours. Therefore I call this theory of mine effectively busted...

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I’ve looked at some replies where folks have asked this question before and the guidance seems to be either the tank won’t adequately supply the Shoebox or why spend the money on a regulator to keep the pressure at 120 PSI. I’m a diver and have a couple tanks around and have used them in the past to fill my S510’s but they obviously won’t fill my Wildcat adequately. I’ve been looking at some options to replace my FX hand pump, including leasing an N2 bottle and buying the reg, getting one of the larger HP tanks and running it to the dive shop once a month/as needed, or getting a Shoebox and buying the HP desiccant filter for 3-400 bucks.

So this popped into my head the other day; what if took an old SCUBA first stage, dialed the low/intermediate pressure to 125 PSI, and hooked that up to feed the Shoebox, negating the need for the filter? Yes I’d have to still get the tanks filled to source the Shoebox, but I was running some numbers and unless I’m doing my math wrong an 80 at 3000 PSI should supply the Shoebox with dry, filtered air for 100 hours. Here are my numbers:

80 cf tank @ 3,000 PSI = 240,000 cf @ 1 PSI (corrected earlier fat finger to 240,000)

240,000 cf / 125 PSI = 1,920 cf

1,920 cf/.3 cfm = 6,400 minutes (this represents a .3 cfm consumption using the Shoebox FAQ against the equivalent volume of the tank at 125 PSI)

6,400/60 = 107 hours

Now if those figures are correct, I should be able to generate enough dry, filtered air with one 80 to top off a 97 cf 4,500 PSI tank from 2,200 PSI around 22 times. 

So at this point I respectfully ask the forum members to review this and let me know where I’ve got bad info or calculations in this little theory of mine. Otherwise it would seem this would be a very effective setup using a lot of what I already own.
 
Chuck I can't say for sure and I cant prove your math wrong or right but I just cant see it happening like that (22 fills). I could envision a few fills maybe 4-5 but that is also just a guess. Somehow I'm thinking more air is going to have to be applied/replentished. With your math and thinking your should then be able to use one of those tanks now filled to 4500# to make even more 4500# fills. Hopefully someone smarter than me can check your math and give you a thumbs up or down. Very interesting non the less though.

Jking
 
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I'm sorry if I'm missing something here. So you want to use a doner scuba tank with 3000 psi, to supply the shoebox and you think that the air will multiply its volume by regulating it down to 125 psi and recompressing it back to 4500 psi with the shoebox and you expect that to top off a larger half full tank, 22 times. I think that you should go back to the old drawing board. I would think that you would be lucky to get more than 1 or 2 top ups at the most unless you are feeding the shoebox with a compressor as well.
 
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I'm sorry if I'm missing something here. So you want to use a doner scuba tank with 3000 psi, to supply the shoebox and you think that the air will multiply its volume by regulating it down to 125 psi...
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Yes, and it’s proven via Boyle’s law; the pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature. The long/short is if you increase pressure, volume is reduced, and if you increase volume pressure is reduced, so 80 cf at 3,000 != to 80 cf at 125.


 
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I’ve looked at some replies where folks have asked this question before and the guidance seems to be either the tank won’t adequately supply the Shoebox or why spend the money on a regulator to keep the pressure at 120 PSI. I’m a diver and have a couple tanks around and have used them in the past to fill my S510’s but they obviously won’t fill my Wildcat adequately. I’ve been looking at some options to replace my FX hand pump, including leasing an N2 bottle and buying the reg, getting one of the larger HP tanks and running it to the dive shop once a month/as needed, or getting a Shoebox and buying the HP desiccant filter for 3-400 bucks.

So this popped into my head the other day; what if took an old SCUBA first stage, dialed the low/intermediate pressure to 125 PSI, and hooked that up to feed the Shoebox, negating the need for the filter? Yes I’d have to still get the tanks filled to source the Shoebox, but I was running some numbers and unless I’m doing my math wrong an 80 at 3000 PSI should supply the Shoebox with dry, filtered air for 100 hours. Here are my numbers:

80 cf tank @ 3,000 PSI = 230,000 cf @ 1 PSI

230,000 cf / 125 PSI = 1,920 cf

1,920 cf/.3 cfm = 6,400 minutes (this represents a .3 cfm consumption using the Shoebox FAQ against the equivalent volume of the tank at 125 PSI)

6,400/60 = 107 hours

Now if those figures are correct, I should be able to generate enough dry, filtered air with one 80 to top off a 97 cf 4,500 PSI tank from 2,200 PSI around 22 times. 

So at this point I respectfully ask the forum members to review this and let me know where I’ve got bad info or calculations in this little theory of mine. Otherwise it would seem this would be a very effective setup using a lot of what I already own.
Click to expand...

Not sure this is correct
 
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Why not just get a compressor? The shoebox was a good booster back in the days but now compressor are getting affordable, shoebox isn't worth the time and investment anymore. You mentioned dry air. Sure your suba tank will supply the booster dry air, but you will always need a filter cause heat in the piston will create moisture. Always need a output moisture filter to keep your tanks dry.
 
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Why not just get a compressor? The shoebox was a good booster back in the days but now compressor are getting affordable, shoebox isn't worth the time and investment anymore. You mentioned dry air. Sure your suba tank will supply the booster dry air, but you will always need a filter cause heat in the piston will create moisture. Always need a output moisture filter to keep your tanks dry.
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I'd bet a Dollar to a donut that you don't own a Shoebox, and never have.

Go thru ALL of the forums and even google and come back and tell us how many Shoebox compressors you can find that have blown up. (I know of one from when I was back on another forum. Lacked maintenance and had thousands of hrs) Then search all the same places for a chinese compressor that HASN'T had one blow up.

Personally I think I'll keep my Shoebox
 
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The Shoebox is a no frills low maintenance HPA mechanism that works time and time again, I would and will buy another but so far mine has lasted 4 years with regular use and only very minor upkeep was needed to maintain this little machine. Big shout out to Tom Kaye the guy who invented this thing, if know one had said it in awhile I will........ Thank you for your contribution to our sport we really appreciate it. Rob maybe someday these guys will see the light but until then we can be entertained by the new and ever growing problems associated with those cheap Chinese missiles.
 
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I spent a bunch of time on a few different forums last fall, researching which compressor to buy. I settled on the Shoebox and have been nothing but pleased with it. The Shoebox has kept my four PCPs quite happy. My only complaint is that the convenient access to trouble-free air makes me wish I had more time to shoot. 

As for the original question about using an scba tank as the first stage, seems like it would work to me. 

I emptied my 19cf scuba tank a few months ago so I could remove the breather tube...... It took a LONG time to get it down to zero psi. Id think that 80cf at 3000psi, regulated to 125psi would feed that shoebox a long time. Id keep the hose from the scuba to the shoebox as short as possible so you don't lose as much air in the hoses each time. 
 
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Why not just get a compressor? The shoebox was a good booster back in the days but now compressor are getting affordable, shoebox isn't worth the time and investment anymore. You mentioned dry air. Sure your suba tank will supply the booster dry air, but you will always need a filter cause heat in the piston will create moisture. Always need a output moisture filter to keep your tanks dry.
Click to expand...

I'd bet a Dollar to a donut that you don't own a Shoebox, and never have.

Go thru ALL of the forums and even google and come back and tell us how many Shoebox compressors you can find that have blown up. (I know of one from when I was back on another forum. Lacked maintenance and had thousands of hrs) Then search all the same places for a chinese compressor that HASN'T had one blow up.

Personally I think I'll keep my Shoebox
Click to expand...

I don't own a shoebox cause I own the hatsan lighting. So I don't see a point of owning a shoebox when my hatsan can top off my tank in under 15 mins and fill a 74cf tank from 0-4500psi in 50 mins. And not wait a whole day or hours just to top off. It funny how people say cheap chinese compressor blows up. Compressor do not blow up. Folks with the yong Heng compressor have burst disk blow or damage piston, or leaking air. That's all. Compressor don't blow up. Lol. I owned a yong Heng for a year and it paid for itself. Switch over to the hatsan only cause my yong Heng won't fill my tank pass 4000psi.
 
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Shinyknight, If you take a look at your accuracy points they are a little on the wrong side of zero. It's because you keep popping off about stuff you know nothing about and usually in a very derogatory manner. You may not give a rip about your accuracy points, some don't but it is a direct reflection of what people think about you and your comments. You can get a lot out of this forum and contribute to it in a positive way if you just give it a chance. Look and listen to what others have to say and try to keep your negative comments to yourself or if you do want to express your opinions just think about them before you reply.

Jking
 
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I don't own a shoebox cause I own the hatsan lighting. So I don't see a point of owning a shoebox when my hatsan can top off my tank in under 15 mins and fill a 74cf tank from 0-4500psi in 50 mins. And not wait a whole day or hours just to top off. It funny how people say cheap chinese compressor blows up. Compressor do not blow up. Folks with the yong Heng compressor have burst disk blow or damage piston, or leaking air. That's all. Compressor don't blow up. Lol. I owned a yong Heng for a year and it paid for itself. Switch over to the hatsan only cause my yong Heng won't fill my tank pass 4000psi.
Click to expand...

Sorry, you're right......I guess this is Blowing "OUT" not "UP"

1556921189_13391250655cccbb652feec5.53665404_pump2_zps517iw9ll.jpg
1556921189_17372864235cccbb654834e9.40988978_pump1_zpsbh2eb4fz.jpg

 
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Why not just get a compressor? The shoebox was a good booster back in the days but now compressor are getting affordable, shoebox isn't worth the time and investment anymore. You mentioned dry air. Sure your suba tank will supply the booster dry air, but you will always need a filter cause heat in the piston will create moisture. Always need a output moisture filter to keep your tanks dry.
Click to expand...


A Shoebox runs around $600 delivered when all is said and done, and after thinking about it I doubt there would be a problem with the Shoebox getting air in the pressure range of the typical first stage setup, so I may be able to just pop my diving rig on the bottle and plug my BCD hose into the Shoebox. Simple and plug and play with what I have today, and I save cash I could put somewhere else.

The air from a quality fill station is pretty dry, and I literally trust my dive shop with my life; diving cold quarries in Ohio can freeze your first stage up if there's moisture in your tank, which leads to not breathing at all. I don't see running the air through a second compression as adding moisture that wasn't there in the first place. It's a closed system after all.
 
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I have been using a scuba tank with a regulator feeding a shoebox for a couple of year now with no problems. With an 80 cf bottle it is almost 100 psi for 100 psi on 60 min scba. There are no moisture issues with the breathing air or you can use nitrogen. They are better setups out there but if you have a shoebox it not a bad way to go. The biggest downside for me is the maintenance on the shoebox. Just a hassle not much money.
 
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I'm sorry if I'm missing something here. So you want to use a doner scuba tank with 3000 psi, to supply the shoebox and you think that the air will multiply its volume by regulating it down to 125 psi...
Click to expand...

Yes, and it’s proven via Boyle’s law; the pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature. The long/short is if you increase pressure, volume is reduced, and if you increase volume pressure is reduced, so 80 cf at 3,000 != to 80 cf at 125.

Click to expand...

This thread has gotten a little off track. Back to your original enquiry. You have misunderstood the point that I was trying to make. The input air for your shoebox is going to be supplied from a doner scuba bottle and nothing else, right?. If that's the case, the same amount of air is going to come out of the output side. Forget about your flawed equation. The claimed CF capacity of a bottle is calculated by the volume, usually in litres times the amount of pressure in psi. divided by atmospheric pressure. The mixture of imperial and metrics confuse things as well. so your 80 CF scuba tank, probably about 11 or 12 litres, is not 80 CF of 3000 psi air. It's 11or12 litres of 3000 psi / 310 bar. 80 CF is the space that that air would displace, uncompressed. I hope that I have been able to explain this as simple as possible, I have a mild brain injury that makes it difficult to put things into words that some people understand. My point is that you can't get something from nothing. All the shoebox does is squash input gas, it doesn't make more.
 
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Chuck,

It's good to see that you are looking for ways to maximize the use of what you already have. Nothing wrong with that! However, you did get your math wrong. The reason it's called an 80 cubic foot tank is because it holds, at 3,000 psi, an amount of air that would take up 80 cubic feet if it was at atmospheric pressure. The tank pressure simply reduces the volume that amount of air occupies. So, for the purposes of your calculations, a cubic foot of air always means a cubic foot of air at atmospheric pressure. Now, your 97 cubic foot tank, when it is at 2,200 psi, is only holding about half of its capacity, around 47 cubic feet or so. I'm not going to be exact here, as the decimal points just confuse the issue. That means we need another 48 cubic feet or so to finish filling the 97 cubic foot tank to capacity. Subtract that 48 cf from your 80 cf tank, and you're left with about 38 cf. Which won't quite be enough for your next top off. You'll be getting less than two top-offs from each 80 cf tank, not 22. How much trouble and cost is it to fill your 80 cf tanks? That will decide the relative value to you. On the plus side, your air will already be dry. The heat of compression does not "create" moisture. If the input air is already dry, the output air will be dry. I hope this helps, and I think it's really cool you're thinking outside the box.

James
 
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OK, time for some high-school physics ......

Boyle's Law says that pressure (P) x volume (V) = constant at a fixed temperature

If you mix two volume of gas, one at P1, V1 and the other at P2, V2 and put it into a container of volume V3 and get a pressure of P3, then 

P1 x V1 + P2 x V2 = P3 x V3

In words, it means if there is no loss like leakage in the process, the amount of "PV" extracted from the donor tank = that injected into the receiving tank. In even simpler words, the number of air molecules taken out of the donor tank is equal to that pushed into the receiving tank. 

Now let's do some math : 

volume of the space inside the 3000 psi 80 cu ft tank = 80 x 14.7 / 3000 = 0.39 cu ft

volume of the space inside the 4500 psi 97 cu ft tank = 97 x 14.7 / 4500 = 0.32 cu ft

where 14.7 psi = pressure of one atmosphere

"PV" extracted from the donor tank = pressure loss x volume = ( 3000 - 125 ) x 0.39 = 1121 psi cu ft

"PV" injected into the receiving tank = pressure gain x volume =( 4500 - 2200 ) x 0.32 = 736 psi cu ft

so theoretically you can do 1121 / 736 = 1.52 fill with your set up but in practice there are bound to be some leakage so I believe you can do slightly more than one fill. 

I don't want to go into any debate over the choice of compressors but I do think established Chinese brands like Yong Heng are good value for money. The scary "blow out" picture posted above must be due to some very abnormal circumstances because pressure simply cannot be built up in the crank case. The picture is seemingly taken from this thread : https://www.gatewaytoairguns.org/GTA/index.php?topic=132637.0 . The original poster of the thread is not the person who experienced the incident so who know what has been done to the compressor by the owner ? The OP concluded the thread by saying : " the breather was plugged or improperly installed.... Blowby pressurized the crankcase, which caused it to fail.... "
 
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