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Author: Subject: Gases and pressure and technical stuff
Dooey99

posted on 23/1/14 at 07:00 PM Reply With Quote
Gases and pressure and technical stuff

Can someone who is an expert in this field help me out.
I am putting plans together to make a mini sub (I know crazy and ill provably never do it) I'm thinking about my ballast tanks, I can get co2 fire extinguishers for nothing so probably use them to fill my ballast tanks with gas once I'm submerged and want to surface, bare with me, if I had an extinguisher with say 2 bar of co2 in how much would the co2 its self weigh at that pressure and what volume of gas would it be at normal atmospheric pressure...





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mookaloid

posted on 23/1/14 at 07:28 PM Reply With Quote
I think if we do have any submarine experts on here they would say that you are certifiably barking mad......

Are you thinking of going in this your self?

Usually I understand that subs use compressors to compress air which is then used for breathing as well as for filling the ballast tanks. How you know if you have got enough air to refloat the sub is pretty critical I would imagine and leaving such a critical thing to some knock off fire extinguishers (so to speak) sounds pretty scary.

on the other hand if you are not going in it then what the hell, give it a go





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SteveWalker

posted on 23/1/14 at 07:42 PM Reply With Quote
Gas at 2 bar (I assume that is barg - ie: bar gauge which reads 0 at atmospheric pressure) is at 3 bara (bar absolute which reads 0 at a full vacuum) and when released to atmospheric pressure (0 barg or 1 bara) will take up three times the volume.

eg:

P1xV1/T1=P2xV2/T2

where:

P1 = 3 bara
V1 = 20 litres

P2 = 1 bara

Ignoring T1 and T2 as we can assume that they are almost identical (otherwise they are the absolute temperatures in Kelvin).

gives:

V2 = 60 litres

However, if you are using it in a sub-you will not be releasing it to atmospheric pressure, but to atmospheric pressure, plus water pressure (approximately 0.1 bar per metre depth) and it's volume will be proportionately less.

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Dooey99

posted on 23/1/14 at 08:50 PM Reply With Quote
Mookaloid

my theory is if it floats when the tanks are full of gas and sinks when they are full of water it will then surface again when i re fill them with gas, i have no idea. i would be piloting her from the inside.

i agree they use compressor to fill there tanks but i just thought extinguishers were easier but i suppose i could just buy an old like 25L air compressor and some how make it run off the batteries i will use to power it, i dont intend to go full bore with the project and give it a generator to charge the batteries and stuff just a couple of volt and amp gauges so i know when the batteries are running out.

Steve

thanks i shall try and get my head round that





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MakeEverything

posted on 23/1/14 at 10:06 PM Reply With Quote
So many variables to do a desktop exercise in a few minutes.

Your design depth will determine;

Ballast tank sizes, which will determine;

Volume of gas required to displace liquid.

Steve is right, in that your depth will have an effect on the volume of gas, and the rate of expansion once released into the ballast tanks. IIRC CO2 expands at an unpredictable rate when released from a CO2 cylinder, which is why submarines use compressed air.

There are a load of other issues such as controlling the gas, safety valves (and a 2nd set of emergency valves), pressure control, descent control, ascent control and all this without the challenges of normal control (Fwd, Back and rudder).





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NeilP

posted on 23/1/14 at 10:51 PM Reply With Quote
I really wouldn't use CO2 anywhere near a trapped bubble of air that you're occupying...

...only upside is that if it goes wrong you'd go out on a nice high!!!





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morcus

posted on 23/1/14 at 11:35 PM Reply With Quote
Theoretically I should have a sensible answer to this question (2 years studying naval architecture, a few friends who work with submarines and a few goes on submarines). I would agree with the above, you don't want to take co2 down with you, and without checking I've got a feeling it wouldn't really work to clear ballast tanks.

Your best bet is a snorkel system so your always drawing real air from the surface, this has many benefits, noteably that such a set up will allow the use of an internal combustion engine, obviously you need an exhaust snorkel as well.

Snorkels will of course limit how far you can dive but in a small sub alot of other factors will also limit you and I'd not want to go very far down in a sub that couldn't make it's own air.





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snapper

posted on 24/1/14 at 06:35 AM Reply With Quote
For every 10meters you decend the pressure increases by 1 Bar
No I have completely forgotten Boyels law and Dalton's laws so am struggling with displacement
But as I know air expands as it surfaces by watching bibles I expelled when Scuba diving I assume you would need air pressure above submerged pressure to displace ballast water, therefore at 50 meters you would need above 6 Bar and that would consume 6 times the stored volume of air
When scuba diving we ran 207 bar and 232 bar tanks of up to 15 litre volume there are tanks of 300 bar or more working pressure available.
All tanks need an internal visual inspection and pressure test and regular intervals.
You'll have to work out the amount of tanks needed to provide enough displacement for multiple dives and extra for emergency
The compressors to pump that pressure are beasts or you can get portable ones but the trade off is how long they take to fil





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coyoteboy

posted on 24/1/14 at 01:00 PM Reply With Quote
quote:

really wouldn't use CO2 anywhere near a trapped bubble of air that you're occupying...



This.

Plus the issue of icing on the valves.

Plus the issue of not understanding the loads on the pressure vessels.

All lead me to think you should stop now.

[Edited on 24/1/14 by coyoteboy]





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me!

posted on 24/1/14 at 02:33 PM Reply With Quote
quote:
Originally posted by coyoteboy
quote:

really wouldn't use CO2 anywhere near a trapped bubble of air that you're occupying...



This.

Plus the issue of icing on the valves.

Plus the issue of not understanding the loads on the pressure vessels.

All lead me to think you should stop now.

[Edited on 24/1/14 by coyoteboy]


+1!

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morcus

posted on 24/1/14 at 04:12 PM Reply With Quote
quote:
Originally posted by snapper
...I know air expands as it surfaces by watching bibles I expelled when Scuba diving...


I know thats meant to say bubbles but the very idea that you were dropping off bibles while scuba divings made me chuckle.





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luke2152

posted on 24/1/14 at 07:27 PM Reply With Quote
Archimedes principal is the one to look at here - if an object is immersed in water the upthrust received is equal to the weight of liquid displaced.

So a fire extinguisher of say 10L volume will give 10kg upthrust in water (less the actual weight of the extinguisher). The pressure in the cylinder is not important as long as it is enough to displace the water pressure.

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paulf

posted on 24/1/14 at 09:38 PM Reply With Quote
A C02 extinguisher contains C02 as a liquid at roughly 50 bar pressure depending on its surrounding temperature.The liquid gas expands to many times its liquid volume and therefore a single extinguisher would probably hold enough gas for a few resurfaces .
Having said that i would not want to be sealed in a vessel underwater with pressurised C02 pipework inside it .
Paul
quote]Originally posted by Dooey99
Can someone who is an expert in this field help me out.
I am putting plans together to make a mini sub (I know crazy and ill provably never do it) I'm thinking about my ballast tanks, I can get co2 fire extinguishers for nothing so probably use them to fill my ballast tanks with gas once I'm submerged and want to surface, bare with me, if I had an extinguisher with say 2 bar of co2 in how much would the co2 its self weigh at that pressure and what volume of gas would it be at normal atmospheric pressure...

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02GF74

posted on 30/1/14 at 07:41 PM Reply With Quote
quote:
Originally posted by luke2152
Archimedes principal is the one to look at here - if an object is immersed in water the upthrust received is equal to the weight of liquid displaced.

So a fire extinguisher of say 10L volume will give 10kg upthrust in water (less the actual weight of the extinguisher). The pressure in the cylinder is not important as long as it is enough to displace the water pressure.


I am not sure I would agree with all of that.

Sure you can displace the water but where will it go? The water will be pumped out of the submarine into the surrounding water - which will be under pressure, the deeper you go, the bigger the pressure, the harder it will be.

There is a reason why you can't buy 1 or 2 man submersibles in your local Halfrods, namely cause you neeed a fair bit of know how to avoid it being a metal coffin at the bottom of a lake, with added co2 to help suffocation.

by the way you confuse co2 and no2 = nitrous oxide or laughing gas.

TBH I would look for another project to occupy your time.





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luke2152

posted on 31/1/14 at 06:49 AM Reply With Quote
Yes i guess i misread the original post. I thought the Co2 cylinders were being used as the actual bouyancy vessel rather then the means of filling it. The point remains that the pressure in the buoyancy vessel need only be higher then the water pressure. Its the volume that's important. Filling to 200bar wont give extra bouyancy over say 5bar (unless w pressure is over 5)
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geoff shep

posted on 31/1/14 at 09:33 AM Reply With Quote
Filling it to 200 bar will give you 40 times as much gas to play with as 5 bar. So you can let a lot more out and it will displace a lot more water.






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luke2152

posted on 31/1/14 at 10:12 AM Reply With Quote
quote:
Originally posted by geoff shep
Filling it to 200 bar will give you 40 times as much gas to play with as 5 bar. So you can let a lot more out and it will displace a lot more water.


Yes that is true. Either way you wont catch me going down in a home made submarine either

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geoff shep

posted on 31/1/14 at 10:14 AM Reply With Quote
It would be interesting to try it - but only in a open-topped sub and with a scuba gear on. And preferably in the caribbean!






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02GF74

posted on 1/2/14 at 11:44 AM Reply With Quote
quote:
Originally posted by geoff shep
It would be interesting to try it - but only in a open-topped sub .



also known as a boat.





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Angel Acevedo

posted on 1/2/14 at 03:11 PM Reply With Quote
You can´t fill a CO2 Cylinder to 200 Bar, at least at room temperature,,





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Simon

posted on 1/2/14 at 04:01 PM Reply With Quote
quote:
Originally posted by coyoteboy

All lead me to think you should stop now.

[Edited on 24/1/14 by coyoteboy]


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