DOES COMPRESSED AIR FORCE WATER OUT EASIER AT DEPTH?

[subdog]

Sorry for so many questions...but again need some help.

This question deals with a sub that pumps water into a sealed ballast tank..then compresses the air.....

Which water level around the hull is easier for the water to be forced out of using the compressed air in the ballast tank (I have a tire pressure type valve that is opened via a servo....and the pump only pumps water one way into the ballast tank):

Should the water come out of the compressed air tank easier...

a. Near the surface;
b. At depth OR;
c. It doesn't matter surface and depth should be the same.

When I am near the surface, it SEEMS that the water leaves the ballast tank VERY, VERY slow when I open the valve to force the water out via the compressed air in the tank. There is no pump in this system to expel the air..the water simply comes out from the compressed air being released.

[bradv]

Compressed air will force out

Compressed air will force out the water more slowly at depth.

The reason for this is that the pressure differential between that of the compressed air in your ballast tank and the external water will decrease the deeper you go. For every 10m down (~33' for the non-metric enlightened) you go, the water pressure increases by 1 atmosphere. So, if you had compressed the air in your ballast tank to (say) just 1 atmosphere, as your sub approached 10m down (a looong way for one of our subs!), the external water pressure would approach this same pressure. This would then only allow a slow trickle of water out of your ballast tank when you released your valve. Go beyond 10m, and the external water pressure would be greater, so opening your valve would actually let water back into your tank instead of expelling it! Back near the surface, your ballast water should squirt out much more quickly.

One of the chaps over here in Oz has seen this in a diving pool, where his sub took some time to come back up when down quite deep. I'm unsure why this may seem otherwise for you - maybe there's a tiny leak in your tank allowing the air pressure to dissipate a little at depth, which may not be apparent at the lesser pressure near the surface but would then reduce your blow rate if you had planed back up?

[subdog]

Thank you..that sure helps.

I

Thank you..that sure helps.

I think I have a leak in the water tight hatch that seals that ballast tank then. I can pump water in extremely well with the pump into the ballast tank at depth, but I can't get the water to pump out except when its in drydock. Sitting there, I cansee it dripping out the bottom quite slow.

[Wheelerdealer]

If the water you take

If the water you take aboard the ballast tank compresses the air above it in a sealed ballast system then what you experience will be correct. As the sub nears the surface, the air is being decompressed by the water leaving the ballast tank, so the water will vaccate the tank more slowly as the sub approches the surface and pressure in the ballast tank decreases. When the ballast tank is full and the sub submerged the air is stored under much greater pressure so you can empty the tank of water at a faster rate.

[subdog]

Yes...its just trickling out on

Yes...its just trickling out on the surface from the sealed ballast tank. The tank is also now very well sealed, so they are no leaks.

You must be very right, so I guess I need to test this system at greater depth.

So far, I was just testing on the surface or in dry dock and I began to wonder why the water was coming out so slowly from the bottom.

What you say makes sense now. Thank you.

[Wheelerdealer]

Try this: fill the ballast

Try this: fill the ballast tank fully and let the the sub settle on the bottom of you test tank. Lift it out and left all the excess water drip away. now command the ballast tank to empty, you'll see the water at first rushes out and then slows to a trickle when the tank is nearly empty (when the sub would be on the surface if you were operating it for real).

[subdog]

If the water you take

If the water you take aboard the ballast tank compresses the air above it in a sealed ballast system then what you experience will be correct. As the sub nears the surface, the air is being decompressed by the water leaving the ballast tank, so the water will vaccate the tank more slowly as the sub approches the surface and pressure in the ballast tank decreases. When the ballast tank is full and the sub submerged the air is stored under much greater pressure so you can empty the tank of water at a faster rate.

A few short questions if you please sir..

I have a U-Boat with a reverse pump, solenoid and water ballast bag in a WTC.

During testing on its sub stand, with its water intake hose in a glass of water, I can hear the pump running both ways as I switch the stick on the transmitter left and right...BUT...it will only suck water from the glass INTO the bag...it will NOT take it back out. The pump is working fine and it pumps the water into the bag like a banshee...when I reverse the pump with the transmitter, I hear the pump reverse and run very well, but the water will NOT leave the bag.

Do I need to have this thing submerged in order for it to be able to pump the water OUT of the bag? Does the principal that you mentioned above apply here for a water ballast system?

[subshop]

ballast pump

what kind of pump are you using? are you sure that the polarity reverses when you move the stick in the other direction? have you tried using the pump by connecting it to you battery's terminals then reversing the connections to see if it in fact reverses? some type of pumps will only pump in one direction no matter which way the polarity is ! neil