hello there
I'm trying to build RC submarine but I have a problem, the air tanks supposed to have two hoses one pointed to the water and the other one tied to a small air filled ball to keep it pointed up, while the tanks have water level sensor to sense weather the tanks are full or not.
my problem is I can fill the tanks using water pump but how to reverse the process and make it empty the water and fill it with air
thanks in advance
Same way they used to in the big subs - vent to let water in and compressed air to blow the water back out again. You have another potential issue though - you say "RC submarine" - unless it has a trailing antenna on the surface, normal radio control signals (it is frequency dependent) will only penetrate the water a fraction of an inch or so.
Suck the air out of the tank into a holding tank then pump it back when required.
Weedpharma
Release air from a compressed air tank.
mohd9011:
hello thereI'm trying to build RC submarine but I have a problem, the air tanks supposed to have two hoses one pointed to the water and the other one tied to a small air filled ball to keep it pointed up, while the tanks have water level sensor to sense weather the tanks are full or not.
my problem is I can fill the tanks using water pump but how to reverse the process and make it empty the water and fill it with air
thanks in advance
so, you have two tanks, two hoses in each. one falls to the bottom of the tank, the other floats to the top.
if you jus put on a fitting on the top of the tank, you do not need the float. ditto connectiong to a hole on the bottom.
as for air, you must have compressed air. at 33 feed deep, sea level, at what is called 1 atmospher or +1 atmosphere, the volume of air will be half that of air on the surface.
in order to expel the water from the tank, the pressure has to be over the external water pressure.
there is no reason you cannot compress the air you have in the tank, you are not required to release it back into the wild.
how deep do you expect to dive ?
mohd9011:
my problem is I can fill the tanks using water pump but how to reverse the process and make it empty the water and fill it with air
Lot's of people have made working model submarines. I have little doubt that Google knows the answer. This is not an Arduino problem.
...R
This is not an Arduino problem.
How to reverse the water pump with arduino ? 
Domino60:
How to reverse the water pump with arduino ?
I don't believe that he worked on is needed on a submarine. to look this ship they only need to be there, to raise the ship you need to introduce compressed air.
but I do agree that remote interface turning the relays on and off to do whatever one wants to do those belong in the realm of the Arduino
A RC submarine is not acting like the real one, take a 2 lit. bottle, only air inside is floating right? put some water inside more than half is going down.
My point is if you use a water pump to pump water inside the tanks and there gonna be as well the air which going to compress the sub will go down after reversing the pump and getting the water out the compressed air remained inside the tank will take all the tank again and the submarine will go up. What I mean is a toy it doesn't need a lot of air or a lot of water to dive and get up.
Domino60:
How to reverse the water pump with arduino ?
Even that is more of a mechanical question, since not all pumps are electrically reversible.
Domino60:
How to reverse the water pump with arduino ?
No doubt that will be relevant. But the OP's question is to do with mechanics " how to reverse the process and make it empty the water and fill it with air"
Personally I would try moving a piston back and forth. Imagine a large diameter syringe with a hose where the needle would go. When the piston is withdrawn the syringe will fill with water. When the piston is pushed in the water will be pushed out.
...R
Here is my idea:
My basic idea is to use the water pump only to pump the water outside, the water tanks will be filled
naturally like a bottle under the water, the air gonna get outside and the water gonna come inside,
after we finish our work/tasks, we use the water pump to get the water outside and the air tank to fill the water tanks with air again.
As you understand there gonna be needed few servos to open the tank 1 and 2 valves and another servo to open the compressed air valve.
at this point the submarine will go down and up 
ps Do not be scared of the paint work
EDIT:
If the OP is a advantage user in any kind of project I suggest him to use a DC Motor and create a
air compressor to compress the air from the Tank 1 and 2 in to the compressed air tank so like that
the air will be never lost.
D.60
Your sub could be slightly positively bouyant, and your diving controlled by thrust vectoring, and/or hydroplanes and forward motion.
That way, assuming you don't jam it hard into the ooze or under a rock, if all else fails, it comes up.
You can even add a fail safe lIke Piccard and Walsh had - iron ballast, held in place by an electromagnet.
(or even something simple like the British used on early limpet mines as a safety timer - aniseed balls that dissolved slowly) .
Handling air is power-hungry and bulky.
since we cannot alter the laws of physics, I would offer a few caviots.
first, no mattter what you do, as you increase depth, your vessel will shrink. on submarines, they would tie a string from one side of the hull to the other and make it taught. then as the depth increased, the slack would make the string bow not sure if they would make it touch the floor or not.
what that means is that the density of the vessel would increase and boyancy would naturally decrease.
I like Robin2's idea of syringe. you alter the physical size of the vessel.
I also like AWOL's idea of a fail safe. an electromagnet that would drop a load when the battery ran out and alter the buoyancy of the vessel. as for pumping, I believe a peristaltic is more efficient in this application than a centrifugal.
IIRC someone who had a model submarine at a show said that they had a ping-pong ball (or equivalent) on a long string. if the submarine lost control it would be released and float to the surface to mark where the sub was.
...R
first, no mattter what you do, as you increase depth, your vessel will shrink
Which is why most of Piccard and Walsh's was filled with petrol. (don't try this at home)
Or the baking soda trick. Add water, and carbon dioxide is produced.
first, no mattter what you do, as you increase depth, your vessel will shrink
I don't think he will try it under 100 meters, at least 5 meters the vessel will not shrink don't be so dramatic
If you build the body out of 3D Printed Hard / flexible plastic with a triangular structure inside there is
no possible way the submarine to shrink even under 50 meters. A real diver with normal diving suit
he can dive no more than 100 meters after 100 meters he needs the fully metal suit because after 100
meters the pressure increase so much that a diver with a normal suit can't stay there.
A water proof factory watch will crack after 100 meters under water.
My point is that OP is building a RC Submarine he is not building a scientific submarine that can dive
under 1km, for a RC submarine 5 meters is way enough and there is no such pressure to shrink it.
D.60
Domino60:
I don't think he will try it under 100 meters, at least 5 meters the vessel will not shrink don't be so dramaticIf you build the body out of 3D Printed Hard / flexible plastic with a triangular structure inside there is
no possible way the submarine to shrink even under 50 meters. A real diver with normal diving suit
he can dive no more than 100 meters after 100 meters he needs the fully metal suit because after 100
meters the pressure increase so much that a diver with a normal suit can't stay there.A water proof factory watch will crack after 100 meters under water.
My point is that OP is building a RC Submarine he is not building a scientific submarine that can dive
under 1km, for a RC submarine 5 meters is way enough and there is no such pressure to shrink it.D.60
unless he is using a metal hull or plastic pipe or something that has a very strong structure, it will shink especially at first 2-3 meters. the pressrue will double at 10 meters.
what is neutral boyant at the surface will be negative a 1 meter.
the human body is mostly water so the water is not the problem, it is the air inside. a wet suit will compress to 1/4 or 1/8 of it's size by the time the diver gets to 10 meters.
if you have a neutrally buoyant device at 1 meter, it will go negative at 2 and unless you have a way to bring it back up, it will sink. it was was noted getting to the depths of the Mariana Trench, they designed the vessel not have any air inside except the human space.
the point is that some form of rescue device would be worth looking into.
Domino60:
A real diver with normal diving suit
he can dive no more than 100 meters after 100 meters he needs the fully metal suit because after 100
meters the pressure increase so much that a diver with a normal suit can't stay there.
D.60
You're not a diver, are you, D.60?