What to be used to keep a valve closed while power is on.

[codyd]

I am making a robot that goes underwater and I don't want to lose it at the bottom. I had the idea of buying a small air cylinder and filling it up to 100 psi so it is heavier than water and i thought i could use a valve with springs on it to make the air flow if the battery dies but I need something that will hold it closed when there is power. I am just looking for what other people think before I do it. I was thinking a servo or solinoid but i don't know if it would overheat if it's underwater for hours at a time.

[SmAsH]

Would a sprinkler valve work?
As for the electronics side of things, you could use a normally open relay so when power is lost it closes a circuit...
A servo would suffice for this, but it requires a microcontroller to operate, if you already have one then great, but if not maybe look at solenoids.

[madchimp]

Several people have posted diy mini sub projects on the net and most of those use drop ballast for emergency surfacing. Basically it's just extra weight on the bottom that if dropped the sub floats to the surface but that only works if the vehicle has positive buoyancy without the weight. But chances are if you are building something that will have an area that is water tight for electronics and such you will probably need ballast anyway.

[chuckels]

check the web for NO solenoids(normally open) clipper, mac, skinner,and other manufacturers have them in many sizes and configurations.

[Artificer]

Get a 3/2 valve. You can use it as a normally open or normally closed.

[antom]

Clippard makes the air reservoir like you described which is a cylinder body without a piston & rod. They also make some small 12vdc air valves that draw minimum current. You would use a normally open that would be held closed when power applied. You can just thread the valve into the cyl. Clippard also makes some small toggle switch valves that could be used to isolate the tank.
My company is a distributor for Clippard. We did carry Humphrey which makes a lot of the same things. Both are high quality but inexpensive.
Try Clippard.com . You can even get prices online.