heat and pressure.

[racedude1984]

I had a thought yesterday. I just happen to think about something odd ball but I wonder if it can also apply in the scuba world. In the automotive world people run nitrous which comes in a tank. People put heat pads on them to build up more pressure.

When your getting your tank filled I see they put them in cold water to lower pressure. HERES MY THOUGHT: If a person is diving deep and starts using are up at a faster rate. Couldn't someone put a heat pad on there tank to bring the pressure up some. I'm not talking putting alot of heat but enough to bring the pressure up. Is there something I'm missing that makes my thought a complete miss?

 

[fppf]

Yes, there not heating the tanks to raise pressure, there trying to prevent it from dropping when its in use. Nitrous is actually a liquid when its highly compressed. When you start venting the tank it causes the liquid to boil and greatly cool. The cooling will lower the vapor pressure of the liquid and you can't get as much gas out as fast. This is also why nitrous tanks are mounted at an angle. The angle provides more surface area for the liquid to evaporate.

A scuba cylinder with air is still in its gas form. There would be no reason to heat the gas.

 

[Davidstealey3]

did not know that about nitrous tanks. i watched some pinks episodes and watched them put a torch to the tank, but i thought they were trying to raise the pressure. guess you learn something new everyday

 

[fppf]

Yeah, heating a nitrous tank WILL raise the pressure, don't get me wrong. But as soon as some gas or liquid is pulled from the tank it will cool.

Heating a cylinder with a torch is like having a death wish. Hot spots can damage the material and cause a failure of the system.

The other reason for a cylinder heater with Nitrous is because the temperature pressure curves of the liquid are very steep. You do get a large pressure change for a small change in temperature. The heater keeps the pressure at a predictable point. This reduces the need for jet adjustments given different seasons as well.

 

[racedude1984]

but heat doesn't change the pressure of a scuba tank?

 

[fppf]

No, heat will increase the pressure in a scuba cylinder as well. But because its a gas in the scuba cylinder there is no phase change. You wont get the benefit like you do a nitrous cylinder.

The heater on a nitrous cylinder is there to keep the liquid at its boiling point, and also to try and keep the pressure output stable as your drawing a huge load on the cylinder. Same goes for propane cylinders, when you use your grill it gets cold, you can see how full it is in the summer by the dew line. If you try to run a house on the same size cylinder it will cool to the point you wont get any propane out of it. That's why home propane tanks are so large, and they lay them on there sides.

Even if you heated a scuba cylinder to 120 degrees F your only going to be able to get another few cubic feet of gas out of the cylinder. The equivalent to maybe 5-10 minutes on the surface, only 1-2 minutes at 100 feet. But in order to heat a cylinder and keep it that warm your going to need to bring a car battery with you and a high power heater.

Best way to get more gas is to get a bigger cylinder.

 

[captain]

but heat doesn't change the pressure of a scuba tank?

Yes it does but not enough to matter very much. A 50 degree change would only change the pressure a couple of hundred pounds.
Nitrous like carbon dioxide and freon are in a liquid state when under pressure in a tank at normal ambient temperature, air is not.

 

[racedude1984]

hmmm very interesting

 

[paddler3d]

but heat doesn't change the pressure of a scuba tank?

Yes it does. I wouldn't say heat changes the pressure, as does any temperature change.

PADI says as a rule of thumb, 5psi/degree Fahrenheit.

The actual science is:

P1V1/T1=P2V2/T2

I believe it is also called the ideal gas law.

All math is done in absolute and if using imperial, convert Fahrenheit to Rankin (add 460 to F).

Most commonly used to figure pressure drop in tank filling. This is why guys will overfill a tanks so when they cool to room temperature, they end up with the pressure they want. You can also use it to figure out why your SAC changed so much when your tank was full before getting in the 40 degree water and you seemed to use a lot of gas quick.

 

[DCBC]

Charles Law can be written: "When the volume is constant, the pressure of a gas is directly proportional to that of the pressure."

In other words, the physical volume of a cylinder doesn't change (appreciably) so as the temperature of the gas increases, so does the pressure. This can typically be seen if you put a full tank into your trunk on a hot day, the pressure increases; when the tank enters relatively cold water, the pressure drops.