**Surprised That Its Possible...****Depth Compensation**- My understanding is that a twin tank J-valve is really just a spring powered over pressure valve that you can manually open. When its not manually opened, it causes up to a 500 psi difference between the reserve tank and the other tank throughout the dive, regardless of depth. You can pull the lever at any time or depth and hear the air equalizing between the tanks.

- I read the info in Basic Scuba and Here's How I Understand it: If you pull the lever when you're deep, the reserve air will expand and give you more as you come up. If you pull the lever shallow, the reserve air will compress and give you less if you go deeper. Is this correct?

Lets walk through what actually happens with the tanks of air in a twin tank, 38 cubic foot system which is full at 1800 psig. With a 500 psig spring in the J-reserve valve on one side of the system, we have:

500 / 1800 = 0.2278 (This is the ratio of reserve air in one tank verses the whole single tank)

0.2270 x 38 ft3 = 10.5556 ft3

So we have in reserve 10.6 cubic feet of air in the single tank. If we let this tank stay at 500 psig, and the other tank goes to zero psig, then we trip the reserve, we will equalize the two at 250 psig. But we will still have in reserve 10.6 cubic feet of air. Now, if we are at nearly the surface, and breathing at a rate of 1 cubic foot per minute, we will have about 10 minutes of air. But if we go to 33 feet (34 feet is Silver Springs), which is two atmospheres, and breath at the same rate as on the surface, we will consume 2 cubic feet of air per minute, which is about 5 minutes of air supply. This is because the air is at twice the pressure, so each breath is twice as dense.

10.6 ft3 / 1 ft3/min = 10.6 minutes (surface)

10.6 ft3 / 2 ft3/min = 5.3 minutes (33 feet of sea water, 34 feet of fresh water)

Now if we were at 66 feet, the air is three times as dense. Therefore,

10.6 ft3/3 ft3/min = 3.5 minutes of air at 66 feet (68 feet of freshwater)

And so on down the depth. Now, if you pull the reserve at depth, and start ascending, what happens? We have the same breathing rate, at a surface air consumption rate of 1 cubic foot per minute. But the pressure at, say 68 feet in freshwater is three times the surface, so that rate is three times what it is on the surface. But as we ascend, the pressure lessens, and air density lessens, and therefore we consume less of that 10.6 cubic feet of reserve air per breath. So yes, as we ascend, we have more available air in reserve.

Now, Fred Roberts, in Basic Scuba, states that the spring release is slightly greater than above. We need to add the ambient pressure to the amount of pressure from the J-valve spring. Here's what he states:

He goes on to break down the use of twin manifolds, and the reserve on each. In each case, he adds 44 psig to the reserve (300 psig spring = 344 psig; 500 psig spring = 544 psig). But here's the thing, at the depth of 100 feet, where the engagement of the spring occurs for a twin tank (500 psig spring) at 544 psig, the total pressure of the tank would be 1844 psig too. If we go through the same division as above, where we use the ratio, we get:

544 psig/1844 psig = 0.2950

If we multiply this by the 38 cubic feet, we get 11.21 cubic feet in reserve.

38 ft3 x 0.2950 = 11.21 ft3

So it appears that there is slightly more air held in reserve at 100 feet. But at 100 feet, we have:

100 ft / 33 ft/atm = 3.03 atm

The pressure is 3.03 times the surface pressure, and so our 1 cubic foot per minute air consumption at the surface would be 3.03 cubic feet at 100 feet.

11.21 ft3 / 3.03 ft3/min = 3.6997 minutes, or 3.7 minutes of reserve air.

If we do the same for the pressure at 68 feet of freshwater, we get:

529.4 psig/1829.4 psig = 0.2894

If we multiply this by the 38 cubic feet, we get 10.9966 ft3 = 11 cubic feet in reserve.

38 ft3 x 0.2950 = 10.9966 ft3 = 11 ft3

So it appears that there is slightly more air held in reserve at 68 feet. But at 68 feet, we have:

68 ft / 34 ft/atm = 2 atm difference between the surface (at 1 atm) and 68 feet.

The pressure is 2 times the surface pressure, and so our 1 cubic foot per minute air consumption at the surface would be 3.03 cubic feet at 100 feet.

11 ft3 / 2 ft3/min = 5.5 minutes of reserve air.

Then, as we surface, the air consumption rate goes down as the pressure goes down. Therefore the amount of air available is greater as we surface. The air doesn't actually compress at depth, but the pressure is increased. Here is a table that Fred Roberts developed in Basic Scuba (page 166):

John