Answers to questions:

1. Is there any danger if your epiglottis opens accidentally, exposing the sinuses to the negative pressure in the lungs?

- No danger; the worst that can happen is that the air in your mouth rushes back into your lungs, but the negative pressure is not enough to cause a reverse equalization on the ears.

2. Blood shift during negative dives with the mouthfill

- Doing the mouthfill before a negative dive allows a much greater depth to be reached. In fact, you can simulate over 200m! This opens up a real danger. Because equalizing is not the limit, chest squeeze / lung fluid / blood shift now becomes the limit, and if you go to far, you can be SERIOUSLY INJURED!!

Please, when doing negative dives, always use a buddy, and never go to the point of discomfort.

You can calculate the depth that you are simulating as follows:

- Find your failure depth during a real dive, i.e. the depth at which you cannot exhale any more air: X

- After fully exhaling on the surface and filling the mouth, if you truly exhaled everything, then when you descend to depth Y, you can calculate the simulated depth as follows:

At depth X (pressure 0.1X+1), you reached empty lungs.

During the negative dive, you reached empty lungs at the surface (pressure = 1atm). You reached a depth of Y, or a pressure of (0.1Y+1), which is (0.1Y+1) times the original pressure of 1.

So, during the negative dive, you descend to (0.1Y+1) times the pressure at which your lungs became empty.

So, if your lungs really become empty at depth X, then assuming the ideal gas law at depth (which is not accurate), the simulated depth would be:

10*[(0.1Y+1)*(0.1X+1)] - 10

So, if you fail at 35m, and you did a negative to 10m with a FULL exhale, then you simulated:

10*[(0.1*35+1)*(0.1*10+1)]-10 = 80m

In my case, my failure depth for normal equalizing is 80m, and I have reached 22m on negative, so the simulated depth is:

10*[(0.1*80+1)*(0.1*22+1)]-10= 278m

Seeing this number, I realize that I probably didn't fully exhale on my 22m negative, because my lungs were only half filled with fluid at the bottom.

However, both of these calculations are too conservative. Near the surface, the ideal gas law holds, i.e. a double in pressure causes the volume to be reduced by half. However, down at 100m+, doubling the pressure does not cause the volume to decrease by half; the ideal gas law no longer holds, so you must use the Van Der Waals equation or something similar. If you do, the result will be an even greater depth, i.e. the above results (simulation of 80m, or simulation of 278m) are actually simulating deeper depths.

Eric Fattah

BC, Canada