Altitude Divers

[TMHeimer]

A question for Altitude Divers. I know that if your SPG reads say 40 feet you are actually closer to the surface than that. I know that the pressure of the atmosphere at altitude is less than at sea level. Does the lower atmosphere pressure affect the pressure the water excerts? If so, does that mean the fresh water pressure is less than 62.4 pounds per cubic ft. throughout the entire body of water? If so, is that why you are shallower than what your SPG reads?

 

[AzAtty]

Your question is somewhat loaded. Water pressure is determined by the gravitational constant ("g" which is approximately 9.8 m/s^2) and the depth (or height) of the column. So no, changes in atmospheric pressure from increasing altitude do not significantly affect water pressure.

However, ambient pressure underwater (a.k.a. "absolute pressure") on a diver is the sum of atmospheric pressure plus water pressure.

Atmospheric pressure is approximately 14.7 psi at the surface of the earth. Atmospheric pressure decreases with altitude. For altitude "h" the formula is P = 33e^(-0.038h). The formula simply says that pressure decreases at a certain rate for each unit increase in altitude.

So, when diving at altitude, absolute pressure is less than it is at sea level.

A Bourdon tube depth gauge (a kind of SPG, really) shows a shallower depth at altitude because it actually "reads" pressure rather than a particular depth. "Zero" gauge pressure (sea level reading) is really one atmosphere. Pressure acting on the tube causes it to uncoil in a certain direction. When the tube uncoils, it moves gears that are calibrated to move the needle a particular distance based on a "zero" of one atmosphere. The needle's movement corresponds to depth readings on the face of the SPG. At altitude, however, gauge pressure is less than "zero" because atmospheric pressure is less than one atmosphere. This causes the tube to move a shorter distance, so the SPG reads shallower. What results is the following: if you dived at an altitude where atmospheric pressure was 0.5 ATM, you would have to go down about 18 feet in fresh water before the depth gauge would even register that you had submerged. At 18 feet fresh water, your gauge would read 1 foot.

Capillary gauges, on the other hand, read deeper than actual. Computers use transducers to show pressure. Both operate differently than a Bourdon tube.

 

[eponym]

1. You can ignore the minor difference between what your depth gauge tells you and what a measured chain would read when diving at altitude.

2. Yes, the pressure of the atmosphere decreases as you ascend. Divers returning to the surface are exposed to somewhat less pressure than those diving in the ocean, and this should be taken into account when planning altitude dives.

3. No, the weight (and pressure) of water is for all practical purposes constant until you get much farther from the surface of the earth than you can while diving.

4. Again, don't think about whether your gauge is correct or not. Think about returning to a fractional atmosphere (say, 9/10s) after diving. The difference between the pressure of dissolved nitrogen in your body and that of the pressure of nitrogen around you on the surface is somewhat greater than it would be had you done the same dive in the ocean.

For that reason we add a "depth correction" factor to our dives. For instance, if I plan to dive to 60 feet in a lake that's at 2000 feet above sea level, I will plan it (look up my max no-deco time on my tables) as though I were going to go to 64 feet. So my no-deco time is shortened somewhat.

I can look those correction factors up on the Navy's altitude correction table, or if I have a dive computer I can just make sure it knows what my surface altitude is and then let it do the math and tell me my new no-deco time limit.

Hope this helps,
Bryan

PS. For much more on this subject 8th Element has a good article and good links.

 

[TMHeimer]

Thanks guys. I doubt I'll ever do altitude diving, but I was curious. So, it a nutshell, since the weight of the water above you (and thus the pressure of the water) basically does not change because of altitude, it is the reduced pressure of the atmosphere that would cause a normal SPG to be off, no?

 

[AzAtty]

That is correct.