Analog depth gauge and altitude.

[landlocked]

Since my depth gauge reads pressure rather than depth, I have noticed a bit of a discrepancy when diving at altitude (usually around 5,000 feet.) Is there a formula that can help compensate for the error in depth readings?

 

[jonnythan]

Well, if you think about it, it's not that big a deal is it? The effective pressure is what you really care about anyway, no..?

 

[Rick Murchison]

Diving at altitude requires corrections for fresh water, for gauge inaccuracy and for the effects of reduced atmospheric pressure on decompression.
Let me use an extreme example - 18,000' - where the atmospheric pressure is one half sea level. (While a fresh water correction may also be required, it isn't a bigee for our example)
Your sealed bourdon tube depth gauge will read -16.5' at that altitude; won't get to zero until you get to 17 FFW - by the time you're at 50FFW, your depth gauge will read 33 feet, but you're at FOUR atmospheres and building a decompression obligation as though you're at 100 feet when diving at sea level! This is a *huge* difference in how you must treat your dive and calculate your decompression obligation when using a regular depth gauge for altitude diving.
At 5,000' the difference isn't as dramatic, but it is still significant - if you dive your depth gauge and use the dive tables or a non-altitude-correcting computer as though you are at sea level you can get into serious decompression trouble in a heartbeat.
Take an altitude diving course if you intend to dive above 1000 MSL.
Rick

 

[jonnythan]

I'm certain you're right about the deco obligations and corrections... but how do you figure that being at 50 feet means four atmospheres? The difference between atmospheric pressure and the pressure at that depth is only about 1.3 atmospheres. Where does the four come from?

 

[Rick Murchison]

Originally posted by jonnythan
I'm certain you're right about the deco obligations and corrections... but how do you figure that being at 50 feet means four atmospheres? The difference between atmospheric pressure and the pressure at that depth is only about 1.3 atmospheres. Where does the four come from?

At 18,000' altitude, one atmosphere is 7.85 psi, or 17FFW. 50' is therefore three ATM of water and 1 of air. This is the pressure that must be used to figure deco obligations, stop depths, etc. Your safety stop, for example, should be made at 7.5 vice 15 feet under these circumstances.
There are other factors that make extremely high altitude diving (above about 10,000') even more dangerous than what I have outlined here - I only use 18K because it's easier to work with the numbers.
Bottom line - if you're contemplating diving above 1,000 feet, please take the altitude diving course, and all this stuff will come clear.
Rick

 

[jonnythan]

Ahh, ok.. I suppose it would be ok if you did your surface interval at 16 feet

Point taken

 

[landlocked]

Originally posted by Rick Murchison
Let me use an extreme example - 18,000' - where the atmospheric pressure is one half sea level. Your sealed bourdon tube depth gauge will read -16.5' at that altitude;
Rick

I've misplaced my formula for determining this. Does anyone have it handy?

 

[Goldminer]

Originally posted by Rick Murchison

At 18,000' altitude, one atmosphere is 7.85 psi, or 17FFW.
Rick

Actualy I believe that I ATM = 14.696 psi or 1.0133 bar period, at least according to my book of conversion factors, weights and measures. It is defined as the mean air pressure at sea level so you being at 18,000 feet is irrelevant. An atmosphere is a constant but, I agree with you that returning to a surface air pressure of .5 atm or 7.85 psi in your example has a significant effect on your calculation of NDLs.

 

[roakey]

Of course, you could dive with the old capillary depth gauges and tables, and then you don't have to even think about altitude. A capillary depth gauge does all the altitude corrections for you because of the physics it's based on!

Roak

Ps. Ok, you would still have to accomodate driving up to altitude.