Why Altitude Corrections for Scuba Diving?

Why Altitude Corrections for Scuba Diving? OK, when we ascend to an altitude where the air is less dense, the water is fresh and less dense than salt water. Why do we need to make corrections to the dive tables to dive at altitude?

Actually, there are several reasons. First the air is less dense. If you remember your basic scuba class (assuming they taught you about dive tables) Haldane stated that the less pressure exerted by the surrounding atmosphere, the less dissolved gas your body could hold without creating bubbles in your bloodstream.

Also if you are diving with a standard  universitycafe gauge instead of a modern dive computer, you gauge lies! Really it’s true. If your gauge didn’t have a peg to stop the needle from going below zero, it would read a minus amount. How much? Approximately minus one foot for each 1000 feet in altitude. Also the gauge (really a pressure gauge calibrated in feet) won’t register the depth correctly. You are diving in fresh water (34 feet=14.7psi or 1 atmosphere). The gauge, as manufactured, is a salt water gauge. (33 fee =14.7psi or 1 atmosphere).

You have a problem with arrival at altitude also. When you ascend to an altitude from sea level you are traveling to a lesser atmosphere. If you remember Henry’s Law, that means less pressure on your body and the liquid in it, therefore less dissolved gas remaining in your bloodstream.

We still haven’t addressed the major player in the need for altitude corrections. When you make a dive at altitude and come back to the surface, you are ascending to a less dense atmosphere than the dive tables to track. Some divers have a hard time getting their heads around this fact. Let me repeat this, you are ascending to a less dense atmosphere than the dive tables track. This is like making dives at sea level and then going up in an airplane!

One more thing, if you need to ascend to a higher altitude to go over a mountain pass to get home, exercise caution. You need to off-gas enough so that you don’t get bent by going over the summit. Divers have gotten bent (Decompression Sickness) by diving at sea level and then going over the ‘Grape Vine’ in the Southern California area on the way home!

OK now you know the problem, what do we do about it? There are several ways. One dive with a modern dive computer that calculates dives at altitude. It also accounts for ascending to altitude and going home and ascending to a higher altitude.

If you are diving with a standard gauge, there are several methods you can use to make corrections to the dive tables. One method is to do several corrections (gauge correction, fresh water correction & finally corrections to compensate for coming up to a reduced atmosphere after the dive).

A better way is to use a table designed to make all these changes in one simple step. For example, the tables referenced in my biography

Different agencies (SDI and others like Padi) have different procedures for altitude corrections to their tables and you should familiarize yourself with them.

Why Altitude Corrections for Scuba Diving? OK, when we ascend to an altitude where the air is less dense, the water is fresh and less dense than salt water. Why do we need to make corrections to the dive tables to dive at altitude?
Dave has been a Scuba Instructor since 1984. He holds certification in NAUI, HSA, SSI & SDI. He is the inventor of a High Altitude Calculator http://www.hsdivers.com/HSDstore and High Altitude Tables He is the co-author (with Dr. Bruce Wenkie) of an article on Altitude Diving published in the Best of Sources by NAUI. He teaches at High Sierra Diver

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