Altitude Diving Lake Tahoe (Safety + Physics)

[rossh]

Hi All,
I have some dive physics questions about Lake Tahoe. I have read a lot about altitude diving, I've even had a short course, but there are a couple questions I guess I just am not smart enough to clear up on my own.

1. My computer (Oceanic Atom 3.1) read 94 feet as my depth yesterday in Lake Tahoe. I understand that this is not the actual depth, but the equivalent amount of pressure to 94 feet of seawater at sea level.
-What was my actual depth?

2. When you ascend, 15 feet in Lake Tahoe is similar to +-15 feet in Monterey, but when you reach the surface you are suddenly at 6300' elevation.
-How is this different than diving Monterey and upon surfacing, getting in a helicopter and instantly flying up to 6300'?

Thanks in advance!

At elevation, the surface air pressure is lower (about 0.82 at 6,500ft). The fresh water pressure is still 1 ATA per 34 ft / 10.5 m. So a total pressure at a real 34ft water depth is 1.82 ATA.


A lot depends on the computer (not familiar with yours). A good computer will be able to work out the actual surface pressure and actual water depth. Then a good inbuilt deco algorithm will be able to compute plans for return to the 0.82 ATA surface. But not all computers are made equal.

If we go back many years, the depth gauge was a mechanical bourdon tube design and it was not able know the surface pressure, and consequently gave depth numbers in sea level values. The depth error for this situation would be around 5ft, and that was within the accepted variation of mechanical depth instruments. The printed NDL / deco tables were also built to sea level pressures. This meant the diver needed to apply correction factors and tables to offset for the pressure differences at elevation. But good computers have now replaced this skill, and no corrections are needed.

 

[Bob DBF]

If we go back many years, the depth gauge was a mechanical bourdon tube design and it was not able know the surface pressure, and consequently gave depth numbers in sea level values. The depth error for this situation would be around 5ft, and that was within the accepted variation of mechanical depth instruments. The printed NDL / deco tables were also built to sea level pressures. This meant the diver needed to apply correction factors and tables to offset for the pressure differences at elevation. But good computers have now replaced this skill, and no corrections are needed.

If you go back before the mechanical bourdon tube depth gauge, we used a capillary depth gauge which, because it is a tube open to air pressure, senses air pressure and gives no depth error at altitude. The water would compress the air in the tube as you submerged and you would read the depth at the air/water interface. I took one down to 130' at Tahoe and it agreed perfectly with my computer while my depth gauge was off in its own little world.

This is another SCUBAPRO capillary gauge

The plastic tube on this is yellowed, when new it would have been clear and easier to read.



Bob

 

[DiverDante]

Most newer dive computers will automatically calibrate themselves for the altitude you're at when turned on. Mine also changes the settings to fresh water, since it assumes that if you're at a higher elevation then it's most likely fresh water. It's good to understand the physics behind it all and have a "plan b", but newer computers are pretty good at guiding you through a safe profile.

 

[boulderjohn]

Most newer dive computers will automatically calibrate themselves for the altitude you're at when turned on. Mine also changes the settings to fresh water, since it assumes that if you're at a higher elevation then it's most likely fresh water. It's good to understand the physics behind it all and have a "plan b", but newer computers are pretty good at guiding you through a safe profile.

Computers deal with altitude differently, and you need to deal with them appropriately.

1. A few years ago, I was diving on the first day of a vacation in Florida, and I put together a regulator set with an old Suunto Cobra (air integrated) to use as a pressure gauge while I used a computer on my wrist for the actual tracking of the dive. While diving it occurred to me that the last time I had used the Cobra was at high altitude, and it was still set to that. I checked and, sure enough, it already had me well into deco, even though I was really not even close. I believe new Suuntos must still be set manually for altitude, but I could be wrong.

2. I used to use a Uwatec as a backup computer, and the model I had adjusted for altitude automatically, even when it was not turned on. The shop where I worked had that model on all their rental gear. Apparently it was so sensitive that it kept adjusting every time there was a decent change in air pressure, and as a result they went through batteries rapidly. I believe that model is no longer made, but there are modern computers that know the altitude before they turn on.

3. One of the most popular computers today, the Shearwater Petrel, must be turned on prior to the dive to read altitude. If you are at altitude and let it turn itself on during the dive, it will not read depth accurately.