Altitude Diving, the Ol' Fashioned Way

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

John C. Ratliff

Contributor
Scuba Instructor
Messages
3,817
Reaction score
3,039
Location
Beaverton, Oregon
# of dives
I'm a Fish!
I just put together a post on the Basic Diving area, where I had mentioned capillary depth gauges as compensating for altitude. I decided to elaborate, and this is what I said:
I mentioned above that the capillary gauge automatically corrected for altitude. The reason is that the capillary gauge will read deep. If you look at the table below, which I copied from various sources in the mid-1970s, you can see that at 4000 feet (about 1200 meters) the capillary gauge reading needs to be multiplied by 0.887 to get the true depth. So if I'm diving a lake at 4000 feet altitude, and my capillary gauge reads 30 meters (98.43 feet), I would log in my dive log 30 meters, then to correct it I would have to do the following calculation:

30 meters x 0.887 = 26.6 meters actual depth
or:
98.43 feet x 0.887 = 87.3 feet actual depth

But if I were using the U.S. Navy Diving Tables, which show for 100 feet a NDL of 25 minutes. But if we used the actual depth of 87.3 feet, the NDL would be 30 minutes.

If we used an accurate oil-filled Bounden tube depth gauge, we would need to add 4.7 feet to the depth reading, and then add a 3% correction factor. So if we were at 30 meters (98.43 feet) we would need to add 1.4 meters (4.7feet) to that reading, for 31.4 meters (99.83 feet), plus 3% for a depth of 34.3 meters (102.3 feet) depth. Note, that this would put the NDL higher, at 110 feet for 20 minutes using the U.S. Navy Diving Tables.

In the 1970s, we only had what was on this sheet for calculating altitude corrections. The Cross Tables for "Theoretical Depth at Altitude for Given Actual Diving Depth in Fresh Water" was a set of tables developed by E.R. Cross. If we had a line-measured depth, or a depth with correction using a depth gauge, we could use this table to determine which depth to use with the U.S. Navy Diving Tables. For instance, at 4000 feet altitude, we could dive to 100 feet, but would be best to use 116 feet as the "theoretical depth" for calculating NDLs or decompression. Table is quite out-dated, and should not be used currently, but I wanted you to see it to see how, in the past, we coped with altitude.

SeaRat

http://www.usu.edu/scuba/navy_manual6.pdf
 

Attachments

  • Altitude Corrections001.jpg
    Altitude Corrections001.jpg
    94.7 KB · Views: 159
https://www.shearwater.com/products/peregrine/

Back
Top Bottom