Altitude Diving Theory

[8thElementDiver]

Hi everyone,

I'm an instructor new to the board. I'm new to altitude diving and since I have a Ph.D. in theoretical chemistry I decided to delve into the theory behind it. I've been asked a lot of questions about it by my students, too, so I put together an article.

If you're curious, take a look and let me know if any of it doesn't make sense to you, or if you think I've made some mistakes. It's at http://www.8thelementdiving.com/scuba_math/altitude.php

I'm also open to ideas about future articles. I love math, science, diving, and writing, so I definitely will be writing more technical articles.

Enjoy! And unlike what Barbie might have once said, math is NOT hard.

Regards,
-Todd Stedl

 

[knotical]

A point you might want to include is that depth gauges are actually pressure transducers, so they introduce a small error when used in fresh water. This error just offsets the correction you propose in your article. For practical purposes, the tables are OK. Your correction would be appropriate if another means of determining depth were used, such as a weighted line.

 

[pir8]

Interesting but I need more time to digest your information. And no its not that math is hard just a different taste to acquire.

 

[Gene_Hobbs]

Nice article! I second the point on gauge correction. It is good info for someone new to have.

As a PADI Instructor, you might also want to add a reference to the DSAT articles published in their 1979 Decompression in Depth Symposia (RRR ID: 4230).

The two papers of interest from that meeting are:
And yet another approach to the problems of Altitude Diving and Flying After Diving. by Bruce Bassett
The theoretical structure and testing of high altitude diving tables. by Richard Bell, Anthony Thompson, and Reid Borowari

There are also some other good papers available:

The theory of high-altitude corrections to the U.S. Navy standard decompression tables. The cross corrections. Bell and Borowari Undersea Biomed Res. 1976 Mar;3(1):1-23. RRR ID: 2748

Diving at diminished atmospheric pressure: air decompression tables for different altitudes. Boni, Schibli, Nussberger, and Buhlmann. [SIZE=-1]Undersea Biomed Res. 1976 Sep;3(3):189-204. [/SIZE]RRR ID: 2750

Automatic compensation by capillary gauge for altitude decompression. MacKay. [SIZE=-1]Undersea Biomed Res. 1976 Dec;3(4):399-402. [/SIZE]RRR ID: 2440

Converting standard air decompression tables for no-stop diving from altitude or habitat. Hennessy. [SIZE=-1]Undersea Biomed Res. 1977 Mar;4(1):39-53. [/SIZE]RRR ID: 2784

Decompression Procedures for Flying After Diving, and Diving at Altitudes above Sea Level. Bassett. 1982. RRR ID: 4531

Diving at altitude: a review of decompression strategies. Egi and Brubakk. [SIZE=-1]Undersea Hyperb Med. 1995 Sep;22(3):281-300. [/SIZE]RRR ID: 2194

 

[Charlie99]

To say what Knotical said, but a bit more bluntly ---- your 34/33 correction for fresh water is incorrect. If you are using a table that is in fsw, then you should be measuring pressure (not depth) in fsw, independent of whether your are in fresh or salt water.

If the depth is being read by an electronic bottom timer or a dive computer, the sensor is measuring pressure, not linear distance. The measured pressure is in whatever units that computer is calibrated for. For some computers it is always fsw. For some, it is always ffw. Some dive computer convert from fsw to ffw when they detect altitude.

----------------------

Other depth sensing gauges require other corrections. For example a capillary type depth gauge ( the cheap ones, that have a greatly expanded scale for shallow depths) will automatically do the conversion to equivalent sea level depth and allow direct entry into the sea level tables using the indicated depth. This is because those gauges are really measuring depth in multiples of the local atmospheric pressure, which is all that things like the cross correction or Theoretical Ocean Depth calculations are doing. So one solution is to simply use the cheapest depth gauge out there, and use standard sea level tables in the normal manner. The analog capillary depth gauge does all of the conversions for you automatically.

Oil filled depth gauges have yet another set of corrections that are needed. These analog gauges have more or less constant angular change for a given depth change. I don't have personal experience with this style gauge, but I think it has just a fixed offset correction to get the real depth. You then use that to enter the Theoretical Ocean Depth tables.

 

[8thElementDiver]

Thanks for the responses.

Based on knotical's and Charlie99's feedback, I'll add a bit of a discussion about using depth guages and computers at altitude. However, I think it's important to note that the 34/33 correction is not incorrect.

1st, many dive computers automatically correct for altitude, and many of those then automatically switch to fresh water pressures for their depth readings. My dive computer, for example, is more than 5 years old and automatically switches to fresh water at altitudes above 2,000 feet. Other computers have manual settings where you can switch between sea water and fresh water. And the way technology is progressing, dive computers will only become more accurate--eventually, some might directly measure the salinity of the water and adjust their depth readings accordingly; others might use GPS to determine whether the diver is diving in sea water or fresh water.

2nd, and more important, is that the tables are to be used for dive planning, not as an after-dive reference to determine whether you exceeded your NDL. The tables should be used with bathymetric maps, depth sounders (or "fish finders") on boats, and whatever other information you can find about the lake. That type of information will always be based upon actual depths, not depths based upon pressure gauge readings. For example, if a state's Dept of Natural Resources mentions that there is an artifical reef at a depth of 87 feet in a particular lake at altitude, then I need to use that depth--in feet of fresh water--to figure out my TOD, and then use that to determine my max bottom time from the tables. Once I actually do the dive, I'll keep that max bottom time in mind as a backup, but I'll primarily rely upon my dive computer to determine how long I stay under.

In summary, the updated tables I provide in the article are correct, and they are more accurate for dive planning purposes than the old tables. But divers may need to apply corrections to their depth guage when they do the dive, in order to accurately read the actual depth in fresh water. That's important information that I'll add to the article--thanks for bringing it up.

 

[rakkis]

I think the point Charlie99 is trying to elucidate is that a linear distance of water is not really what determines nitrogen uptake; it is the weight resulting from that thickness of water. The courses stress depth because it is a slightly more intuitive concept.

So while it is neat to have an exact depth in fresh water, it is not useful from a decompression standpoint. What matters is the pressure resulting from it. Of course the difference is so minute that at the end, it doesn't really matter.

As an example:

Let's say your salt water computer shows a max depth of 129' while you're diving in fresh water. If one were to convert to the actual depth, one would find out you went to 133'. Did you go beyond recreational diving or not?

I will argue that what matters (and what is calculated on the tables) is nitrogen uptake due to a certain pressure (which happens to correspond to a certain WEIGHT of water...not depth). So even though you were physically at 133', your body only absorbed nitrogen as if its max depth was the equivalent of 129'.

Again.... a moot point when looking at the big picture, but one certainly worth pointing out in light of the point of view you propose.

 

[GrandpaScuba]

A point you might want to include is that depth gauges are actually pressure transducers, so they introduce a small error when used in fresh water. This error just offsets the correction you propose in your article. For practical purposes, the tables are OK. Your correction would be appropriate if another means of determining depth were used, such as a weighted line.

That depends on how the guage is calibrated. Most Uwatec computers are actually calibrated for freshwater and read 3% deeper in saltwater.

 

[nereas]

That depends on how the guage is calibrated. Most Uwatec computers are actually calibrated for freshwater and read 3% deeper in saltwater.

I believe this is true of SUUNTOs as well.

My SUUNTO and my UWATEC always agree, but they are both several years old now. I do not know what the newer models do.

 

[Scuba.Pete]

Off the subject...but I just realized what is meant by '8thElement'
Creative!