How much elevation change is significant?

[Frank O]

Recently I got a GPS receiver for the first time, and have been amusing myself by measuring my speed on bike rides, mileage driven to work, etc. In addition to a bunch of other information, I've learned that my house is at an elevation of 550 feet, and the place where I work is up against the foothills at about 1,200 feet.

Sometime back I recall that another diver told me a story about how he had very mild DCS symptoms after a dive that worsened significantly during a drive up to a community at an elevation of 1,400 feet -- so much so, in fact, that he had to turn back partway. (He was subsequently treated in a chamber.)

Now, I've heard the usual cautions about going up into the mountains immediately following a dive. But given that going to an altitude of 8,000 feet is a change of about 1/3 atmosphere (or so I've heard), then an elevation change on the order of 1,500 feet is less than a fifth of that -- or, say, a 15th of an atmosphere, mas o menos.

My question, then, is this. How small of an elevation change could make a difference -- whether immediately after normal diving ... or if there was a suspicion of mild DCS ... or if you were, say, discharged from a chamber and told not to fly for a few days? My gut instinct is that a change from sea level to 1,500 feet is 'way down in the noise, but I'd be interested to know if this isn't the case. (Yes, I know that deco theory is an imprecise science, so there are no hard-edged numbers to answer this question; I was thinking more of ballparks.) Thanks for any input.

 

[BRW]

FO,

Every 1000 ft of elevation increases your actual dive
depth by roughly 3.6 % as far as using sea level tables.
This has been widely used, accepted, and tried at altitude
for dissolved gas tables (USN, Buhlmann, etc), and holds
with slight modification for bubble tables up to some
10,000 ft elevation.

Best has a book called Diving Above Sea Level that explains
altitude modifications, travel rules, flying-after-diving, etc.
Plus it's covered in other books by Best like Technical Diving
In Depth, Basic Decompression Theory And Application,
and Reduced Gradient Bubble Model In Depth.

So, for a a dive to 50 fsw at 5000 ft elevation, the dive
is treated as 1.18 x 50 fsw = 59 fsw as far as tables,
meters, and dissolved gas software. For bubble calculations,
the depth is more, something like 63.5 fsw for tables, meters,
and software employing bubble models.

BW

Recently I got a GPS receiver for the first time, and have been amusing myself by measuring my speed on bike rides, mileage driven to work, etc. In addition to a bunch of other information, I've learned that my house is at an elevation of 550 feet, and the place where I work is up against the foothills at about 1,200 feet.

Sometime back I recall that another diver told me a story about how he had very mild DCS symptoms after a dive that worsened significantly during a drive up to a community at an elevation of 1,400 feet -- so much so, in fact, that he had to turn back partway. (He was subsequently treated in a chamber.)

Now, I've heard the usual cautions about going up into the mountains immediately following a dive. But given that going to an altitude of 8,000 feet is a change of about 1/3 atmosphere (or so I've heard), then an elevation change on the order of 1,500 feet is less than a fifth of that -- or, say, a 15th of an atmosphere, mas o menos.

My question, then, is this. How small of an elevation change could make a difference -- whether immediately after normal diving ... or if there was a suspicion of mild DCS ... or if you were, say, discharged from a chamber and told not to fly for a few days? My gut instinct is that a change from sea level to 1,500 feet is 'way down in the noise, but I'd be interested to know if this isn't the case. (Yes, I know that deco theory is an imprecise science, so there are no hard-edged numbers to answer this question; I was thinking more of ballparks.) Thanks for any input.

 

[Frank O]

Every 1000 ft of elevation increases your actual dive depth by roughly 3.6 % as far as using sea level tables.

Thanks, Bruce. Since I'm interested in this topic, I'll check out the "Diving Above Sea Level" book.

As a practical matter in situations I typically face, it sounds as though the impact is trivial. If I do a dive at Malibu to a depth of 30 or 40 ft and then drive to work at our 1,200-foot elevation (as I plan to tomorrow morning), the elevation gain would add less than two feet to the effective depth for the purposes of dissolved-gas calculations. Since I never get anywhere close to NDL limits, this as I expected is 'way down in the noise.

What about a relatively modest elevation change potentiating the worsening of mild DCS symptoms, as in the case recounted by the other diver I mentioned?

 

[BRW]

FO,

All of your questions are answered in DASL -- effects of
dive depth, length of dive, wait interval before ascending,
etc, etc. Plus the effects of repets.

BW

Thanks, Bruce. Since I'm interested in this topic, I'll check out the "Diving Above Sea Level" book.

As a practical matter in situations I typically face, it sounds as though the impact is trivial. If I do a dive at Malibu to a depth of 30 or 40 ft and then drive to work at our 1,200-foot elevation (as I plan to tomorrow morning), the elevation gain would add less than two feet to the effective depth for the purposes of dissolved-gas calculations. Since I never get anywhere close to NDL limits, this as I expected is 'way down in the noise.

What about a relatively modest elevation change potentiating the worsening of mild DCS symptoms, as in the case recounted by the other diver I mentioned?

 

[Dr Deco]

Hi Frank O:

Imprecise Science

There are some aspects of decompression that are imprecise, especially when treating a group, but individual cases are not as “imprecise” as one might think. If this individual is experiencing DCS pain, then clearly there has been sufficient gas phase formation. A distance of 1,500 vertical feet is not a trivial amount; expansion can be significant enough to elicit pain. Small distances up or down in a hyperbaric chamber can do this to an individual with DCS pain.

People are certainly aware that they can feel the expansion of free gas, for example, in the middle ear. On a trip in an elevator in a tall building, the pressure change [expansion of gas] is very obvious.

Dr Deco :doctor:

 

[pasley]

Sometime back I recall that another diver told me a story about how he had very mild DCS symptoms after a dive that worsened significantly during a drive up to a community at an elevation of 1,400 feet... (He was subsequently treated in a chamber.)...

Ahhh, the memories. Actually, the story went like this. I did a Dive on Wednesday to 108 FSW. On Friday I call DAN with really minor "maybe" symptoms in my ankles (I had dismissed them as from being rolled by a wave on entry). By Friday, they were worse and going up my calf now. DAN says go to chamber. I go and take one ride in the cigar tube. Doctor suggested a second ride the next day. I told him I would not be comming in and he did not argue so I blew off the second ride (OK I'll say it "Idiot" :bonk: ). At this point I really still did not believe I was bent. A week later (Thursday), per my discussions and agreement of the doctor at the time of treatment, I went to Casino Point for two shallow dives with 1.5 hours SI between the two. Then on Saturday I drove to Hemet, elevation 1,400 feet. A fact I had never really paid attention to. Arrived in Hemet with no doubt in my military mind that I was in fact bent, and returned to Long Beach for the recommended two rides in the chamber and no diving for a month. That was August 24, 2001. 95 dives later and doing fine.

I also heard from a dive partner about her husband who got bent and was unable to drive to their home near Big Bear if memory serves for over a week without aggravating his symptoms.

1,400 feet is not much, but it did hurt.

 

[Frank O]

Ahhh, the memories.

Hey Melvin, well, I did cloak the story in heavy anonymity. ;-) Your story had stuck in my mind after you related it many moons ago, and after I realized this week that I make a similar elevation change every time I go from seashore to workplace, my question bubbled out. (So to speak.)

I'd always thought that 1,200 or 1,400 feet wasn't much to be concerned about, but I guess as your experience shows, this isn't always the case ...

 

[Dr Deco]

One should be concerned if they already have DCS symptoms (pain or neurological).

Dr D

 

[pasley]

Hey Melvin, well, I did cloak the story in heavy anonymity. ;-) ...

True you did. But I am not shy about talking about my DCS, and admitting my mistakes. I figure others can learn by them.

I truely had not thought much about the 1,400 feet either until it became an issue for me. If I had not already been bent, it probably would not have mattered, but then again.... I always suggest to people who have to go up the hill to get home from a dive to relax and enjoy a little food and take there time. Give it a few hours after the dive before tackling the hill. I figure it could not hurt and might help.

See you in the water.