Weight belt equation

[glbirch]

Here’s a web site that goes into detail, should you be so inclined:
http://www.mindspring.com/%7Edivegeek/altitude.htm

This would be one of those "It's on the internet, so it must be true" things. I don't care what this guy's PHD is in, water density/pressure is not going to vary significantly with a change in altitude. Once you leave the surface and start descending, you gain another ATM of pressure for every 34 feet (give or take). If you believe this guy, you'll be adding another ATM of depth every 11 feet or so at the top of Mt Everest.

This, IMO, is a much more realistic discussion: http://www.scuba-doc.com/divealt.html

The altitude mods on dive tables are there because once you leave the water at altitude you are entering a lower pressure area than the table was calculated for. It would be like climbing out of the ocean and jumping on an airplane immediately.

 

[O2BBubbleFree]

This would be one of those "It's on the internet, so it must be true" things. I don't care what this guy's PHD is in, water density/pressure is not going to vary significantly with a change in altitude. Once you leave the surface and start descending, you gain another ATM of pressure for every 34 feet (give or take). If you believe this guy, you'll be adding another ATM of depth every 11 feet or so at the top of Mt Everest..

This, IMO, is a much more realistic discussion: http://www.scuba-doc.com/divealt.html.

Um, I'm confused. Your guy's approach is more basic, but he quotes my guy, then simplifies. He dosen't contradict. So if you don't trust the credentials of my guy, how can you trust your guy, who uses my guy as a basis?

In my mind an even simpler and more realistic approach would be to get the conversion tables and training required in using them. That's what I've been trying to say from the beginning.

The altitude mods on dive tables are there because once you leave the water at altitude you are entering a lower pressure area than the table was calculated for. It would be like climbing out of the ocean and jumping on an airplane immediately.

Good point. I never made any attempt to fully explain all aspects of diving at altitude (anybody seen the original topic lately?), just wanted to point out that there are many extra considerations.

So, just to further illustrate, here's another consideration for you: At 6000' the pressure changes much more drastically in the first 20' of depth. Combined with the shallower recommended safety stop, holding your depth can be a bear! Especially in 7mmx2 suit with all accessories.

 

[glbirch]

Um, I'm confused. Your guy's approach is more basic, but he quotes my guy, then simplifies. He dosen't contradict. So if you don't trust the credentials of my guy, how can you trust your guy, who uses my guy as a basis?

In my mind an even simpler and more realistic approach would be to get the conversion tables and training required in using them. That's what I've been trying to say from the beginning.

The second set of calculation are a different approach than the first even though the author state he is merely simplifying, and do not depend on the density of water changing. (He doesn't say he's contradicting the original, but his math says otherwise.) So in 'simplifying' the second guy has removed an assumption that doesn't fly. They both get the same answer in the example used, but I think the second set of equations is more correct in the route used to arrive there. If you plug different numbers in for depth (say 200'), you can see that the depth correction is different depending on which set of equations you use (12' for one, 6' for other?). Not sure I trust the second guy either , but his math looks good.

Good point. I never made any attempt to fully explain all aspects of diving at altitude (anybody seen the original topic lately?), just wanted to point out that there are many extra considerations.

So, just to further illustrate, here's another consideration for you: At 6000' the pressure changes much more drastically in the first 20' of depth. Combined with the shallower recommended safety stop, holding your depth can be a bear! Especially in 7mmx2 suit with all accessories.

Definite that you should get training for altitude diving. Another short article here on altitude diving from someone who I do trust: http://divermag.com/archives/june2000/divedoctor_june00.html
http://divermag.com/archives/august2000/divedoctor_august00.html

Aqua Ho: I don't think there's an equation that will give you the answer you're looking for. Going from a 14mm suit to a 7mm suit (or whichever) you will lose x amount of neoprene which will affect your weighting. But x will vary depending on the size of your suit and what percentage of your total buoyancy it accounts for. If you know how much weight you need to be neutral diving in a bathing suit, then the rest of the weight accounts for the wetsuit buoyancy and multiplying that by the change in thickness should give you something around the number you are looking for.

 

[Tamas]

....actually the chart worked for me as well.

Thanks again!

 

[Groundhog246]

I confess that my above explanation was a bit simplistic. My point was that there are many considerations to diving at altitude, and it shouldn't be done lightly or without proper education.

On a less technical scale, if you look at your dive table there should be a note mentioning that the table should not be used above 1000'. If you go to your LDS and look at a conversion table that gives theoretical depth at various altitudes, you will see that the differences are more than a straight 'add 1' at all depths' assumption can account for. For example, diving at 6000' with an actual depth of 20' your theoretical depth for dive planning is 25', and with an actual depth of 100' your theoretical depth will be 124'!
.

I think I agree with what you're trying to say. I certainly agree with the need for appropriate training/tables. But the reason it changes is not a change in water density, but a change in surface air pressure.

nuff said.

 

[yc92602]

For me the difference between salt and fresh is 8#, and I don't consider that insignificant. Don't forget hood, gloves, and booties, if you use 'em.

I think what he means is the difference caused by the wetsuit. This 8# difference is mainly due to your body.

 

[fldivenut]

I think what he means is the difference caused by the wetsuit. This 8# difference is mainly due to your body.

Yes that's correct. The main source for the weight difference between SW and FW is your body and its very personal. I have 6# for instance. And this is not only based on your body weight but your body composition as well so the best thing to do is to try it out with no wet suit and an empty tank (200psi or so).

Any theoretical calculations are IMO only good for starting points. And for practical purposes it doesn't work to do a weight check when you jump in (start your dive) either since your tank is full and on eth first dive your gear is dry and traps some air that will lift you a few lb worth.

Most people that doesn't dive very frequent are over weighted and thus have possible problems with bouyance and drag causing higher air consumption and ear/sinus problems.
--A

 

[LeFlaneur]

Well, just getting in the ballpark was all I wanted so I can start adjusting from there. I guess the big surprise for me is how much more weight one needs to add for thicker neoprene. I had no idea it was such a difference.



Now... if I were diving at 6000' wouldn't the effect of gravity be slightly weaker at that altitude... and wouldn't the air bubbles in the neoprene expand due to the lower external pressure...? I'm just seeing if I can start another arguement.

 

[glbirch]

Well, just getting in the ballpark was all I wanted so I can start adjusting from there. I guess the big surprise for me is how much more weight one needs to add for thicker neoprene. I had no idea it was such a difference.



Now... if I were diving at 6000' wouldn't the effect of gravity be slightly weaker at that altitude... and wouldn't the air bubbles in the neoprene expand due to the lower external pressure...? I'm just seeing if I can start another arguement.

Do you really want me to pull out Newton's gravitational formula and beat you with it, or will you take my word that the difference in gravitational pull due to altitude change is not significant.
Yes, as was mentioned the bubbles would expand due to reduced air pressure. I suppose if you wanted to account for that you'd need to come up with a value for the elasticity of neoprene.

 

[pma]

I have a question. Surely the fact that AL tanks are positively buoyant at the end of the dive is a red herring. The way that it is always stated has always implied to me that if you are diving with steel tanks then you don't need to worry about it, as they are still negatively buoyant at the end, but diving with AL tanks you have to adjust for.

Surely what matters is that at the end of the dive you are going to be carrying the weight of the air less weight than you were when you started the dive.

Whether you are using AL tanks or steel or anything else doesn't matter, as the weight of the tank stays the same, but you have to adjust for the air that you have used, as you will be carrying that much less weight at the end of the dive.

I hope I made myself clear, and if I am misguided please tell me why.

Thanks

Peter