Weight belt equation

[Charlie99]

.

 

[Groundhog246]

OK, that makes sense. Now I'm curious and will have to find a scale and weigh my tanks. Wonder if I could borrow a fish scale and weigh them full in water.

 

[eponym]

Well, I've never met the man, so I can only go from his written work. From his web page:
The barometer reads 24.61 inches (625 mm) Hg. Thus, at this altitude, 24.61 inches (625 mm) Hg (not 29.92 inches (760 mm) Hg) is the atmospheric pressure! ...

Thus, every 27.9 feet of fresh water (not 33 fsw) at this altitude corresponds to one atmosphere of pressure at this altitude.

Water pressure is a function of the weight (mass) of the water column over you. The weight of the water column depends on the density of water and the force of gravity. If we assume that gravity is not changing then density must be?

I'm not offended, but even if his answers are correct I question the methodology.

When Taylor says, "every 27.9 feet of fresh water (not 33 fsw) at this altitude corresponds to one atmosphere of pressure at this altitude" he means that for every 27.9 feet you descend at this site, you increase your pressure by an amount equal to the pressure at this site's surface, which was 625mm (or 833 mBar, which is quite low compared to nominal sea-level at 100mBar). It's just another way of saying that if the surface pressure is low, you add another surface pressure's worth of pressure in less depth than the standard 33 (34 fresh) feet. No water compressibility is implied. Please let me know whether I've made that point clear. Thanks.

I think Dr. Ware looked at how Taylor's procedure initially treats depth analysis in terms of local atmospheres, which are eventually factored out, leading Ware to simplify the analysis considerably. I haven't taken the time to compare the two approaches over a range of cases but I plan to do so.

And why Texas?

Sorry for the obscurity - it was just that Texas was the location for the member who first suggested compressibility of water as a factor in diving. My bad, as that attempt at humour verged on the ad hominem. I apologize to SB in general.

 

[eponym]

Guilty as charged, 3dent. I'm sorry for that and I have no excuse - neither late nights nor caffeine nor temporary narcosis . . .
Please accept my apology.

-Bryan Saint Germain

PS Nice sigline there!

 

[O2BBubbleFree]

And why Texas?

My fault here, I think this slam was aimed at me, since I started this mess. And with that let me apologize for implying that the different dive tables required for diving above 1000' are due to water compresibilty. I know that's not the case.

Water pressure is a function of the weight (mass) of the water column over you. The weight of the water column depends on the density of water and the force of gravity. If we assume that gravity is not changing then density must be?

Water pressure may be the same, but a standard depth gauge won't measure it accurately. Most computers will, though.

Also, if I understand it right, gas dynamics are influenced by water pressure and by the divers internal tissue pressure, which is affected by the barometric pressure the diver has recently been exposed to. Post dive barometric pressure is also a factor in off-gasing. That's why altitude divers have different considerations if they have been 'at altitude' less than 24 hrs, if they have driven over any mountain passes in going to the dive, etc.

Again, let me empahsize to all those in the stands, if you are considering diving at altitude be aware that the standard dive tables cannot be used without modification, dives are calculated differently, safety stop depths and times are different, NDL limits are different, etc. etc. As always, please get the proper education for the type of diving you are planning.

 

[O2BBubbleFree]

Guilty as charged, 3dent. I'm sorry for that and I have no excuse - neither late nights nor caffeine nor temporary narcosis . . .
Please accept my apology.

-Bryan Saint Germain

PS Nice sigline there!

As I stated in the reply to your pm, no prob!

Thanks, and, thanks.

 

[pma]

No. The standard AL80 is indeed positive at 500psig. If you are changing between type of tank, there is normally an adjustment involved.That is the buoyancy change you will see during the dive, along with whatever compression your gear goes through. The positive buoyancy of an aluminum tank is a static problem.You will often find a person telling you that a given tank has no buoyancy shift as it empties. I have been told that by many very experienced instructors. They were wrong.

The bottom line is that tank buoyancy is part of the non-changing portion of your buoyancy. It may be different for each tank, but it will not change measureably during a dive. For instance, I need to take twelve pounds off if I go from double AL80s to double steel 72s.

The gas in the tank is part of the changing portion of your buoyancy. It is one of the things a BC needs to be able to deal with. I start the dive quite a bit heavier with a big tank than a small tank, but if I run the gas down to the pressure I weighted for in both cases, I will be neutral.

Don,

dispite appearing to disagree with me it appears that you actually agree from your final statement. I was not saying that the AL tanks were not positively buoyant at the end of a dive, but just pointing out that the thing that is important is that you finish a dive being more buoyant than when you started, and this is due to the amount of air that you have used, and not to whether or not the tank is AL or ST.

Peter

Sorry for the long sentence, in a hurry.

 

[glbirch]

When Taylor says, "every 27.9 feet of fresh water (not 33 fsw) at this altitude corresponds to one atmosphere of pressure at this altitude" he means that for every 27.9 feet you descend at this site, you increase your pressure by an amount equal to the pressure at this site's surface, which was 625mm (or 833 mBar, which is quite low compared to nominal sea-level at 100mBar). It's just another way of saying that if the surface pressure is low, you add another surface pressure's worth of pressure in less depth than the standard 33 (34 fresh) feet. No water compressibility is implied. Please let me know whether I've made that point clear. Thanks.

I think Dr. Ware looked at how Taylor's procedure initially treats depth analysis in terms of local atmospheres, which are eventually factored out, leading Ware to simplify the analysis considerably. I haven't taken the time to compare the two approaches over a range of cases but I plan to do so.

Sorry for the obscurity - it was just that Texas was the location for the member who first suggested compressibility of water as a factor in diving. My bad, as that attempt at humour verged on the ad hominem. I apologize to SB in general.

Many thanks. The little light in the back of the fridge finally came on. Now I see how Dr. Taylor is using ATA and ATM in relation to atmospheric pressure at altitude. With that undersood the math works, although I think it is much more complex than required. Dr. Ware certainly simplifies the equations to something that is slightly more elegent (IMO). My apologies for any confusion I may have caused to anyone, to 3dent for doubting his reference and especially to Dr. Taylor if he ever happens to read this.

eponym: I've worked both sets of equations for a range of altitudes from sea level to fifty thousand feet, and water depths from zero to one hundred and two feet. They match to within a couple of decimal points. I'd be glad send you a copy if you're interested.

 

[eponym]

glbirch exclaimed:
Many thanks. The little light in the back of the fridge finally came on.

I really like that image, glbirch. I don't know why, maybe I'm just still chilled through from today's pool sessions, watching over kids (human, not goat) trying out scuba.

If you can send me your results via email, please do. I'd enjoy seeing them.

-Bryan

 

[O2BBubbleFree]

I really like that image, glbirch. I don't know why, maybe I'm just still chilled through from today's pool sessions, watching over kids (human, not goat) trying out scuba.

If you can send me your results via email, please do. I'd enjoy seeing them.

-Bryan

And I was getting a kick out of the goat diving image, 'til you ruined it...