Calculating Weight Requirements

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I've got a doubles question. If you do your 10-15' buoyancy checks using 500psi GAUGE pressure, you really have 1,000 psi. I did my buoyancy checks using 500 psi gauge pressure. Should I also be checking it at 250psi GAUGE pressure which is really 500psi?

I usually make sure I'm at my safety stop with 500psi gauge pressure, but wondered about
using 250psi instead or in addition?

I've gotten different answers from different doubles divers. What's the consensus?

Thanks.


Hopefully some smart people will comment, as I understand, the intermediate pressure of most regs are about 135 psi, and some run at 170 psi. Once you are down to 100 ft, that is equivalent of 149 psi, and 210 psi respectively. Running your tank till it hit 250 doesn't seem to be very safe, as I am not sure, how many breath you will really have?

Also, can you answer me - will the reg continue to breath once the intermediate pressure dropped below the recommended pressure for the second stage?
 
fisherdvm, I think you're misunderstanding me. I use 500psi GAUGE pressure as the least amount of air remaining I want in my doubles set at a 15' safety stop. With doubles, that gauge pressure really means that you have 1,000 actual psi in your tanks, total. (Assuming one's gauge is calibrated accurately).

I would never consider having only 500psi at 100 ft. depth if that's what you're asking? Or did you mean 100 psi?

At a 15' safety stop with 250psi Gauge pressure a doubles diver has 500psi in their tanks total.
(250 psi in doubles is the same as 500 psi in a single tank). Again, assuming one's gauge is calibrated accurately.

So, my question was, should I also do a safety stop buoyancy check at 15' with 250 psi Gauge pressure rather than the 500 I've been doing?
 
Also, can you answer me - will the reg continue to breath once the intermediate pressure dropped below the recommended pressure for the second stage?

The reg will continue to breath, but with an unbalanced regulator the inhalation effort will increase.

Jim, 500psi total makes sense to me.. doubles or singles it's all about the air you need and not draining the tanks empty (and as you said accounting for SPG inaccuracies).

There is no need to reduce weights so much that you cannot be in control until you surface.

When in doubt a diver is far better off with 2-4 extra pounds than 2 too few.
True, I didn't mean to imply being out of control above 10 ft is OK. Rather, when set for 10ft you should easily be in control all the way to the surface, especially if you maintain horizontal trim - that's my experience and if you find yourself losing buoyancy, it's trivial and common sense to add a pound or two.
 
Hudson,
That's what I've been thinking. My next weight check, I'm thinking of trying a 10' and 5' stop with 250 gauge pressure. (500). My usual tech mentor buddy has had me finish at the surface with 500 gauge pressure/1,000 actual psi, so I'd like to see how much weighting difference there'll be. As soon as the snow storm goes 'way...
I've been asking around and most people have told me to base it on what I'd intend finishing up the dive with, minimum at the surface-but...obviously on shore dives
with a shallow/slope exit it can vary so I'm a little confused which to aim for (500 gauge or actual psi) weighting wise, or if it'll even make any difference in the amount of wt. (between 15' and 5') I dive a neoprene drysuit, so I'm thinking it will.
 
Hold your breath and float at eye level is only designed to get you close. If you got a new wetsuit, expect it to lose many pounds of buoyancy over the first 10-20 dives due to collapse of the closed cell neoprene. Also, it has been my experience that if you do the test in the first few minites of getting in the water, you do NOT have to add 5 pounds for lost air weight. The reason for this is that when you initially get in the water, your equipment and wetsuit probably have 5 lbs worth of air bubbles trapped in it. The best test is to repeat the experiment at the end of your dive with an empty tank, when all bubbles have been squeezed out. Once all of this is done, take 2 lbs out after every couple of dives and see what happens.
 
Estimate and then use trial and error. After a couple of dives dragging your ass along the bottom followed by two uncontrolled ascents you'll know how much weight you need :wink:
 
Let's first define the correct amount of weight: you should be neutrally buoyant at 10ft with a near empty tank, half a breath (err on the "too much" breath side to gauge this). The other alternative is to try for neutral buoyancy at the surface, but you have no reason to hold a depth between 10ft and 0ft, and in my experience if you are neutral at 10 ft you will have a nice and very slow ascent to the surface.

That's a good way to get bent or killed.

The last thing you want is an uncontrolled pop to the surface right into a boat prop, or an unplanned ascent when you had a deco obligation.

You need enough weight to allow becoming neutral at any point below the surface, with the least amount of gas you would ever have in your tank.

The buoyancy check taught in most OW classes involves finding what it takes to just sink you with an empty (as empty as you ever expect it to be) tank and a normal breath of air. There is no calculation that will get you the correct weight. The only way to find it is to do the test.

A pound or two over won't cause much trouble, but a few pounds too light will give you an impossible-to-control safety stop and final ascent. The last 15' is the most important part of your ascent and slower is better.

Terry
 
My usual tech mentor buddy has had me finish at the surface with 500 gauge pressure/1,000 actual psi,

. . . (500 gauge or actual psi) weighting wise,

If your gauge says 500, your tank should contain 500 (more or less).

What's the 500 <-> 1000 thing?

Terry
 
That's a good way to get bent or killed.

The last thing you want is an uncontrolled pop to the surface right into a boat prop, or an unplanned ascent when you had a deco obligation.

You need enough weight to allow becoming neutral at any point below the surface, with the least amount of gas you would ever have in your tank.

The buoyancy check taught in most OW classes involves finding what it takes to just sink you with an empty (as empty as you ever expect it to be) tank and a normal breath of air. There is no calculation that will get you the correct weight. The only way to find it is to do the test.

A pound or two over won't cause much trouble, but a few pounds too light will give you an impossible-to-control safety stop and final ascent. The last 15' is the most important part of your ascent and slower is better.

Terry

I agree with much of what Terry has said, but differ on the concept of "correct weight" since some people try to tweak it too closely.

IMO, not only is it better to have an extra pound or two than be underweighted, as Terry pointed out, it's also better than having "correct weight" in some circumstances.

Some people try to achieve what they think is "perfect" or "correct" weighting, practically to the ounce, but they don't seem to consider that during much of the dive they are still dealing with the imperfect situation of having the extra weight of tank air (beginning with 6.4 lbs of air in a full al80).

Their "correct" or "perfect" weighting really only gives them only a slight advantage during most of the dive compared to someone who has a couple extra pounds. The slightly easier buoyancy control and less wasted air is a minuscule advantage really.

What about the disadvantages? Having "correct" or "perfect" weighting has disadvantages at the end of the dive in circumstances where being able to get negative is desirable, such as in surge or with boat props spinning above, as in Terry's scenario.

In that last scenario, positive buoyancy is clearly a problem, but so would being neutral with "correct weight" because in both cases one would have to propel oneself away from the surface rather than just fully vent the bcd and drop down.

A prop spinning a few feet from one's head gives a new appreciation for what really constitutes "correct" weighting! Some overweighting begins to look pretty good right about then! :D

Dave C
 
If your gauge says 500, your tank should contain 500 (more or less).

What's the 500 <-> 1000 thing?

Terry

Don't confuse pressure with volumn. A set of doubles at 500 psi has the same volumn of air as a single with 1000 psi. So if I normally surface with 500 psi in a single I could do the same at 250 psi with doubles. In both cases I surface with the same amount (cubic feet) of air remaining.
At 99 feet ambient pressure is 58.8 psia so at 250 psig (264.7 psia) tank pressure I am still 206 psi above ambient and would continue to get air until tank pressure reached 58.8 psia (44.1 psig) althought inhalation might become more difficult depending on the regulator.
 
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