dry suit weighting question

[msimp2005]

if all you have is this forum to worry about and to get mad over i truly feel sorry for you.

 

[msimp2005]

if all you have is this forum to worry about and to get mad over i truly feel sorry for you.

this is meant for fire diver:mooner:

 

[Lobzilla]

Had a "duh" moment last dive.

My suit buoyancy was determined 'net' - after I finagled all gas out in the vertical position. If you use this number for dead-nuts weighing, an ascent in horizontal trim is going to be challenging as there is gas trapped in the suit that will not come out easily in the horizontal position; not much, but enough to make the ascent 'interesting'.

A better approach would be to do the first dive in a new configuration slightly over-weighed, descend and add gas, ascent horizontally with stops, close the dump valve shortly before surfacing and then figure out on the exit how much weight could be dropped. From that, one can calculate the 'gross' buoyancy for suit and undergarments. (I like to have the suit buoyancy as a separate number so I can use other gear without having to do trial-and-error again)

 

[Bubbletrubble]

A better approach would be to do the first dive in a new configuration slightly over-weighed, descend and add gas, ascent horizontally with stops, close the dump valve shortly before surfacing and then figure out on the exit how much weight could be dropped. From that, one can calculate the 'gross' buoyancy for suit and undergarments. (I like to have the suit buoyancy as a separate number so I can use other gear without having to do trial-and-error again)

I'm confused. Why would you have to "close the dump valve shortly before surfacing"?
Most of the drysuit exhaust valves I've seen have an auto-dump setting. Set it and forget it underwater. I see no reason to change exhaust valve settings just to do a weight check as you near the surface.

Yeah, it does make sense to do the weight check for a drysuit in a horizontal position. This just goes back to basic fundamentals in proper weighting. The key thing to remember is that the parameters under which you conduct the weight check should replicate what you will experience at the end of the dive with a near empty tank and empty wing as you approach the surface.

 

[SubMariner]

Had a "duh" moment last dive.

My suit buoyancy was determined 'net' - after I finagled all gas out in the vertical position. If you use this number for dead-nuts weighing, an ascent in horizontal trim is going to be challenging as there is gas trapped in the suit that will not come out easily in the horizontal position; not much, but enough to make the ascent 'interesting'.

A better approach would be to do the first dive in a new configuration slightly over-weighed, descend and add gas, ascent horizontally with stops, close the dump valve shortly before surfacing and then figure out on the exit how much weight could be dropped. From that, one can calculate the 'gross' buoyancy for suit and undergarments. (I like to have the suit buoyancy as a separate number so I can use other gear without having to do trial-and-error again)

With all due respect, you need to spend more time actually doing buoyancy checks in the water you are going to be diving with the gear you are going to be using rather than spend all this time on complicated weighting experiments in an effort never to have to do buoyancy checks. :shocked2:

ANY calculations are simply a starting point. But what you cannot "calculate" is how much air a new diver retains in h/h lungs while diving which ultimately plays a big part in the weighting process: newer divers tend to use/need more weight than experienced divers for this very reason.

It's also the reason why log pages have spaces for weighting.

Pax,

 

[Lobzilla]

I'm confused. Why would you have to "close the dump valve shortly before surfacing"?
Most of the drysuit exhaust valves I've seen have an auto-dump setting. Set it and forget it underwater. I see no reason to change exhaust valve settings just to do a weight check as you near the surface.

Yeah, it does make sense to do the weight check for a drysuit in a horizontal position. This just goes back to basic fundamentals in proper weighting. The key thing to remember is that the parameters under which you conduct the weight check should replicate what you will experience at the end of the dive with a near empty tank and empty wing as you approach the surface.

Sorry for not being clear enough.

The point of closing the valve close to the surface was to capture the gas that remains in the suit during an ascent in horizontal trim. If I ascend horizontally and leave the valve open after surfacing, the residual gas in the suit will have escaped by the time I reach the ramp. If I do the weight check then, I will underestimate the buoyancy of the suit in the ascent phase of a 'real' dive.

On the dive in question, I checked at the ramp with all the air finagled out of the suit and had enough weight to accommodate for the consumption of back-gas; it was going to be a short dive. What I did not account for was the gas that the suit retains when ascending horizontally.

 

[Lobzilla]

With all due respect, you need to spend more time actually doing buoyancy checks in the water you are going to be diving with the gear you are going to be using rather than spend all this time on complicated weighting experiments in an effort never to have to do buoyancy checks. :shocked2:

ANY calculations are simply a starting point. But what you cannot "calculate" is how much air a new diver retains in h/h lungs while diving which ultimately plays a big part in the weighting process: newer divers tend to use/need more weight than experienced divers for this very reason.

It's also the reason why log pages have spaces for weighting.

Pax,

Thanks. While I have about two orders of magnitude fewer dives than you, I think that I have progressed past the point of holding my breath in anxiety and flailing my fins on descent/ascent.

The question of the OP was how much weight you need for a new drysuit. Pretty simple: The negative buoyancy of all your gear and lead needs to be equal (or slightly more) than the positive buoyancy of the suit with the diver in it.

The problem is that we do not know the positive buoyancy of the suit-ed diver; but we can get that number rather quickly and learn something in the process.

Let's start with the negative gear buoyancy that can be easily determined by hanging the gear on a luggage scale and submersing it in a pool. We can do this with the gear fully assembled or item by item. I prefer that latter, as I can then calculate the buoyancy for any combination of the individual items.

The tank(s) should be nearly empty since we want to determine the weight needed to 'hang' at the end of the dive when the weight of the backgas is (almost) gone. If the tanks are not empty we simply calculate and subtract the gas weight. The weight of air or Nitrox [in pounds] equals the actual tank pressure [in psi] multiplied by the rated tank capacity [in cuft] divided by the rated pressure of the tank [in psi] multiplied by .075. Subtracting that number from the measured tank buoyancy gives us the buoyancy of the empty tank(s). You may be surprised how much that differs from some tank specs floating around on the web.

So, after fifteen minutes at the pool's edge we have a precise number for all the items with negative buoyancy.

Now, we do the trial-and-error weight check with that gear and the new suit/undergarment. From the additional lead needed and the known negative buoyancy of the gear we can calculate the positive buoyancy of the suit.

Why did we bother to get the numbers if we still need to do the trial-and-error check?

Because the suit/undergarment buoyancy we just calculated is not going to change if we use a different tank, swap the stainless backplate for aluminum, add a canister light, etc. By having the individual buoyancy numbers for each item we can now combine all items in ANY way we want and still know how much lead is needed. (BTW: 10# of lead has a negative buoyancy of only about 9#. Why?)

There is nothing wrong with this calculated approach; I just failed to take the residual gas and increased buoyancy of the suit after a horizontal ascent into account.

Math never fails; but 'garbage in garbage out' applies when cranking numbers.

The next time you fly commercially, ask the pilots whether they calculate the actual take-off weight or whether they just look in their log book for a similar flight. If the answer is the latter, I would choose a different airline

 

[bj139]

With a 7mm wetsuit and 5mm hooded chicken vest I needed 22lbs in fresh water. My first drysuit dive (Whites Fusion w MK2 undergarment) I used 25 lbs and felt over weighted. I added one puff of air to the suit on descent. My next drysuit dive I had -6lb SS backplate and HP117 steel tank( -6lb at 2500 psi). I carried 10 lbs extra lead (total 22lbs). I added no air to the suit and felt slight squeeze at 50 ft; not uncomfortable. I was still slightly over weighted since I was letting the last bit of air out of my BC at 10 ft. at end of dive and could sink on exhale. At that point my tank should have been about -3 lbs. 6+3+10=19. So my suit should be 19lbs buoyant with the medium weight undergarment. These calculations all seem to have tradeoffs. If you need to add air to your DS because of squeeze or you breathe your tank down more than anticipated you should have carried more weight. That must be why there are rocks on the bottom

FYI. There is a buoyancy calculator on SB that seems to get you pretty close.
http://www.scubaboard.com/forums/bu...ems/158370-ultimate-wing-lift-calculator.html

 

[11x]

well 24lbs seems to be the magic number. thanks for all the help