Calculating Weight Requirements

[dave4868]

Many people seem to seek "perfect" weighting, that is, the absolute minimum weight and minimum bcd/drysuit air that would still allow one to hold neutral buoyancy at a specific safety stop depth and at a specific psi of remaining tank gas.

Others, like myself, prefer "excess" weight, despite the trade-off of "excess" bcd/drysuit air, because we want to always be able to achieve negative or neutral buoyancy even at the surface with near zero psi. Rationales include:

• Safety (being able to maintain or maximize a more critical safety stop; more easily avoiding hazards on the surface)
• Dive profile (cruising in the shallows at the end of a dive, working in shallow water)
• Buoyancy control (quicker changes may be desirable, rather than contorting the drysuit position to squeeze out that last little bit to get neutral)
• Warmth (more air in the drysuit)

Some of the rationales may come down to personal preferences for particular benefits and trade-offs, but can someone provide a good argument or rationale for choosing the following commonly used criteria?

• achieving minimum "perfect" weight
• neutral at a safety stop of 15-20'
• 500 psi remaining

Is it primarily to reduce the gas in the bcd and/or drysuit to make buoyancy control easier during part of the dive?

And does one have to assume there is only a remote chance of needing to hold a stop while draining a tank close to zero psi?

Am I missing something? (Wouldn't be the first time....)

Dave C

 

[captain]

Many people seem to seek "perfect" weighting, that is, the absolute minimum weight and minimum bcd/drysuit air that would still allow one to hold neutral buoyancy at a specific safety stop depth and at a specific psi of remaining tank gas.

Others, like myself, prefer "excess" weight, despite the trade-off of "excess" bcd/drysuit air, because we want to always be able to achieve negative or neutral buoyancy even at the surface with near zero psi. Rationales include:

• Safety (being able to maintain or maximize a more critical safety stop; more easily avoiding hazards on the surface)
• Dive profile (cruising in the shallows at the end of a dive, working in shallow water)
• Buoyancy control (quicker changes may be desirable, rather contorting drysuit position to squeeze out that last little bit to get neutral)
• Warmth (more air in the drysuit)

Some of the rationales may come down to personal preferences for particular benefits and trade-offs, but can someone provide a good argument or rationale for choosing the following commonly used criteria?

• achieving minimum "perfect" weight
• neutral at a safety stop of 15-20'
• 500 psi remaining

Is it primarily to reduce the gas in the bcd and/or drysuit to make buoyancy control easier during part of the dive?

And does one have to assume there is only a remote chance of needing to hold a stop and while draining a tank close to zero psi?

Am I missing something? (Wouldn't be the first time....)

Dave C

I don't see a where the weight of 500 psi in an 80 is an issue. The difference between 500 psi and 0 is about 1 pound. I can swing my bouyancy at least 5# to 7# with lung volumn. I always prefer to use the least weight necessary.

 

[Kingpatzer]

Just to drop back in here -- I'm the OP btw -- and say thanks for all the input, but it's seems like many folks are missing the question I asked ...

I am not asking about getting perfect weight.

I'm saying that I've heard people comment that there are a half dozen ways to calculate how much weight one needs, but all I've ever seen explained is the PADI method of jumping in with a thank, taking a breath, floating at eye level and adding 4-5 more pounds.

What I am asking is what the other methods that I keep hearing mentioned as existing but never see explained are.

So, what I've seen outlined so far is to take a nearly empty tank, a bunch of extra weight and try and get neutral at 10-15', ball parking based on gear, measuring each piece of gear, and the eye float method. Are there others?

 

[dave4868]

Just to drop back in here -- I'm the OP btw -- and say thanks for all the input, but it's seems like many folks are missing the question I asked ...

I am not asking about getting perfect weight.

I'm saying that I've heard people comment that there are a half dozen ways to calculate how much weight one needs, but all I've ever seen explained is the PADI method of jumping in with a thank, taking a breath, floating at eye level and adding 4-5 more pounds.

What I am asking is what the other methods that I keep hearing mentioned as existing but never see explained are.

So, what I've seen outlined so far is to take a nearly empty tank, a bunch of extra weight and try and get neutral at 10-15', ball parking based on gear, measuring each piece of gear, and the eye float method. Are there others?

There are other methods, depending on what one is trying to achieve, which is why my post isn't really a hijack. Sorry if it appears to be one.

The PADI method in my 1994 OW manual (pages 115-116) actually doesn't mention adding any weight after getting that "floating at eyeball" with a full tank weighting.

They also state it's an approximation of "optimal weighting" and go on to say that one of the reasons it's optimal is that "When you return to the surface, you'll have slight positive buoyancy."

They probably intend to add a little safety for the new diver with such a method.

The method gets tweaked by all of us in different ways, depending on our preferences and goals, which is what my post was getting into.

In the "empty tank" in-water method you described, one doesn't even need to measure each piece of gear, just put them on and adjust weights to get the level of buoyancy the way you want it.

Another variation: full tank, adjust weighting at the surface to the desired buoyancy, then add weight to offset the calculated maximum weight of air you plan on using.

Final in-water tweaking will be needed, especially if you want "perfect" weighting at a certain safety stop and not at the surface. Easy peasy.

Lots of methods.... and lots of rationales, I might add.

Dave C

 

[Web Monkey]

What I am asking is what the other methods that I keep hearing mentioned as existing but never see explained are.

So, what I've seen outlined so far is to take a nearly empty tank, a bunch of extra weight and try and get neutral at 10-15', ball parking based on gear, measuring each piece of gear, and the eye float method. Are there others?

The only thing that's 100% accurate is an actual measurement of how much weight it takes to let you hold a stop just below the surface, under the worst circumstances you would need to handle.

The worst case would be that you're almost out of air in all your tank(s) and breathing like a hoover.

Since this will typically leave you significantly overweighted, many people choose to take the measurement in slightly less severe conditions, and assume 500 PSI in the tank and normal breathing.

None of the methods that involve various calculations will be as accurate as an actual measurement in actual conditions.

Just to muddy the water a little, whatever weight you come up with isn't etched in stone, and will be effected by your general anxiety level, the water conditions (currents, etc.), your trim while diving, how "gassy" you're feeling, and even the actual weights you use. SCUBA weights are not precision devices, and if you do a weight check with one set, then rent a different set, it's entirely possible to be off by a pound or two.

If you wear a drysuit, your weight requirements will vary tremendously, depending on the thickness of your underwear and how "fluffy" (warm) you feel like being on any given day.

The short answer is you need to get in the water and try it, then add, remove and move weights over a number of dives to get you the weight and trim that works for you.

Terry

 

[captain]

I think what most are saying is you can't really do it with pencil and paper. It's in the water trial and error. The most you can reasonably calculate is the difference between what you use in salt vs fresh water when you actually know one or the other.

 

[medic001918]

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.

Assuming you're using an isolator that's open, you don't really have 1000 psi of gas. You have 500 psi. For the purposes of weighting, you would want to treat it as one large tank. This is how I understand it anyway.

Shane

 

[jim T.]

Hi medic,
I had used psi in place of volume in the early posts in this thread (which I knew better to do and felt rather foolish about). It was really weighting for that volume that had me confused. I also hadn't taken Reg IP levels into consideration and learned some things about that. That's the great thing about forums.

My original post was really meant (essentially) to ask about whether or not to weigh test at the volume of one tank's worth at 500psi or two tanks at 500psi.

 

[LarryHinDC]

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.

I'm not a doubles diver, but I think that the answer is to do the check at whatever pressure is considered "empty" for your rig. For singles, that's the 500 psi mark.

 

[jim T.]

Just a follow up, as everyone was so helpful regarding my questions and forgiving regarding my volume vs. psi mistake.

I did a weight check at 500 p.s.i. in the HP 100 doubles. I was able to drop 5 lbs. off of the belt as I'd added the can light and was slinging the 30cf pony. I was neutral at 15 ft.
with no air in the suit or wing. I was able to hold a 10 ft. stop at both 500 psi and 250 p.s.i. gauge pressure so that answered that question! Thanks for the help and thanks to the OP for my sort of highjack...
Everything totaled 23.5 lbs.
Jim