Calculating Weight Requirements

[jim T.]

Thanks Capt. You beat me to it. (Although I though I explained the 500-1,000 thing in a previous post or two?)

Web Monkey, On most people's doubles sets, there is only one spg gauge used for both tanks to reduce failure points and extra danglies. Some people use two gauges but it's the vast minority from what I've seen and read. I see from your profile that you dive/live in the Great Lakes. Isn't that the common gear set up there?

Yes, "perfect" weighting is what I'm guilty of aiming for. I already found it at 20lbs.
with the doubles. Diving was effortless-like flying, I could maintain my safety stop
and shallower using the 1,000 ACTUAL psi remaining. I'm pretty sure I could do this when I was down to actual 500 psi (250 gauge pressure) once, but I was working on some other new skills, had already done a 20' safety stop and finished the dive swimming up slope so I realized I hadn't paid enough specific attention to be sure.
So I want to try it out.

Now that I've added an HID light (2.5lbs submerged) and my 30cf pony (about 6lbs. submerged) I figure i'm about 8.5lbs overweighted.

I'd like to get as near to my "perfect" buoyancy/weighting I had at the original 20lbs.
without the can light and pony and then "over weight" by no more than 3lbs. or so.
I have to admit, I wouldn't mind being able to add a bit more air to my neo drysuit at depth for warmth. (I mostly use my wing for buoyancy control).

My choices with the weights I presently have at hand: I have a 4 lb. soft V weight between my tanks. If I start by taking that off, I'd be down to 24 1/2 lbs. total. If I take a 5lb wt. off my belt instead I'd be down to 23 1/2 lbs. total.

Part of my decision is whether to keep the v weight over my lungs or to go with taking it off and having more ditchable weight. I can swim up what I have with no air in the suit or wings but I don't like the concept of "balanced" rig on a deco hang or at the surface. So more ditchable appeals even though I like the feel of having the v wt. over my lungs. I like to keep my ditchable wt. to a minimum of 30% of total and maybe even 50%.

For my present recreational diving, I like having it ditchable. For upcoming deco training I'd prefer to keep the v wt. I think...I'm still a tad foot heavy so if I go to more ditchable,
losing the v wt. I have the additional joy of maybe having to go through "all new" trim experimentation again.

I'll be doing some additional training in the spring, so my instructor may have other ideas but for now...advice is appreciated from people who regularly dive doubles.

Thanks.

 

[gcbryan]

Jim T. you don't have 1,000 psi in your doubles when you have 500 psi according to your gauge. You have 500 psi in each tank but it's still 500 psi. You have the volume that a single tank would contain at 1,000 psi but not the pressure (psi). Whether you are comfortable trusting the accuracy of your gauge when it gets to the 250 psi range is up to you.

 

[dave4868]

Thanks Capt. You beat me to it. (Although I though I explained the 500-1,000 thing in a previous post or two?)

Web Monkey, On most people's doubles sets, there is only one spg gauge used for both tanks to reduce failure points and extra danglies. Some people use two gauges but it's the vast minority from what I've seen and read. I see from your profile that you dive/live in the Great Lakes. Isn't that the common gear set up there?

Yes, "perfect" weighting is what I'm guilty of aiming for. I already found it at 20lbs.
with the doubles. Diving was effortless-like flying, I could maintain my safety stop
and shallower using the 1,000 ACTUAL psi remaining. I'm pretty sure I could do this when I was down to actual 500 psi (250 gauge pressure) once, but I was working on some other new skills, had already done a 20' safety stop and finished the dive swimming up slope so I realized I hadn't paid enough specific attention to be sure.
So I want to try it out.

Now that I've added an HID light (2.5lbs submerged) and my 30cf pony (about 6lbs. submerged) I figure i'm about 8.5lbs overweighted.

I'd like to get as near to my "perfect" buoyancy/weighting I had at the original 20lbs.
without the can light and pony and then "over weight" by no more than 3lbs. or so.
I have to admit, I wouldn't mind being able to add a bit more air to my neo drysuit at depth for warmth. (I mostly use my wing for buoyancy control).

I think using the term "actual psi remaining" may just confuse people, since it mixes pressure and volume concepts.

I'm more curious about the feeling you get that:

Yes, "perfect" weighting is what I'm guilty of aiming for. I already found it at 20lbs. with the doubles. Diving was effortless-like flying, I could maintain my safety stop and shallower using the 1,000 ACTUAL psi remaining.

Here's the way I look at it.

From your profile (double hp100's), you are apparently starting the dive with 200 cubic feet of tank gas. That means you'll need to have air in your bcd or drysuit to offset a little over 13lbs of negative buoyancy, the weight of the air in the tanks that would eventually be gone when you get down to 500psi.

Are you able to notice a difference in the "feeling of flying" when 5lbs overweight? 10lbs? 2lbs? Remember, there's a 13lb swing in your buoyancy during "perfectly" weighted dives.

For me, thinking in those terms about overweighting and perfect weighting puts them into a different perspective.

Now, please indulge me a little longer....

Why not figure your perfect weighting on zero (0) psi? Wouldn't that better accomodate worst case scenario?

Hopefully, the reason isn't to avoid having to wear 2.3 lbs of weight to offset the 29 cubic feet of gas that those tanks hold at 500psi.

Tweaking has its limits, especially when there's a 13lb swing in buoyancy anyway.

Just my humble opinion.

Anyway, we haven't even discussed yet whether to have "excess" air in the drysuit for more warmth! Yikes!

Dave C

 

[hudson]

I wouldn't sweat 1-2lbs but once you start going over that you are just exacerbating a non-optimal situation, that is having air in your BC. It's a buoyancy problem, not a drag problem. The more experienced/comfortable you are with buoyancy control, the less it matters but for a newbie like me it could be a big deal.

 

[jim T.]

gcbryan, yes, of course, you're right concerning volume vs. psi. Sorry!
I only am willing to go to 250 gauge pressure when doing buoyancy checks, etc. right off of
the beach in 10-15' of water and only very rarely. I realize that the gauge's calibration is
not dependable in that range.

dave4868, same sheepish apology. Hudson,Yes, I was able to tell a difference with 5lbs. over which frankly surprised me, although I could certainly get used to it. Of course the 8.5lbs I'm now over with the new can light and 30cf is more than sluggish.

Would you guys personally get rid of the 4lb. V weight or the 4-5 lb. belt weight to adjust for the additional light and pony?
One is ditchable one is not. Eventually I'll have a soft overhead/deco to consider but for now it's recreational diving.
Would you have a definite preference for one over the other?
Sounds like 3lbs.over is the generally accepted maximum for most divers, here. I'd rather not consider the light and pony ditchable if at all possible.

 

[Web Monkey]

Thanks Capt. You beat me to it. (Although I though I explained the 500-1,000 thing in a previous post or two?)

You might have but they're not equivalent. Double 95's with 250PSI are much less useful for breathing than a single 95 containing 500PSI. Once you get down to where the tank pressure is below the regulator's IP, it's going to become hard (or impossible) to breathe.

Web Monkey, On most people's doubles sets, there is only one spg gauge used for both tanks to reduce failure points and extra danglies. Some people use two gauges but it's the vast minority from what I've seen and read. I see from your profile that you dive/live in the Great Lakes. Isn't that the common gear set up there?

I can't speak for "most." People around here dive doubles with all sorts of different configurations, including one or two computers and/or spgs.

Now that I've added an HID light (2.5lbs submerged) and my 30cf pony (about 6lbs. submerged) I figure i'm about 8.5lbs overweighted.

That's only true if you don't you don't use the pony. The pony will weigh about 3 pounds less when empty.

I'd like to get as near to my "perfect" buoyancy/weighting I had at the original 20lbs.

"Perfect" will be difficult, since no matter which way you slice it, you're carrying about 18 pounds of breathable gas (assuming double 95's or similar), which means that at a minimum, you'll be 18 pounds "overweight" when you start.

Terry

 

[jim T.]

Web Monkey, I see your point. Thanks.
My "perfect" weighting was based on my safety stops at 15'-20'. I usually finish up shore dives
by spending a fair amount of time swimming around at that depth after coming up a slope.
I guess I also meant perfect in regards to not having to use a lot of air in my wing at deeper depths to compensate for too much weight/achieve neutral buoyancy. Am I making sense?

Regarding the pony. I use it now, only as a bail out bottle. I intend having it full at the end of my dives. So that brings up another question. I should plan on being 3lbs. overweighted at the end of the dive in case I should lose that gas on a dive or use it in an emergency yes?

I'll be going through all of this with my AOW instructor when I get together with him in the spring but want to dial things in properly in the meantime. (He's away doing Cave Cert. training). Thanks for the help guys.

 

[Web Monkey]

Web Monkey, I see your point. Thanks.
My "perfect" weighting was based on my safety stops at 15'-20'. I usually finish up shore dives by spending a fair amount of time swimming around at that depth after coming up a slope.
I guess I also meant perfect in regards to not having to use a lot of air in my wing at deeper depths to compensate for too much weight. Am I making sense?

Wanting it makes perfect sense, but violates the laws of physics.

No matter what happens, if you're perfectly weighted, (given the stated configuration) you will be about 18 pounds negative at the start of the dive, which will require adding about .3 cubic feet (volume at depth) of air to your wing.

Only you can decide if it's "a lot", however regardless, it is what it is. Physics is a harsh mistress.

Regarding the pony. I use it now, only as a bail out bottle. I intend having it full at the end of my dives. So that brings up another question. I should plan on being 3lbs. overweighted
in case I should lose that gas on a dive or use it in an emergency yes?

That's your call, however any gas that would make you positively buoyant if used, is essentially unavailable if you don't have enough weight to cover it.

Becoming positive on a rec dive is annoying and possibly dangerous, but could be fatal on a deco dive, so in reality, you need to be negative enough at the start to offset the loss of all the gas you're carrying.

Terry

 

[jim T.]

Thanks Terry. So it sounds like I was thinking correctly about the pony. I want to treat every dive as the decompression dive it is. If I have to pass off the pony to a buddy, overstay to cut someone out of an entanglement etc. I want to be weighted for that event.

 

[MauiScubaSteve]

In shore dives significant time can be spent near the exit point safely burning air while exploring in shallow bright water.

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.

Completely agree with 2-4 extra is better. Or I should say for recreational diving (no-stop), I feel I am perfectly weighted when I can take a normal breath and maintain my normal pace at 10' deep with 300psi.

Many shore entries in Hawaii have a hundred yards or more of 14' deep or less water before the 4' deep water we stand up in. If you finish your 15-20' SS with 600psi, you might find a number of interesting critters without endangering yourself in the final 300psi of your dive. Or what if the current/surge is significant at 15' and you are working hard and breathing full breaths to maintain heading/depth/position? Or the surface chop from the afternoon trades might be quite bumpy, with significant surface current, so the bottom is much easier and therefore much safer. And how about James Bond when the bad guys are shooting at him; what if he just popped to the surface when he got to 400psi?

How much air do you need to get you and your buddy safely up from 8'