How to estimate the amount of weight with new gear

[inquis]

So if you are weighted right, when you hold a normal breath your head should be just awash

That method relies on your lungs being matched to the non-reserve gas weight in the tank. A better approach, IMHO, is to find the lead that makes your head awash after a normal EXHALE. (This is also more consistent and easy to identify.)

Then increase the lead by the gas weight: 5 lb (AL80), 7 lb (HP100), 9 lb (HP120). (Air weighs 0.08 lb per cubic foot. I knock off a pound for the reserve.)

In metric, add 2 kg (11.1 L/206 bar), 3 kg (13L/237 bar), 4 kg (15L/237 bar).

 

[therookie]

I will 100% bow to your humble opinion @inquis . Just look at our respective dive counts

 

[inquis]

Lol, I actually got it from the manual for @rsingler's weighting spreadsheet that you linked above. Cheers!

 

[fatgeorge]

Which is why most around here will tell you "go buy your own gear so you don't have to fuss with this"...

I'm considering buying a carbon backplate and travel wing, and will eventually be buying a wetsuit (latter is harder to pack with carryon-only travel though). Regulator I'm still planning to rent as it doesn't affect buoyancy (I think).

While I travel a lot, it's always carry-on-only so I have limited space, and there is no reasonable diving here in Bay Area - I've read about Monterey but the water is cold and at this moment I have no plans to ever try drysuit, so no local diving. Mexico is warm though and just 3hr flight south.

 

[rsingler]

Sounds like it's worth a quick reprise on why this subject keeps coming up, and why there are so many "right ways" to check your weighting.

The objective is to be neutrally buoyant at 10-15 feet at the end of a dive with zero air in your bcd.

Why? Because carrying any gas in your bcd at 10-15' makes holding the stop  much more difficult. If you inadvertently go up a couple of feet, gas expansion magnifies the problem. So instead of relaxing as you offgas, you're expending effort (and dwindling tank gas) venting and refilling your bcd to hold at that shallow depth. With no gas in your bcd and proper weighting, you just hang there, easy-peasy.

So why is it so difficult to figure out your weighting, and why are there a dozen different methods?

Because neoprene compression is significant, and buoyancy loss is significant at 15 feet.
Plus, you're trying to estimate buoyancy at the beginning of a dive, in preparation for the end of a dive when you'll be five pounds lighter from emptying your tank.

The past "standard" method of weighting yourself to rest at eye level with a full tank, only applied to (the very common) 5 mm wetsuit with a standard recreational rig, whatever that was.
Except, newer soft stretchy neoprene is more buoyant.
Except, many wetsuits claim to be 5 mm (as an indicator of warmth), but in fact have much less neoprene lined with a water-retaining layer that traps body heat.
And if you have a different neoprene requirement, then your buoyancy change at 15' will be different, requiring a different standard to check your weighting at the start of a dive.

In short, it's complicated.

If you have the ability to empty a tank, empty your bcd, and try things out repeatedly at 15', adding or subtracting a pound of lead at a time, that's the gold standard.

If you're trying to figure everything out in advance of your first dive with a new set of gear, it maybe requires a complicated tool.

 

[Jai Bar]

Once you dialed the weights you need for specific gear configuration (e.g. wetsuit thickness, backplate, tank type etc.), write it down.

It will help you as a starting point for the next time you go diving, then you can fine tune it for the next dives.

 

[AliLuka]

Hello.. do you do a bouncy check before diving?
Maybe today you dive in fresh water and tomorrow you're diving in salty waters.
Without much of calculations, you need to do a bouncy check. Take 2 kg as example and get to the water. Remove all the air from your BCD, keep only half the air in your lungs and don't kick. See where the water level Is. If you're floating this means you need one more kg and repeat. (Positive bouncy)
If sinking this means you need to remove a kg and repeat. (Negative bouncy)
Till you find the water level is at your mask level then you have the neutral bouncy. But how to go down?? Exhale the other half air in your lungs and enjoy your dive.
It's advised to be a little negative not neutral bouncy when diving with aluminum tanks.
Be aware that steel tanks are heavier than aluminium.
Still, doing the bouncy test is the key since you're renting equipment and maybe today you dive with 5 MMS suite and tomorrow it's 7mms
Maybe you use an old 7mms but feels like it's 6 because it's very old.
Just do the test like you do equipment check before diving
Hopefully my answer was simpl

 

[therookie]

In short, it's complicated.

If you have the ability to empty a tank, empty your bcd, and try things out repeatedly at 15', adding or subtracting a pound of lead at a time, that's the gold standard.

If you're trying to figure everything out in advance of your first dive with a new set of gear, it maybe requires a complicated tool

Thanks so much for this post! It answers a few questions that I had about your spreadsheet, and also really helps me understand the iterations I went through working out my weighting.

I'm purely a vacation diver using rented equipment (other than mask, snorkel-when-I-bother-to-use-it, fins and computer) and there are no "local dive shops" other than at my usual dive destination, so it all has to be done at the start of each trip, either before the first dive or on the first safety stop (or both). I've been gradually reducing weight as I've gained experience in the water; then we reduced it to what I thought was too far but in retrospect was probably actually spot on; then on my last major trip I decided I was going to rent a 3mm shorty wetsuit which changed the buoyancy characteristics all over again...

... but the weight that I wound up with that does indeed allow me to "just hang, easy peasy" at the safety stop with zero air in the BCD (and that I think was my problem with thinking I had taken too much weight off - I needed to learn more tricks to get the BCD really zero and not just "I think it's empty now") is a perfect match for this genius spreadsheet. A month after my last major trip I had the chance to sneak in a couple of two-tank mornings on a recent trip to Roatan for other reasons, and when my usual shop asked my weight again I replied without hesitation and it was still absolutely perfect.

For the OP - with everything else (exposure, water type, etc) being equal, if I switch the calcluations from an AL80 to a 15L steel the differences in required lead can be enormous. The different buoyancy characteristics for the seven 15L European tanks in the spreadsheet (or even restricting it to just the HP ones) surprise me - clearly not all tanks are created equal, so even just knowing the cylinder capacity isn't necessarily enough information...

 

[rsingler]

Thanks so much for this post! It answers a few questions that I had about your spreadsheet

Well, it's probably also worth pointing out the reason the tool was created in the first place, which was NOT as a weight calculator.

It's easy to find "errors" in the weight results it gives you (though I'm glad it's turned out well for you). The whole subject is so complex that not having all the correct input data will give a spurious result.

But that was never the real point. What the tool shows so nicely on the Wetsuit tab is that you can simply jettison a significant portion of your weight, should your bcd tear on coral, or should the corrugated hose connection come loose, and you won't rocket to the surface and get bent.

Instead, you can swim up at modest initial effort, and eventually at minimal effort, to a point at which you are neutral in the water, and can prepare your deceleration strategy for a mildly positive final ascent as originally suggested by @johndiver999 in a contentious thread many years ago.

The tool shows the math behind buoyancy, and is a small effort at reducing through education, the tendency toward overweighting so prominent in the training and resort communities.

 

[therookie]

This conversation has definitely motivated me to look more closely at the tabs on the spreadsheet after "QuikResults", that's for sure...

The beginnings of the first glimmers of understanding about why proper weighting is so important are finally beginning to penetrate my rookie brain. It's not just about improving my gas consumption and the ability to hold the safety stop with less effort; it's also about the proper reaction in the (admittedly rare) event of a BCP/wing failure... My wife's rule of thumb for all things (not just scuba) is that if you are well-prepared for something it is even less likely to happen - it's the stuff you didn't prepare for that bites you - so this information makes both me and her happy!

Thanks so much @rsingler for all the time spent on the spreadsheet, and providing some additional guidance/clarification in this thread (and no doubt others). Greatly appreciated!