Question Shallow post-dive weight check in pool?

[Erik H]

Hello everyone. I want to dial in my required weight in preparation for a liveaboard trip to the Bahamas that I will soon be doing. I haven't done a weight check yet as I don't have that many dives under my belt, and have been switching between rental drysuits and wetsuits in different conditions. I recently got an aluminium backplate and wing setup and own a 5mm full wetsuit, both of which I will be using on the liveaboard trip. Ideally, I want to do an "end-of-dive" weight check with a cylinder at 50 Bar. I know that you're supposed to do the check at safety-stop depth (5m/16ft), but I only have access to a pool that is 4 meters deep (even shallower if I'm staying buoyant off of the bottom). Is it possible to do a "proper" weight check even at shallower depths? Anything I need to think about when doing a weight check at the "wrong depth"? Another factor is that I'll be doing the weight check in a pool but will be diving in salt water. To complicate things further, I can only get rental steel tanks here, but will be diving with al80 tanks in the Bahamas though they apparently have al100s. Any suggestions on how to get a good weight check done with all things considered?

Thanks All!

 

[Outbound]

Just do it at the start of your first dive in the Bahamas. Let your buddy know you need to do a weight check before you descend, and do it the way you were trained. Then, to really dial it in further, do another one at the end of your dive after breathing or purging your tank down to 50 bar/500 psi at your safety stop.

 

[inquis]

Based on Boyle's law applied to the bubbles in the neoprene and assuming 5.5 kg surface buoyancy for that 5mm suit, the buoyancy at 3.5 m is about 0.4 kg more than at 5 m.

At average salinity, buoyancy increases by 2.4% of total dry weight (you+rig+tank+etc) compared to fresh water. That's about 2.3-2.7 kg for single-tank diving, mostly depending on how much you weigh.

There's about 0.9 kg buoyancy difference between a Faber HP100 steel and Luxfer AL80.

Consequently, if you weight yourself to be neutral at 3.5 m with HP100 in fresh water, a first cut adjustment for 5m/salt/AL80 would be around 3 kg (= -0.4 + 2.5 + 0.9 lb).

Refine after another check on Dive 1 of the trip. (Mention this to the DM, so he won't freak out when you're draining your tank at the safety stop. )

 

[Erik H]

Just do it at the start of your first dive in the Bahamas. Let your buddy know you need to do a weight check before you descend, and do it the way you were trained. Then, to really dial it in further, do another one at the end of your dive after breathing or purging your tank down to 50 bar/500 psi at your safety stop.

When you say "at the start of the dive", do you mean the process of dealing in the weight while at the surface with an empty BCD and floating eye-level with a half breath in? While I've thought of doing that, it just seems for me to be a less accurate way of checking.

I was actually never really "trained" or taught to do a weight check and never done one at all. The reason why I haven't done a weight check at all is because I have been very inconsistent in which exposure suits and BCDs I've used (a variety of different rentals for different diving situations). I've gone from neoprene drysuits and thick undergarments for 1°C water, to diving tropics with a 3mm shorty, and always with rental equipment.Thus, I haven't had the opportunity to do a check, and know that the equipment I'll us on one dive may not be the same for the next dive. Doing any weight check had seemed like a waist of effort till I could get my own equipment as I have just done. So far I have just been guesstimating my weighting but have felt that I've been overweighted.

I have also thought of the possibility of doing the weight check at the end of the first dive like you suggested. But as I've understood, that process could and might require removing some weights mid-dive till I'm at the correct weight. Doing that at the end of the dive in open water would likely be infeasible, which is partly why I asked about doing it in a pool so that I could temporarily place extra weights at the bottom. Also, I would like to be able to do other weight checks in the pool as I get other exposure suits and equipment.

 

[Erik H]

Based on Boyle's law applied to the bubbles in the neoprene and assuming 5.5 kg surface buoyancy for that 5mm suit, the buoyancy at 3.5 m is about 0.4 kg more than at 5 m.

At average salinity, buoyancy increases by 2.4% of total dry weight (you+rig+tank+etc) compared to fresh water. That's about 2.3-2.7 kg for single-tank diving, mostly depending on how much you weigh.

There's about 0.9 kg buoyancy difference between a Faber HP100 steel and Luxfer AL80.

Consequently, if you weight yourself to be neutral at 3.5 m with HP100 in fresh water, a first cut adjustment for 5m/salt/AL80 would be around 3 kg (= -0.4 + 2.5 + 0.9 lb).

Refine after another check on Dive 1 of the trip. (Mention this to the DM, so he won't freak out when you're draining your tank at the safety stop. )

Thank you for all the numbers. I am admittedly a math nerd myself and like to have numbers and formulas to refer to.

If I've understood right, doing a weight check at a shallower 3.5m is feasible and won't screw up the numbers when compared to a "proper" check at 5m? I've understood that wetsuits do compress and loose buoyancy depending on depth, and that different tanks have different buoyancies depending on steel vs. aluminium as well as reserve pressure. I guess I wanted some assurance that doing a check in a pool at 3.5m can be done and isn't completely stupid thing to do as it's not at 5m...

As long as I can figure out the numbers and weights in the pool, I could convert them to something usable for the dive trip? I end up renting steel 12L tanks when diving here, so I don't have much experience with Al80s.

 

[rjack321]

The first dive(s) on any trip are typically shallow and an opportunity for the crew and yourself to get familiar with the people and gear you're using. I would talk to the DM and ask if there's a suitable time/place near the end of the first day of diving to do a weight check. (possibly the first dive, possibly the second)

 

[grantmac]

Just do a surface weight check with full tank then add 75% of the air weight on top of what you need. Gets you VERY close.

 

[Erik H]

Ok, then doing a weight check on the first dive seems like it's possible, and maybe even a regularly done thing on liveaboard dives? I'll still try doing a check in the pool as deep as I can now that I see that it is feasible at a slightly shallower depth, but have the first dive as a fallback option.

Thanks!

 

[MidOH]

I just start the 1st dive heavy. Remove weight if I'm too sinky at the end of the dive.

I have a 30# wing in warm water, where I need 15 tops. I err towards too much weight, rather than too little. Use it for trim.

 

[dberry]

First, most every dive op and liveaboard has you do a check-out / weight check dive. And there's no shame in asking for a weight-check in the middle of the week, too.

I'm a math nerd, also, and early on I tried to calculate my exact buoyancy for different gear set-ups. Eventually I went the empirical route and just did jumped in and adjusted as needed. I have a sense of where to start and still use the first day weight-check to dial it in for any small changes. Note that a DM (if you have one in the water) will typically carry 1-2 lbs to give as needed (or take a few lbs back from you.) Most DMs can eyeball you and your gear and get you pretty close at the start.

That said, logging your gear and required weight is great way to get calibrated.

As you get more experienced you'll likely start using less and less weight - just enough to keep the stop. Diving overweighted is typical when you start out, but incredibly annoying after you've honed your skills and trim.

In a perfect world of warm-water diving I'll be in swim trunks + skins with maybe 2-4 lbs of lead on my cam-bands (Aluminum plate + travel wing), despite carrying way too much floaty "bioprene".

Disclaimer: as mentioned above, the more neoprene you wear while diving deeper you have to watch out for being too heavy at depth due to the neoprene compressing and losing buoyancy. IMO it's not too likely to be a problem in the Bahamas, unless you're covered in 7mm.