Dive boat etiquette and buoyancy check

[Cio]

Doing a weight check on the way in will potentially hold up the whole group. If everybody else is waiting at the bottom of the mooring burning through their air, you won't be very popular on the boat.
My plan would be to overweight on the first dive and do the check between dives once the boat is moored at the second site or sort out the issue before hand. If you try to do a buoyancy check at the end of a dive you will have an empty, buoyant tank so not the solution.

I have never heard the notion that a weight check with an empty tank is a bad idea, in fact it is the most accurate way to do it.

 

[Texasguy]

I have never heard the notion that a weight check with an empty tank is a bad idea, in fact it is the most accurate way to do it.

But very impractical to bring an empty tank to a dive.

 

[scrane]

Doing a weight check on the way in will potentially hold up the whole group. If everybody else is waiting at the bottom of the mooring burning through their air, you won't be very popular on the boat.
My plan would be to overweight on the first dive and do the check between dives once the boat is moored at the second site or sort out the issue before hand. If you try to do a buoyancy check at the end of a dive you will have an empty, buoyant tank so not the solution.

I think this is the worst advice I've ever read on ScubaBoard. It doesn't make sense in many ways.

 

[Alurpal]

Doing a weight check on the way in will potentially hold up the whole group. If everybody else is waiting at the bottom of the mooring burning through their air, you won't be very popular on the boat.
My plan would be to overweight on the first dive and do the check between dives once the boat is moored at the second site or sort out the issue before hand. If you try to do a buoyancy check at the end of a dive you will have an empty, buoyant tank so not the solution.

The goal is to be neutrally buoyant with empty bc, at 15ft, with an almost empty tank.

1cf of air weighs 0.0765 lbs so with al80, difference between full tank and 500psi is about 4.9 lbs. So, I can test neutral buoyancy on surface with a full tank and then add about about 5lbs to what gets me there.

 

[Ayisha]

While I agree with most of this, doing a buoyancy check with a near empty tank is actually a good way. You should be able to sit at 15' deep with an empty tank if you are weighted correctly with little/no air in your bcd.

This is good advice, but at 10 feet/3m is even better.

If you're doing the weight check after the dive with a nearly empty tank and BCD at 15 feet, you could still be slightly underweighted.

Doing the check at 10 feet and making sure you can make a slow, controlled ascent all the way to the surface will ensure you are not underweighted.

 

[custureri]

But very impractical to bring an empty tank to a dive.

You don't need an empty cylinder to do a weight check. This is an out of date approach. The only thing a person needs to know is their weighting in whatever exposure suit they put on. The rest is static.......

Al80 empty is +4.7lbs without the valve, most standard pro-valves weight 1.1-1.3lbs. +4.7 - 1.3 = 3.4lbs. Call it +4 for good measure. Most regulators weigh ~2lbs negative. No Jacket BCD is actually neutral, most weigh 3-4lbs positive. It's literally understanding math & basic physics.

Simple formula: positively buoyant minus negatively buoyant = delta. If delta is "+", add weight till it reaches 0. If delta is "-", you're good to go as long as it isn't more than 2-4lbs over.
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For the OP, If you are in Fort Lauderdale, just come by our shop and we will do a proper weight check for you. It takes about 2-3 minutes and doesn't require gear. This way you can skip the guessing games and enjoy your dives. Free of charge.

 

[inquis]

It's literally understanding math & basic physics.

And guesswork as to wetsuit buoyancy -- the largest uncertainty in my experience. Unless you also measure that for them?

 

[yle]

I’ve been diving my new gear in freshwater and plan to do a boat dive in Fort Lauderdale in a couple months. All my previous diving has been in saltwater with rental gear on boats and I would purposely overweight but now I’m more concerned with buoyancy and trim.

My question: is it okay to ask the crew if I can do a quick buoyancy check before descending on a dive boat since I’ll be in different density water than I’m used to? Or is this something I should sort out before the dive?

Thanks!

Saltwater is about 3% denser than freshwater, so the upward buoyant force provided by saltwater is about 3% more than in freshwater.

When you're neutrally buoyant in freshwater, the upward buoyant force is equal to your weight... i.e. the weight of you and all your gear.

This means the increase in the buoyant force when you move to saltwater will be about 3% of your total weight (not the lead weight... your total weight, you and your gear.)

Many people weigh roughly 200 lbs with their gear... although some weigh more and some less. So adding 6 lbs (i.e. 3% of 200 lbs) is a rough guideline for going from freshwater to saltwater. You can get a better estimate if you know your total weight. Scuba gear (BCD, tank, regulator...) weighs about 50 pounds. If a person weighs 200 lbs without scuba gear, their total weight while outfitted for the pool (including fins, wetsuit, lead...) could approach 300 lbs. This person would need to add 9 lbs of lead going to saltwater.

 

[inquis]

Tell me this is not a brilliant solution.

Ok, this is not a brilliant solution.

I missed how you get around wetsuit compression at depth, or the fact you should still have a fair bit of gas in reserve weighing you down before you leave the bottom. Can you elaborate how you know how much of what you took down is "extra" to be discarded before the next dive?

 

[custureri]

And guesswork as to wetsuit buoyancy -- the largest uncertainty in my experience. Unless you also measure that for them?

I literally addressed that when I said: The only thing a person needs to know is their weighting in whatever exposure suit they put on. The rest is static.......

So, to be even more specific:

-Put Suit on. Walk in water, or use back of boat, pool, etc.
-Start with 2lbs, hold to chest, exhale completely.
--Did you sink? Did you Float? Were you neutral? Neutrality is NOT eye-level, it is when you are just BELOW the water line suspended with half lung volume. Eye-level = still positive.
-If you didn't sink, grab 2 more. Repeat until you stay on bottom.
-Determine weight (again, takes only a few moments), then apply that in the POSITIVE column to add; whatever you needed.

I have a more in depth write up, i'll edit link to site. Currently running updates with the site as we speak so blog will be temporarily down.