Need a second (or more) opinion on my buoyancy math

[tuk]

I have a dive tomorrow and I'm not only a little rusty (no open water dives in over a year but some pool time for gear testing) but I'm also diving new tanks. I did some bath tub tests and I'm coming up with 35lbs of weight. This seems a little high but I'm setup sidemount and I have a pretty thick suit for the cold. My old tanks are Worthington HP100s and the new ones are Luxfer AL80s. Anyway, here are the numbers I came up with using weights and a luggage scale in my bathtub:

* Luxfer AL80: +4lbs (x2 and using 4 instead of 4.1 since I'm diving freshwater)
* Long hose setup: -2.4lbs
* Short hose w/ cobalt computer: -2.6lbs
* Gloves + Boots: +2lbs (tested with a 2lb weight and a plastic bag)
* Xdeep Stealth 2.0: -1lb (I soaked the luggage scale, so this is a guess since it's almost neutral)
* Aqua Lung SolAfx 8mm (M): 25lbs (really? more than 3lbs per mm?)
* Scubapro Jet Fins w/ spring straps (L): -1lbs (combined. also an estimate because the scale is dead and I'm using numbers I found elsewhere. could be more neg than that)
* Me: +7lbs (5'7", 160lbs, tested in a bath tub... as close as I could get in a bath tub)

4 + 4 - 2.4 - 2.6 + 2 - 1 + 25 - 1 + 7 = 35

I'll have a couple flashlights and I haven't taken the wing into account but it's almost nothing without air. So the math seems to indicate that 34-36 is about right. Since I like to err on the side of heavy, I'm loaded up with 36. (I'm almost positive I'm not going to be anywhere near trim and will end up floating ass over tea kettle...)

Does 36lbs sound crazy or am I just biased by only ever using steel with this suit? Before you ask, I don't know how exactly how much weight I was using with the HP100s. Best guess is in the low to mid 20s. I may have been in better shape (not a +7) but diving salt [+(N * 1.025), right?]. Maybe around 28... which would make this all make sense...

I won't check again until the morning but hopefully someone with more experience can double check my work and laugh where appropriate.

HAPPY NEW YEAR!

 

[RainPilot]

25 lbs for a 8mm suit seems about right, my 7mm was 20 new (size medium)
Just remember that 4.1 positive for the AL80 is tanks only, the valves are a pound or 2 negative. I usually have around 2 lbs positive empty in salt.

You will also have 12 lbs of gas weight at the start of the dive, plus losing 80% of the suit buoyancy at depth means being at least 30 lbs over at the beginning of the dive, keep an eye on your wing size.

 

[Patoux01]

Sounds like an insane amount to me... Do a proper weight check at the end of the dive.

 

[mainedvr]

Sounds like an insane amount to me... Do a proper weight check at the end of the dive.

I agree the math will get you in the ball park but the weight check will be what puts you spot on.

For example my last dive I dove a new undergarment and doulbe steel 100s did my weight estimate and was definitly off on the light side as I could tell by how it felt trying to decend. if I had low or empty tanks on a blue water accent I would have struggled to hold a stop. Luckily I knew this could be an issue and the dive was one that would not put me in an unsafe situation.

Good luck

 

[tuk]

25 lbs for a 8mm suit seems about right, my 7mm was 20 new (size medium)
Just remember that 4.1 positive for the AL80 is tanks only, the valves are a pound or 2 negative. I usually have around 2 lbs positive empty in salt.

You will also have 12 lbs of gas weight at the start of the dive, plus losing 80% of the suit buoyancy at depth means being at least 30 lbs over at the beginning of the dive, keep an eye on your wing size.

I hadn't even though about valves! There goes 4lbs. Thank you.

The wing is rated at ~25lbs (16kg) so it should be OK on lift.

if I had low or empty tanks on a blue water accent I would have struggled to hold a stop. Luckily I knew this could be an issue and the dive was one that would not put me in an unsafe situation.

Good luck

That's why I'm shooting for over rather than under. New setup (tanks at least) and a spot I don't know. I don't want to be That Guy.

 

[mainedvr]

I hadn't even though about valves! There goes 4lbs. Thank you.

The wing is rated at ~25lbs (16kg) so it should be OK on lift.



That's why I'm shooting for over rather than under. New setup (tanks at least) and a spot I don't know. I don't want to be That Guy.

I hear that

 

[redacted]

I suspect 25 is to much for the wetsuit since it is a medium. And I am pretty sure 7 for you is way too high. I bet you will drop at least 10 lb from that estimate.

 

[boulderjohn]

Just for comparison, if I am diving sidemount (Hollis SMS 75) with Worthington steel LP 85s with my dry suit in fresh water, I don't use ANY weight. When I was diving sidemount with AL 80s in fresh water and a 5 mm wet suit, I used 4 pounds.

 

[dumpsterDiver]

is your dive in fresh or salt water?

 

[Steve_C]

Believe he said fresh.

I question how you tested boots and gloves. The plastic bag gives more air space. As soon as you start to descend the water will force out a lot of air. I rinse my gear after a dive. Before a dive my gloves will float up in the water. After a dive they float much lower.

Are you naturally bouyant? If I were in a lake and you handed me a 7 pound weight there is no way I would float. No even sure that I could float holding my breath. But then I am almost neutral in fresh water.

Same for wet suit. It is much less buoyant on dive 2 and at the end of dive 1 than it was on the start.

I gather from your math that you are diving either doubles or side mount. Is that correct?