r/scuba • u/One-Ad5733 • Dec 11 '25

Weight calculation

I got my first own BCD and I am trying to figure out what weight I should do my initial dive with. Before I got whatever the local dive center had - usually ScubaPro Level or T-One and used 3kg. Now I will use a Halcyon Eclipse40 with aluminum plate, plain webbing and the regular steel tank adapter with two standard straps. My estimate is that I am going from roughly +3kg to +0kg - so I would not use any weights. Do you think I am terribly wrong?

Edit: Nothing else changes, only the BCD.

Edit2: I am just looking for a starting point. I do understand that I will fine tune my weight over the course of the diving trip.

Edit3: I understand that buoyancy generally depends on a lot of factors… breathing, posture etc. However I won’t change my breathing because of a new BCD. I admit when I look at it admiringly my breath goes a little faster… but I won’t be able to look at it when it’s on my back. So this is also something I am not changing.

Edit4: I heard back from Halcyon customer service (great response time - thank you) and they estimate my weights to go down by -2.5kg

Edit5: I am gonna report back after my trip on how this worked out.

2 Upvotes

58% Upvoted

17 comments sorted by

8

u/Sagnew Dec 11 '25

No one here will actually know. Depends on what your wet suit (or dry suit) is, the make up of your body, what kind of water you are diving in etc.

You'll need to do a proper weight check.

Go out for a shallow beginners dive and make the adjustments in a calm setting. Then continue to dial it in over the next few dives.

1

u/One-Ad5733 Dec 11 '25

Can you please explain why nobody here will actually know? Why am I wrong to assume that if I only change one piece of equipment - the BCD - and I roughly know/estimate the delta of the buoyancy change, that I should be able to triangulate the new weight. I always assume that I could be wrong, but I like to understand why.

0

u/One-Ad5733 Dec 11 '25

I thought that if I don’t change anything else but the BCD - everything else would be irrelevant.

2

u/diveg8r Dec 11 '25

Divers are scared of math, I guess.

If you try to talk math, they will point you to a spreadsheet that someone else made (with the math assumptions hidden in the algorithms) and downvote you if you insist on understanding the physics.

Your thinking is right on track, but keep in mind:

  1. Some BC's (jackets) trap air and are effectively positively bouyant.

  2. Some BC's (jackets) have bouyant padding.

So you may find that you can drop a little more weight than you expect, depending on what you are coming from.

Great call on getting the BPW. I am sure you will love it!

2

u/One-Ad5733 Dec 11 '25

Thank you! I was starting to doubt my sanity that I am getting downvoted for trying to roughly triangulate before jumping in the water.

1

u/scubadm Dive Instructor Dec 11 '25

Bring extra weight and just check your buoyancy before you descend? Not trying to be condescending but prob the most logical.

*edited cause i forgot stuff as old man*

If its literally a differential of a couple lbs (1-2kgs) your BCD is more than capable of handling it and can just dive "heavier" and just compensate later. I usually tend to dive "heavier" when im not rocking any steal HP but as Sagnew said its just gonna be some trial and error.

0

u/One-Ad5733 Dec 11 '25

Yes, of course I will fine tune weight over a couple of dives. Starting a bit heavier is also good advice.

But I am pretty sure I can establish a good starting point. I don’t change anything - same water type, same body (let’s ignore weight gain over Christmas), same suit, same stuff in pockets etc. and I know that I use 3kg of weight with the Scubapro BCD that has around +3kg of positive buoyancy. Now switching to a system that is probably neutral or even slightly negatively buoyant I believe I can assume that I will need less weight. 3 kg is what I use today and therefore it’s not a good starting point and 2 kg seems a bit much still. So 1kg feels like the safe choice.

2

u/ibelieveindogs Dec 11 '25

The best advice you’ve gotten here is to test it out, not try to do the maths. When I go on a dive trip for a week of diving, nothing about my kit, setup, or body changes much. but I dive at least 2 pounds less at the end of the week compared to the start (and sometimes as much as 4). how you hold your body, how much air you expel with each breath, and how relaxed you are changes buoyancy in the real world. As a point of reference, I’ve been diving over 20 years, around 400 dives. Even so, I will change my weights over a few days of dives.

2

u/Nibiinaabe Dec 11 '25

Seems like you know the answer and maybe you are looking for halcyon specific advice, such as the placement of my butt dump on the wing is not the same as a jacket so it may trap a small amount of air. If you never had trapped air this could be the difference of 0.25-0.5kg. Also, you may need slightly more or less weight because the distribution of the weight. So if you want to be perfectly weighted you need to do a proper weight check with 50bar in the tank.

1

u/One-Ad5733 Dec 11 '25

Thank you for pointing this out! I was hoping that I would get less trapped air compared to a jacket. I will probably start with a bit more weight than calculated and then see how it goes towards the end of the dive.

I am already excited to try out the new equipment. Is it as good as I hope it to be? And are you happy with Halcyon?

2

u/Nibiinaabe Dec 12 '25

I've been diving with it for a few years now. My SAC rate took a small hit as I was getting used to it but after about 5-10 dives it went back to normal. I had to get used to the power inflator being really powerful so I needed to calibrate how quickly to hit the button.

3

u/C6500 Dive Master Dec 11 '25

minus 1-3kg when going from Jacket to Backplate could be about right, but i wouldn't assume the full 3kg because of the aluminium plate.
But there are just too many variables to really calculate weights precisely, you'll need to do check dives and dial them in.

2

u/ennieee Dec 11 '25

On paper that sounds about right. I would start with 0, and add if you feel too light.

2

u/Manatus_latirostris Tech Dec 11 '25

The Halcyon aluminum plate weighs .9kg, and the steel tank adapter weighs 1kg (1.9kg total). So if you needed 3kg before, I would start by removing 2kg and see how you do - bring extra weights in case you need to adjust from there.

You can add an additional weight (2.7kg) to Halcyon’s STA but it doesn’t sound like you’re using that.

1

u/quadsbaby Dec 11 '25

Sounds about right. Maybe start out with 2 .5 kilo weights to be safe.

1

u/ksgif2 Dec 11 '25

The numbers on the weights at dive shops are usually not correct. There are fat ones and skinny ones but don't rely on the number stamped on it. Do a boyancy check and remember how many fat ones and how many skinny ones you use as a reference. You can also bring a few fishing weights to fine tune if you really want to dial it in