Weighting question

[Teamcasa]

That's actually the kind of tank I just bought.

Teamcasa,

I'm doing a really minor dive on Saturday and will do a weight check as you suggest when I go it. I just don't want to be so overweighted that I jump in and sink right away, you know?

Thanks,

Kristopher

I know, no one like to be overweighted. But really, the best starting point is using what you already know (your last best weighting) and add or subtract based on the tank you are switching to.

 

[knfmn]

I am 235lbs, I dive with 3mm + boots and gloves, al80, weight at 16lbs that is more then enough. Even In a crushed neoprene drysuit I only wear 26lbs.

So where are you getting that formula...? You know there is a bunch of info on this site to help you calculate the weight you might need. I definitely would not go with 10% + 4lbs. Unless the boat captain plans on using you for an anchor...

I now weigh 229. I last dove when I weighed ~245. I'm diving with a 7mm, hood, gloves and an AL80. I used 34 pounds of weight then, and might have been overweighted by 3-4 pounds. Losing weight and having a heavier cylinder has led to my confusion about weighting.

Kristopher

 

[Bubbletrubble]

Rent the same amount of weight you would normally use with the AL80.
If you do this, I'm fairly certain you will have more than enough lead to be properly weighted in your new configuration.

FYI, you've changed two important variables in your configuration: your BCD and your tank.
Due to your recent weight loss, your inherent buoyancy may have changed as well.
Do a proper weight check at the beginning of your first dive.
Don't forget to compensate for the weight of the gas in your full tank.
Pay attention to how much air is in your lungs during your weight check.

Have fun.

 

[Teamcasa]

I now weigh 229. I last dove when I weighed ~245. I'm diving with a 7mm, hood, gloves and an AL80. I used 34 pounds of weight then, and might have been overweighted by 3-4 pounds. Losing weight and having a heavier cylinder has led to my confusion about weighting.

Kristopher

OK, lets say you were over 3-4 lbs and now you weigh a little less and your switching to a water heater (the 120).

Starting @ 34
- 4 based on the previous dive.
- 2 for your weight loss
- 6 for the tank switch
= 22
I'd put on 25 and check, you should be pretty close.

 

[Brendon]

I now weigh 229. I last dove when I weighed ~245. I'm diving with a MM, hood, gloves and an AL. I used 34 pounds of weight then, and might have been overweighted by 3-4 pounds. Losing weight and having a heavier cylinder has led to my confusion about weighting.

Kristopher

One thing you didn't include was height, so I'll give you the benefit.. Assuming your in the 6' range + or - an inch or 2 and being in average shape and muscle mass. lbs will put you on bottom. Everyone dives with different weight.. Some like to be more negative.Thats OK to an extent.. To much will give you bad posture in the water and mean you will have to sling up more weight when your ready to dive. I carry an extra lbs, not for myself as much as others not weighted correctly. You only want enough weight to get you down and keep you neutral at 15' for a safety stop... If you practice that. If your going on a dive soon and still not sure. Go with 26, but break up the weights so you can remove a couple at the surface if necessary, and continue your dive. Hope that helps You were right when you said 10% I didn't realize you were using 7mm... You made find with this new tank that you can remove additional weight. I use a DUI harness, belts like these make it easy for changing your weight when needed. Not saying go out and by one.. However if you change tank types frequently, this will make life easy. Even weight integrated BCD's work well.

 

[Charlie99]

Do you know how much weight you need with an AL80?

If so, then ignore all the crude estimates related to body weight, and just make an adjustment based on the difference in buoyancy between your "steel 120" and the AL80.

The typical AL80 tank is about 4 pounds positively buoyant when empty.
If your steel 120 is a Faber 120 with 3180psi fill pressure and an empty weight of 48.3 pounds, then when empty it will have a negative buoyancy of -7 pounds.

If that is the tank you have, then you will need to removed 11 pounds of lead from your current weighting.

If that is the tank you have, it is 16 pounds negatively buoyant when full.

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan is one of several listings of specifications for tanks.

 

[lulubelle]

I went through the same process when I got my steel tanks. I was glad that I went out to the quarry before hitting the ocean and did a freshwater weight check. I did it with a buddy to whom I could hand off increments of weight at the end of our dives when my tank was at 500. The formulas were not accurate at all for me. I added plus 4 when I hit the ocean and was spot on.

Then after a while I had some weight changes and was traveling and would be diving an AL 80. So I borrowed and AL80 and back to the quarry I went. Plus 5 when I hit the ocean and I was spot on.

I'm probably going to hit the quarry again soon before I go offshore again. I've lost some weight and been working out a lot, so in theory, I should be less buoyant.

Those formulas are only starting points. Do the checks. I'm not carrying anywhere near what they say I should be given my weight and my gear.

 

[Colliam7]

According to the linked specification page, the two most popular AL80's are not exactly the same, and some AL80's are way different. . . . Looking at the 2 linked charts, there is also a wide variety of buoyancy characteristics of HP120's as well. I own Asahi 120's, which are ~3 lbs negative empty. Faber's non-LP steel 120 is 7.7 lbs negative empty, but that is not really an HP tank (3180 psi). Worthington's (119 & 120 cft) are 2 lbs negative empty. . . . What tank(s) are we talking about?

Really good point, and thanks for bringing it up. I have been using the numbers for years, and specifically mean a Luxfer 80 and a PST 120 (my tanks). So, caveat emptor! With the demise of PST, and the ambivalent position of Luxfer with regard to AL scuba tanks, the world is more complicated. (Sigh)

The bigger issue: it is difficult to predict on an a priori basis the precise weight that a particular individual in a particular rig will need, even if formulas for estimation are available (why weight checks with different gear confiurations are appropriate). However, the OP is using a particular formula which provides a starting point.

Do you know how much weight you need with an AL80? If so, then ignore all the crude estimates related to body weight, and just make an adjustment based on the difference in buoyancy between your "steel 120" and the AL80.

And that would be much better, which is why I asked initially about the weight that the OP had used before (? presumably with an AL80).

My suggestion was / is for the OP's specific question - if you have to rent weight before a dive, better to have too much available than not enough. If you rent 25 lbs and need 15, it won't break the bank. If you rent 15 and need 25, and noone else on the dive has extra weight, ... You don't have to use it all if an initial weight check suggests you and your rig don't need it. In the case of the OP referred to, if you go from an AL80, irrespective of manufacturer, to a HP120, irrespective of manufacturer, the probability is you will need less added weight. Whether it is an Asahi, Faber, or Worthington 120, or PST for that matter, a HP120 is likely to be very slightly - moderately negative when empty, while most 3000 psi AL80s are lilely to be several pounds positive when empty. So, reducing the amount of weight you rent by 4 lbs, because you are planning to use a HP steel instead of an AL80, isn't a bad idea.

 

[fnfalman]

Hello all,

I'm in the process of buying my first set of gear and just got my BC and my first tank the other night. The tank is a steel 120, which I've never dove with before. I've got a dive lined up Saturday and have to rent my weights, so I was hoping for some help on getting my weighting right. Here's the calculations I've been able to find. Body Weight X 0.5. 4Lbs for an AL80. 1 lb for every mm of thermal protection. Unfortunately, I'm not sure what to do with a steel tank. Do I take off the 4 Lbs since they are still negative when empty?

Thanks so much for your help,

Kristopher

First of all, here's where you'd go to find buoyancy characteristics of the Faber tanks.

Cylinder Specifications

The Faber FX117 series has a negative buoyancy (that means it will sink) of -9.12lbs when it is filled with air or gas, and a positive buoyancy of 0.16lbs when it's empty (that means it would float if it were empty and needs a hair of weight to sink it). HOWEVER, the data sheet does not include the tank valve, and tank valves are ususally around 2lbs (some are slightly more and some are slightly less, but close enough). That means your tank with valve in place and full with air/gas has the equivalence of wearing 11lbs of lead, and when it's empty has the equivalence of wearing 2lbs of lead.

You want to weight yourself so that by the time your tank runs low (around 500psi or so), you can do a safety stop with no air in your BC and simply stay in place by breathing in and out.

Since that all your gears are new, you don't really know what your dive weight is going to be. So, instead of trying to do the calculations with these formulas, just dive with 10% of your weight, then add or subtract as needed. 10% of body weight while wearing a 7mm wetsuit is pretty close guesswork. I weight 200% and my dive weight (tank (Faber M-series super heavy, steel plate backplate wing, single tank adaptor all added together) is 18lbs, which is a hair less than 10% of my body weight but that's because my wetsuit is well worn in. When new, 7mm wetsuits are super buouyant but they do lose their lifts with use.

My regular diving buddy has the same setup and he runs an actual 10% of his body weight because his wetsuit is newer than mine.

I'd suggest start out at 10% of your body weight plus 4-lbs. If you sink like a rock when you deflate your BC, then the next dive take out 4lbs. If you sink but not fast, then maybe take out 2lbs. Naturally if you don't sink then add 2lbs and see.

The point of the exercise is that with a full tank and an empty BC at the surface, you barely sink under the water when you fully exhale your lungs.

I don't know if you're diving off a boat or what, but if you were then let the deck hands/dive master know that you need to do a weight check and ask them to hand you weights as need. This is how I help other divers do weight checks. Have them attempt to descend and I'll observe. If necessary, I'd ask the deck hands to give me weights to add to the diver that's doing weight check.

 

[knfmn]

First of all, here's where you'd go to find buoyancy characteristics of the Faber tanks.

Cylinder Specifications

The Faber FX117 series has a negative buoyancy (that means it will sink) of -9.12lbs when it is filled with air or gas, and a positive buoyancy of 0.16lbs when it's empty (that means it would float if it were empty and needs a hair of weight to sink it). HOWEVER, the data sheet does not include the tank valve, and tank valves are ususally around 2lbs (some are slightly more and some are slightly less, but close enough). That means your tank with valve in place and full with air/gas has the equivalence of wearing 11lbs of lead, and when it's empty has the equivalence of wearing 2lbs of lead.

You want to weight yourself so that by the time your tank runs low (around 500psi or so), you can do a safety stop with no air in your BC and simply stay in place by breathing in and out.

Since that all your gears are new, you don't really know what your dive weight is going to be. So, instead of trying to do the calculations with these formulas, just dive with 10% of your weight, then add or subtract as needed. 10% of body weight while wearing a 7mm wetsuit is pretty close guesswork. I weight 200% and my dive weight (tank (Faber M-series super heavy, steel plate backplate wing, single tank adaptor all added together) is 18lbs, which is a hair less than 10% of my body weight but that's because my wetsuit is well worn in. When new, 7mm wetsuits are super buouyant but they do lose their lifts with use.

My regular diving buddy has the same setup and he runs an actual 10% of his body weight because his wetsuit is newer than mine.

I'd suggest start out at 10% of your body weight plus 4-lbs. If you sink like a rock when you deflate your BC, then the next dive take out 4lbs. If you sink but not fast, then maybe take out 2lbs. Naturally if you don't sink then add 2lbs and see.

The point of the exercise is that with a full tank and an empty BC at the surface, you barely sink under the water when you fully exhale your lungs.

I don't know if you're diving off a boat or what, but if you were then let the deck hands/dive master know that you need to do a weight check and ask them to hand you weights as need. This is how I help other divers do weight checks. Have them attempt to descend and I'll observe. If necessary, I'd ask the deck hands to give me weights to add to the diver that's doing weight check.

Thank you for this post. I think this was very helpful and much more realistic than trying to dive with the same amount of weight that I was using with an AL80, when I already felt I was overweighted then. I'm actually going to be diving off a dock, so I'll probably rent ~30 pounds and do a weight check from the dock, adding or taking off as need be.

Thanks,

Kristopher