How do you calculate weight with steel tanks ?

[Glen Rushworth]

New to the board, trying to determine which steel tank to buy. On rental Al 80's, I'm properly weighted (after buoyancy course/51 shore dives to date) with 34 lbs. No problems carrying this set up now. Male, age 57, in a Bare Nexgen dry suit and diving cold water. I'm an air hog, so need more air. Cold water doesn't help air consumption. Last dive water temp 39 F. Air determines the length of my dives, never NDL.

This is the problem: I understand that you can remove 6.4 lbs of lead (the difference) when the buoyancy on an empty Al 80 tank is +4.4 ( positive ) and say a Worthington X 8-119 is -2 ( negative ). That I get.

What I don't understand is at the start of the dive. AL 80's are -1.4 (negative) and the Worthington is -10.9 (again negative) The difference is 9.5 lbs. How do I apply this difference compared to removing the 6.4 at the end . Do I add them together ? and then would I be underweight at the end by removing a total of 15.9. Or is 6.4 the total and I would just drop like a rock at the start. This last part doesn't make sense.

I know that I'll need to get my carried weight narrowed down again after new tanks. I'm just trying to understand the math behind it. This would help me determine which tanks to get. I appreciate any and all help. Thanks

 

[paddler3d]

If you buoyancy is spot at the end of your dive with on with 34#'s of lead using an Al80 and the difference in buoyancy empty between the tank you want to dive and are currently diving is 6.4#'s, ditch the 6.4#'s off of your belt.

With steel tanks, you'll likely be a bit heavy at the beginning of your dive, but it is the end buoyancy that is important.

 

[Glen Rushworth]

Thanks Paddler3d. Your answer makes sense to me. Problem is I was hearing that I'll be able to drop up to 15 lbs. ( from reliable sources ) That was the part that I didn't understand. I also read recently on Scubaboard about a guy that went from AL 80's to Worthington 100 's and dropped 10lbs. This is more than the empty bouyancy difference between the tanks .

Thanks again

 

[ligersandtions]

Thanks Paddler3d. Your answer makes sense to me. Problem is I was hearing that I'll be able to drop up to 15 lbs. ( from reliable sources ) That was the part that I didn't understand. I also read recently on Scubaboard about a guy that went from AL 80's to Worthington 100 's and dropped 10lbs. This is more than the empty bouyancy difference between the tanks .

Thanks again

It's more likely that the guy you read about on SB didn't have his weighting down with the Al80. If you have your weighting down (i.e. you can hold your final stop with no air in your wing and little air in your drysuit, if applicable, with <500psi left in your tank), then your weighting difference should be ~6 pounds, as you mentioned in your post.

As Paddler mentioned, you will be more negative at the start of your dive than you would be with the Al80, but what determines your weighting is your buoyancy at the end of the dive. You want to be able to maintain all stops and do a controlled ascent....it's better to be slightly overweighted than slightly underweighted.

 

[muddiver]

You will also find that it depends on the exposure protection you wear as well. I usually weight to be neutral at 60 feet of depth and I end up being positively buoyant on the surface, yet close to neutral at 20 feet of depth, if I am wearing a 6 mil farmer john wet suit.

When I change to my dry suit every thing changes because I am not dealing with a change in volume as I go deeper, you add gas to the suit and volume of the suit stays relatively the same.

Ideally you can do a buoyancy check with nothing but your exposure protection on and you will have your baseline for being neutral. Then you adjust from there for the cylinder you are going to use.

 

[Glen Rushworth]

Thanks for all the info everybody, I'm assuming you are saying " slightly overweight " at the start of the dive due to the -9.5 lbs , the buoyancy difference between full Al 80's and the X 8 119, will actually be -3.1 lbs due to the removal of my 6.4 lbs. The 6.4 is calculated from buoyancy differences at the end of the dive. Correct ?

 

[paddler3d]

he 6.4 is calculated from buoyancy differences at the end of the dive. Correct ?

Yes.

At the end of your dives you want to be able to hold your SS. You need to compensate for the 'swing' on the tank, going from full of air to not so full of air or less buoyant to more buoyant.

 

[Glen Rushworth]

Thanks to everyone for all your help. I'm still a little perplexed as to why I was told that I will be able to take off 15 lbs. Next step is to ask them. Thanks again

 

[fnfalman]

If you're properly weighted for end of dive as discussed above, you're not going to "drop like a rock" at the beginning of the dive because you've already taken 6-lbs off your weight belt. So, yes, when you use the steel tank, you are going to be more negatively buoyant at the beginning of the dive when compared to the AL80. BUT because you've already taken 6-lbs off, you're not going to be THAT negatively buoyant.

 

[ligersandtions]

Thanks to everyone for all your help. I'm still a little perplexed as to why I was told that I will be able to take off 15 lbs. Next step is to ask them. Thanks again

Heh, because someone doesn't know what they are talking about. It's not uncommon....you have to learn to figure out who knows what they're talking about and who thinks they know what they're talking about.

Do ask the person why they think they can drop 15 pounds, and if they've done weight checks at the end of the dive (shallow, <500psi in the tank) with both set-ups and are properly weighted. Maybe they changed two things at the same time (added a steel tank and changed the drysuit for a 3mm shortie???).

Best of luck! Have fun diving with the steel tank