Tank Change Buoyancy Question

[runway1]

I'm switching from a Steel 100 to a Steel 130. Full/empty numbers are:
100.....-14.76 to -7.26 pounds..delta = 7.50 pounds (start/end of dive)
130.....-11.70 to -2.00 pounds....delta = 9.20 pounds (start/end of dive)

Delta full = 3.06
Delta start/end = 1.7
Delta empty = 5.26

So, what's the weight difference? Would you go by the weight deltas from start/end, weight delta when full or weight delta when empty number?

 

[pasley]

100.....-14.76 to -7.26 pounds..delta = 7.50 pounds (start/end of dive)
130.....-11.70 to -2.00 pounds....delta = 9.20 pounds (start/end of dive)

Assuming your numbers are correct here is what it looks like to me.
At the start of the dive you are going to have 3.06 pounds less negative buoyancy.
At the end of the dive your difference is -5.26 pounds less negative buoyancy.

So at the end of the day (and the dive) sitting at 15 feet you will need to replace 5.26 pounds of negative buoyancy that you use to have. so that will be 5 pounds more of lead. But as with all things scuba, the final test is in the water, you will need to do a buoyancy check at the start and end of the dive.

 

[battles2a5]

I agree. Assuming you are properly weighted now, you need to add 5.26 lbs. With a larger capacity tank you are going to be more negative at the start of a dive, that is just a given. You should be more concerned with your weighting at the end of the dive so you can maintain your last stop.

 

[don Francisco]

Pasley, has it right. I would correct weighting for the end of the dive where I'm going to have to hover at my lowest weight (negative bouyancy).

There are small discrepancies in the math, probably because the tanks don't hold exactly 100 and 130cuft respectively. Also since you're considering a tank to tank comparison you don't have to adjust for the weight of your remaining air @.075 per cuft, since I assume you'll be ending with similar reserves.

Assuming you had the correct weight dialed in, you'll be very close to dead on with the adjustment, though the extra 2#s may have you riding a bit lower in the water at the start.

 

[runway1]

Pasley, has it right. I would correct weighting for the end of the dive where I'm going to have to hover at my lowest weight (negative bouyancy).

There are small discrepancies in the math, probably because the tanks don't hold exactly 100 and 130cuft respectively. Also since you're considering a tank to tank comparison you don't have to adjust for the weight of your remaining air @.075 per cuft, since I assume you'll be ending with similar reserves.

Assuming you had the correct weight dialed in, you'll be very close to dead on with the adjustment, though the extra 2#s may have you riding a bit lower in the water at the start.

I tend to agree with Pasley as well. Seems I would want to envelope the biggest change since I'll see that change sometime during the dive. In this example, I'll see the biggest change and the end so I should adress that from my current weighting......it seems to me, anyhow.

 

[DA Aquamaster]

Swing weight depends on the tank volume. It affects the lift you will need early in the dive, but at the end of the dive what counts is being able to hold a 15' safety stop or a 10' deco stop with a near empty tank.

So...assuming I was properly weighted to be neutral with 300 psi or so in a 100 at 10 ft with no air in the wing...I'd just add 5 lbs when switching to the 130 and call it good.

If I had no bench mark (ie. not properly weigthed with the 100, or also changed a whole bunch of other stuff) I'd weight myself to be neutral at 10' with a FULL 130, then add 10 lbs to accommodate the swing weight.

 

[Web Monkey]

The tank weight is pretty much irrelevant (unless you get one of those bizzare boat anchors that's hugely negative).

What matters is the weight of the gas. You need to carry enough lead to be neutral with an almost empty tank. Whatever your buoyancy is with the tank full is compensated for by adding air to your BC.

Terry

I'm switching from a Steel 100 to a Steel 130. Full/empty numbers are:
100.....-14.76 to -7.26 pounds..delta = 7.50 pounds (start/end of dive)
130.....-11.70 to -2.00 pounds....delta = 9.20 pounds (start/end of dive)

Delta full = 3.06
Delta start/end = 1.7
Delta empty = 5.26

So, what's the weight difference? Would you go by the weight deltas from start/end, weight delta when full or weight delta when empty number?

 

[spectrum]

The difference in the empty buoyancy numbers is what determines your weight change.

The air in the tank will require use of the BC to establish neutral buoyancy and only the magnitude of this changes with cylinder. At the end of the dive when either cylinder is running low and you are approaching the surface is when you need your true weight.

Pete

 

[captndale]

"Dry" air weighs 7.5 lbs/100CF. The buoyancy swing in your tanks is due entirely to the loss of the weight of the air in them as you breathe it out.