Steel Tanks: Math!

[CompuDude]

Ok, two questions I hope someone could me out with, and just double-check my math.

I just bought a second used tank, a Faber HP100. (Interesting that technically it's a HP3180 +10% to get 3498 psi, which is req'd to hit 100cf!)

My previous tank is an older PST HP100 (the 3500psi, full DIN tank, not the newer 3442 E7 series tanks).

In reading up at these two sites:
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html
and http://www.techdivinglimited.com/pub/tanks.html

...it looks like the new Faber tank has these charateristics:
Buoyancy Empty: -7.26 lbs
Buoyancy Full: -14.76 lbs

...while my PST HP100 has these:
Buoyancy Empty: -1.3 lbs
Buoyancy Full: -8.8 lbs

So here's Question #1: So am I to assume that I actually will need to drop (or add) 6 pounds from my belt when switching from tank to tank, even though they're both considered HP100s?!?!

Question #2: Using the PST100 as an baseline, if I was borrow a buddy's Alum.80 for a dive, assuming all my other gear is the same, if I've been reading SB posts (and those charts) correctly it looks like I essentially need to add 5-6 pounds to my belt when going from the PST HP100 to a generic Alum.80. Is that correct?

My weight is really nicely dialed in right now with the PST HP100, so I want to use that as a baseline. Hmm, now I'm starting to wonder if I should sell the Faber and try to find another PST instead. I prefer galvanized anyway... Or even a Worthington HP100, which looks to be within 1lb difference to the PST, as opposed to the 6lb difference with the Faber.

 

[do it easy]

I'm not a tank guy, but I'll throw this out:

1) I think you're right about the 6 lbs difference- it's basically the difference between the empty weights

2) I believe the Luxfer 80 is 4.4 positive when empty, so you would add 5.7 lbs (difference between 4.4 and -1.3. Yes, high school algebra just might save your life!) when switching to the AL80 from the PST100.

Good luck!

 

[BHB ScubaTroll]

I'm not a tank guy, but I'll throw this out:

1) I think you're right about the 6 lbs difference- it's basically the difference between the empty weights

2) I believe the Luxfer 80 is 4.4 positive when empty, so you would add 5.7 lbs (difference between 4.4 and -1.3. Yes, high school algebra just might save your life!) when switching to the AL80 from the PST100.

Good luck!

I dive faber blue 133 with no weight and no wetsuit. I am 6.00 ft and 265. The reason I bring this up?... everyone is different. What works for you, may not work for others. Reduce/increase weight based on your situation. if you need less weight, reduce it on the next dive.... until it works for you

 

[lamont]

Ok, two questions I hope someone could me out with, and just double-check my math.

Looks like you got it all right. The fabers are pretty heavy tanks. And that is why its best to have tanks that have similar buoyancy characteristics when empty, so you don't have to futz with your weight.

 

[Packhorse]

...it looks like the new Faber tank has these charateristics:
Buoyancy Empty: -7.26 lbs
Buoyancy Full: -14.76 lbs

...while my PST HP100 has these:
Buoyancy Empty: -1.3 lbs
Buoyancy Full: -8.8 lbs

So here's Question #1: So am I to assume that I actually will need to drop (or add) 6 pounds from my belt when switching from tank to tank, even though they're both considered HP100s?!?!

It all depends on the total external volume or the displacement of the tanks.
If the heavyer tank displaces less water then you dont need as much weight.

 

[CompuDude]

It all depends on the total external volume or the displacement of the tanks.
If the heavyer tank displaces less water then you dont need as much weight.

Then why bother listing weights in the specs? I'm pretty sure it does make a difference.

 

[CompuDude]

Looks like you got it all right. The fabers are pretty heavy tanks. And that is why its best to have tanks that have similar buoyancy characteristics when empty, so you don't have to futz with your weight.

Cool. That's what I thought.

That said, I just did some closer looking, checked the DOT code, and realized that the tanks that I thought was a Faber is actually a older, white PST tank. I didn't even know that PST made white tanks... I thought that was a Faber thing. Apparently the guy I bought it from did, too. It was hydro'd just a couple of months ago (April), so I'm not overly concerned about the merits of the tank itself. It IS an older tank, however... manuf. date is 9/92. Still steel tanks last forever if you take care of them, and this is in decent shape, although the paint is a bit beat up.

Did PST ever make painted tanks, or was this tank painted by a previous owner at some point in it's history?

Regardless, I guess it's likely to be fine from a buoyancy standpoint, assuming the older tanks have the same or similar specs to my other somewhat newer PST tank. (Still not the E series, but just the previous generation to those).

 

[CompuDude]

I dive faber blue 133 with no weight and no wetsuit. I am 6.00 ft and 265. The reason I bring this up?... everyone is different. What works for you, may not work for others. Reduce/increase weight based on your situation. if you need less weight, reduce it on the next dive.... until it works for you

Same size tanks, different weight full and empty. Same exact gear config. So the SOLE difference is the weight of the tanks. There's the difference. No need to experiment when the variables are that tightly controlled.

 

[lamont]

Then why bother listing weights in the specs? I'm pretty sure it does make a difference.

Its correct that it all has to deal with displacement, but the empty buoyancy numbers should take that into consideration. The difference in empty buoyancy between the tanks is the difference in what is going to go on your weightbelt, just like you computed.

 

[BHB ScubaTroll]

Same size tanks, different weight full and empty. Same exact gear config. So the SOLE difference is the weight of the tanks. There's the difference. No need to experiment when the variables are that tightly controlled.

Ok,
Used to need 12 lbs. before the Faber... now, 0 lbs.

Tank empty empty bouyancy is not -12 lbs, and the positive bouyancy of the al S80 doesn't make up the difference.

I mentioned this so you understand that not everything will always work out the way you plan it.

Yeah, I did the math.

I found myself descending like a rock. And, using air in the BC on ascent with as little as 4 lbs of weight

The math may not work. It didn't in my case.