Trapped air

[doctormike]

It's not all that complicated.

When looking at tank specs to figure out how much lead you need to add or remove as you change from one type of tank to another, there is only one spec you need to look at ----- buoyancy when empty.

Exactly. I was just blathering on at length about the science behind that useful rule of thumb...

 

[Walter]

From a physics point of view, this is certainly correct............

Excellent post!

 

[scubadiver888]

You've got the concept down. Next concept, buy the NOAA Diving Manual and toss out the PADI book. If you don't want to spend that much money for a book right now, get Graver's SCUBA Diving, not as complete as the NOAA book, but much better than what you're using now.

If you go to Google Book Search and search for NOAA Diving Manual you can see the 1991 edition of the book for free.

I tried finding the Graver book but no one online in Canada seems to have it. I'll search around for it but if I cannot find it I'll have to buy it from the US.

 

[doctormike]

Excellent post!

Thanks!

 

[Ayisha]

When looking at tank specs to figure out how much lead you need to add or remove as you change from one type of tank to another, there is only one spec you need to look at ----- buoyancy when empty.

Ok Charlie and Doctormike,

So when going from an AL 63 to a HP steel 80, how much less lead do I need?
Doctormike said the HP steel 80 is -7 lbs, and the AL 63 would be +__ empty buoyancy?

Thanks! :blinking:

 

[doctormike]

Ok Charlie and Doctormike,

So when going from an AL 63 to a HP steel 80, how much less lead do I need?
Doctormike said the HP steel 80 is -7 lbs, and the AL 63 would be +__ empty buoyancy?

Thanks! :blinking:

Well, let's use this table to figure that out! We'll do all our calculations for the end of the dive with an empty tank.

Let's assume that you are using a Luxfer 63 with a service pressure of 3000 psi. The empty buoyancy of that tank is +2.6. If you go to a steel Faber HP 80 (with a service pressure of 3180 psi), your tank will have an empty buoyancy of -7.2.

That means that all other things being equal, you would need 9.8 lbs less lead with the HP steel 80 than with the AL63.

Note that on the dock, the HP80 weighs 32.5 lbs, while the AL63 weighs 26.6 lbs. So while going with the steel tank has only added 5.9 lbs to your tank-schlepping burden, it has let you leave 9.8 lbs of weight back in the shop.

Now for extra credit, what about the air? These calculations were for empty tanks, which is more important. Being underweighted at the beginning of the dive just means you will have a hard time descending, and you might need to go back for more weight (or pick up a rock). Being underweighted at the end puts you at risk for a runaway ascent, and you don't want that, right? So make sure that you are OK for the end of the dive, once you have ditched the air that was helping you stay down at the beginning...

Air weighs 0.08 lbs per cubic foot at sea level (14.7 psi). In the HP80, assume that this 80 cf tank holds 80 cubic feet of air when filled to service pressure (not strictly true, but close enough). This air weighs 6.4 lbs, (80 x 0.08), which is lost in our buoyancy equations when going from the beginning to the end of the dive. In the AL63, the air would weigh a little over 5 lbs, so at the beginning of your dive, you could give the steel 80 another 1.4 lbs of negative buoyancy over the AL63 - it is carrying extra weight in the form of extra air.

 

[Ayisha]

Thanks Doc! :10:

I only wear 16 lbs with my AL 63 with a 2-piece 7 mm full wetsuit, hood, gloves, etc. in very cold fresh water. So when I use a HP steel 80 all else being equal, I'll be able to wear just over 6 lbs of lead. That's awesome! Finally those 2 Kg/2.2 lb weights actually help...

 

[doctormike]

Thanks Doc! :10:

I only wear 16 lbs with my AL 63 with a 2-piece 7 mm full wetsuit, hood, gloves, etc. in very cold fresh water. So when I use a HP steel 80 all else being equal, I'll be able to wear just over 6 lbs of lead. That's awesome! Finally those 2 Kg/2.2 lb weights actually help...

Yup, and another 17 cubic feet of air as well...!

However, remember there are a number of different AL 63s and steel 80s on that chart... see if you can find your brand there for more accurate calculations.

And don't forget to do some buoyancy testing to get the weighting completely right with your new gear...

Have fun!

Two weeks 'till Bonaire for me...