Diving 72's & 50's?

[Skinsfan1311]

I'll be diving, in Cayman, this summer. I was reading the FAQ, and under equipment the Dive Op uses,(Red Sail/Tortuga Divers), and they use 72's,(Lexfer sp?), and 50's. I've only ever dived AL80's.

I've rented BC's, regs & weights, (I'm going to purchase prior to the trip) I wear a 2mm shorty and use ~ 12 lbs of weight.

Do these tanks hold significantly less air than the AL 80's, and how should it affect my buoyancy? My thoughts are that they hold less air, and I'll need a tad more lead, and my bottom time will be slightly decreased. Thanks!

Edit....after a little reading, it appears as if the 72 may be a steel tank, so I'm going to guess that I'll lose some lead, and have the same volume of air? Please bear with me, as I have around 20 or so dives, and am still soaking all of this in. Thanks, again!

 

[gt3073b]

An AL80 nominally holds 80 cubic feet of air at its working pressure of 3000 psi but due to marketing guys rounding up, it actually only holds 77.5 cubic feet. The Luxfer 72 holds 69.6 cubic feet at its working pressure. You'll have 90% as much air as an AL80. Dropping down to a Luxfer 50 (which holds 48.4 cubic feet) will only give you 62% of the air of an AL80.

Check out this link for actual tank specs and weight.
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

It should help you estimate the changes to your weight belt when switching tanks. For example if you are weighted properly at the end of the dive (500 psi in tank) and use 12lbs with an AL80, the tank is 4.4 lbs positive empty. The 72 is 3.6 lbs positive empty so you might be able to drop 1 lb when you switch to the 72. The 50 is 1.3 lbs positive so you could probably drop 3 pounds switching from the 80. This is just a starting point and is not exact science. These numbers really are based on an empty tank, not 500 psi. Being one pound overweight is better than one under for your safety stop.

Have fun on your trip!

 

[Skinsfan1311]

Thanks! I appreciate the reply....

 

[The Chairman]

The 72 was rated at 2250 psi as well. Many (if not most) facilities pump these guys up to 3,000 psi on a consistent basis. Consequently, you may actually have 90+ cubic feet of gas on your back!

This is an observation on my part and is NOT an endorsement for the overfilling of tanks!

 

[DA Aquamaster]

Luxfer made AL72's which were a different breed of cat in more ways than 1.

I think we are talking about steel 72's instead.

The generic steel 72 is a 2250 psi tank that holds 71.2 cu ft at 2475 psi (a 10% overfill) and holds 64.7 cu ft at the normal service pressure of 2250 psi.

Bouyancy wise there is some variation as they were made by more than one company to similar but not identical specs, but you can expect to drop 4-5 lbs of lead compared to when you dive an AL80.

There are also shorter 3000 psi Faber 71.4 cu ft tanks. Again the 71.4 cu ft is at 3300 psi. (a 10% ovefill) and it holds 64.9 cu ft at 3000 psi.

Bouyancy wise you can drop about 8 lbs of lead compared to an AL80.

If the tanks are filled to 2475 psi (or 3300 psi for the Faber tank) you only lose about 6 cu ft compared to an AL 80, and the capacity is in fact only 1 cu ft less than the capacity of an under filled AL80 at 2800 psi. So depending on how much air you will have compared to what you had in the past depends a lot on the quality of the fills.

 

[Charlie99]

It might really be the aluminum Luxfer 72. I've seen those in some rental and dive op inventories. Both the AL72 and the AL50 are 6.9" diameter, so it would be easy to swap back and forth between them.

You can compares specs at http://www.huronscuba.com/equipment/scubaCylinderSpecification.html
The AL72 is about 1 pound less buoyant than an AL80 when empty. The AL50 is about 3 pounds less buoyant. In other words, you can drop 1 to 3 pounds of lead from your belt compared to your AL80 configuration with the same wetsuit.

 

[Skinsfan1311]

Thanks everyone, for your replies! I can't wait for Cayman!!!!