Weight difference of 72 Steel Tank vs. 77 Steel

[GregBS]

In the past, I have always used a 72 Steal tank; however, recently I rented a 77 Steal tank while diving at Catalina. Anyone know the weight difference between these two steal tanks at 600 psi? Tyler, filling tanks at Catalina, said the 77 steal is 4 pounds negative over the 72. Is this correct? I definitely was overweighted when using the 77.

 

[Nemrod]

That is about right, the 77cf steel tanks is somewhat more negative.

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

and

http://www.deep-six.com/page66.htm

Steel 77: ------------------ Steel 72

@2400 PSI ------------------ @2400 PSI
DIAMETER: 7.25 inches---------6.9 inches
LENGTH: 23.2 inches ----------- 24.5 inches
WEIGHT EMPTY: 32.5 lbs -------- 29.5 lbs
BUOYANCY FULL: (-)6.8lbs -------(-)0 plus or minus .5
BUOYANCY EMPTY: (-)1.0 lbs ------- (-)5.50 lbs plus or minus .5

approx from memory

N

 

[captain]

N, I think you have your buoyancy numbers switched. Steel 72 -5 to -6 full 0 to -1 empty.

 

[Nemrod]

Yeah, I was having a momentary brain cloud, the steel 72 holds about 5.75 pounds of air at 2,400 psi. I used 2,400 when I calculated, the standard fill is 2,250 psi and the plus ten fill is 2,475 psi.

Air weighs about 4.0 pounds per 50cf. So a 72 cf tank will swing just under 6.0 cf from full to empty.

Thanks for correcting me, if you have some better numbers please post them. I just calculated it while at work and got my aluminums and steels mixed up to a poor result for either. I cannot find a calculator so y'all can get it exact, go for it.

The plus or minus in weights results from variations in weight over many years or production.

The steel 77 is 2,440 psi service, the steel 72 is 2,250 psi service with plus 10% is 2680 psi for the steel 77 and for the steel 72 is 2,475 psi. Again, from memory so take it for what it is.

N

 

[GregBS]

Thanks, Nemrod and Captain. I look forward to when I'm experienced and proficient enough that I can rattle off figures from memory.

 

[GregBS]

Trying to make sense of the numbers. It seems the difference in weight is really only about one pound. If 72 = (-)5.5 full and (-).5 empty, and the 77 = (-)6.8 full and (-)1 empty and even judging based on 4lbs per 50 cf, from 72 to 77 is rather negligable.

So the remark by the guy providing tanks at Catalina that there's a (-)4 lb difference in the two tanks is not correct. I shouldn't drop 4 pounds from my weight belt when switching from 72 to 77. So, it seems I'm just going to have to do a buoyancy test to determine my weight difference when switching between the two tanks.