Need a little tank help.

[DavidHickey]

I recently got a Steel E8 -119 cf tank. I have not had it in the water yet. I have a few questions that are probably really dumb but I'll ask anyhow.

1st with my 80 cf Aluminum I wear 24 pounds of weight with my 7mm suit. From what I can see the 80 cf should be negative 1.5 lbs full and positive 4.4 empty. The E8-119 should be negative 10.5 full and negative 2 empty. So basicaly unless I'm thinking wrong that means I should drop 6 pounds and go down to 18 pounds in my 7mm. Is this correct.

2nd question. I'm a little confused on the CF measurements. Usually at the end of a dive while doing our safety stop with the 80 cf tank I will run my air down to about 500 psi before getting out. Now if I'm diving with a 119 cf do I acutally have more air left at 500 psi than I would in my 80 cf with 500 psi or is the biggest difference just the E8-119 being a high pressure tank were you can put more into start with. And will it affect my computer at all "Sunnto Cobra".

I'm sure they are dumb questions but all I've ever dove with was 80 cf Alum.

Thanks

 

[pants!]

1) Yes, but be sure you fine-tune.

2) 500 psi is a good metric to keep using, though you *will* have more available gas in the 119@500psi than the 80@500 psi.

 

[Doc Intrepid]

What Pants! said.

Here's a matrix of PST tank weights and volumes at various pressures (Table 2 - last three columns):
http://www.lloydbaileysscuba.com/PST E Series Tanks.htm

Your Cobra will not be affected. But, it will only provide tank pressure in psi gauge. 500 psi remaining from 119 cu ft is more than 500 psi remaining from 77.4 cu ft. Its up to you to know the volumn variance at the same psi reading. This will factor into your gas management planning.

 

[spectrum]

2nd question. I'm a little confused on the CF measurements. Usually at the end of a dive while doing our safety stop with the 80 cf tank I will run my air down to about 500 psi before getting out. Now if I'm diving with a 119 cf do I acutally have more air left at 500 psi than I would in my 80 cf with 500 psi or is the biggest difference just the E8-119 being a high pressure tank were you can put more into start with. And will it affect my computer at all "Sunnto Cobra".

I'm sure they are dumb questions but all I've ever dove with was 80 cf Alum.

Thanks

You have the concept of the bigger tank but lets put some numbers to it all assuming perfect fills.....

Aluminum 80....
3000 PSI / 77.4 CUBIC FEET = 38.75 PSI PER CUBIC FOOT
500 / 38.75 = 12.9 CUBIC FEET OF AIR AT 500 PSI

E8-119....
3442 PSI / 119 CUBIC FEET = 28.92 PSI PER CUBIC FOOT
500 / 28.92 = 17.28 CUBIC FEET OF AIR AT 500 PSI

DELTA......
17.28 - 12.9 = 4.48 CUBIC FEET MORE AIR IN THE E8-119 AT 500 PSI

WHAT DOES IT MEAN TO YOU?
LET'S ASSUME A SAC OF .66
15 FOOT STOP = (APROX) .5 ATM DEEP + SURFACE ATM = 1.5 ATM
.66 * 1.5 = 1 CUBIC FOOT OF AIR IS NEEDED FOR EACH MINUTE AT 15 FEET.

4.38 CF of extra air at 500 PSI / 1 CFM = 4.38 minutes
THE E8-119 ALLOWS 4.38 MINUTES MORE AT 15 FEET TO EXIT WITH THE SAME VOLUME OF AIR THAT AN AL80 HAS AT 500 PSI. NOTE THAT THE E8-119 WILL NOW BE DOWN TO 373 PSI WHICH GIVEN GAUGE INACCURACIES AT THE LOW END MAY BE HAZARDOUS TO YOUR WELL BEING.

Pete