Low pressure tank question

[cold diver]

I dive LP95s and I was just thinking..... When the DM says back on the boat with 750LB on your gage they are usually dealing with HP80s and 3000PSI tanks. My LPs are full with 2400 PSI so when I am back on the boat with 750, I have just over half used tanks. Does anyone know of a formula for figuring air left in tanks at a given PSI so I can figure out what a comparable pressure is on a LP tank? Thanks

 

[kelemvor]

Rated Cuft/service pressure with the + (for LP)*actual pressure

so if it's an lp95 that's (95/2640)*750=28.9cuft left in there. about equivalent to 1100 in an al80 I think.

If you get that sucker filled in cave country they will stuff it with almost 130cuft of gas.


Have you ever had a captain or assistant actually check your pressure when you got back? I haven't.

 

[davetowz]

LP 95 is 95 CF @ 2650 psi. Formula is capacity in cf / working pressure. so 95/2650=.036 This is called a tank factor. To determine CF left take the psi in tank x tank Factor. ie... 500psi x .036= 18 CF. Typical AL 80 is 78CF @ 3000psi IIRC. TF of .026 so AL 80 @ 50 PSI= 13 CF. Calc your way to your answer.

 

[tbone1004]

so Kelemvor is right, your tank holds 95cf at 2640psi if it is a Faber, 96.6 if it is PST, 93.3 if it is Worthington. HP80's hold 82cf at 3500psi, but that is very unlikely what you are dealing with. AL80's hold 77.4cf at 3000psi, so when back on the boat with 750 is said, 77.4*750/3000=19cf of gas. That for you is 19*2640/95=535psi, so come back with 600 and you're coming back with the same cf as the guys in the 80's.

Now, that said, you need to really look at rock bottom calculations to make sure 20cf is enough, but that's another discussion.

 

[Basking Ridge Diver]

so if it's an lp95 that's (95/2640)*750=28.9cuft left in there. about equivalent to 1100 in an al80 I think.

Your formula is correct but your result is off just a bit (95/2640)x750 = 26.99 cu ft due to rounding.

OP all the boat or DM is saying is come back on board with a reserve - they are asking you to keep the reserve in mind should something happen. Remember they are looking to avoid an accident or emergency before it happens - should you choose to ignore that advice and something happens you just need to be prepared to self rescue or go to your reserve.

 

[geoff w]

...
OP all the boat or DM is saying is come back on board with a reserve - they are asking you to keep the reserve in mind should something happen. Remember they are looking to avoid an accident or emergency before it happens - should you choose to ignore that advice and something happens you just need to be prepared to self rescue or go to your reserve.

I think it's a little more than that - DMs want divers to plan to come back with some reserve, knowing there are some who will push the limits and come back with much less than they are told to, either because they can't plan when to return or didn't pay attention to their SPG and cut it too close, or think they are better divers than they actually are. DMs hate it when divers they are responsible for run out of air underwater! Plus, at the low end of the scale, SPGs tend to be inherently less accurate, so coming back with 750 PSI showing on your gauge might actually end up as much less. DMs know everyone doesn't take the best care of their equipment, and SPGs typically aren't serviceable

 

[Basking Ridge Diver]

I think it's a little more than that - DMs want divers to plan to come back with some reserve, knowing there are some who will push the limits and come back with much less than they are told to, either because they can't plan when to return or didn't pay attention to their SPG and cut it too close, or think they are better divers than they actually are. DMs hate it when divers they are responsible for run out of air underwater! Plus, at the low end of the scale, SPGs tend to be inherently less accurate, so coming back with 750 PSI showing on your gauge might actually end up as much less. DMs know everyone doesn't take the best care of their equipment, and SPGs typically aren't serviceable

My experience has been - not a single DM or Instructor has ever checked my SPG. They have cursed and looked at my PDC swearing I was some how in Deco when I wasn't - but they could not work my PDC and subsequently left me alone. But never has a single person ever checked my supply of air or gas after a dive....

So maybe they work differently on the West Coast or the islands? Have you ever had to show your SPG to a DM?

 

[MaxBottomtime]

I would find another boat.

 

[Bob DBF]

I have head a DM say to come back with 500#, but I've yet to see one check. I believe it is shorthand for "I know you are excited and want to have a good time but for Christ sake watch your F'n air"



Bob

 

[archer1960]

I have head a DM say to come back with 500#, but I've yet to see one check. I believe it is shorthand for "I know you are excited and want to have a good time but for Christ sake watch your F'n air"
Bob

This ^^^. On a trip to Cozumel last fall, the DM said to signal him when either of a buddy pair was down to 500, and then both go up. He never checked anybody's air once they surfaced, but he did frequently ask for the group's air readings during the dive, especially the first couple of days. The first 3 dives, I was by far the biggest air hog in the group, being newly OW qualified, but on the 2nd dive of the 2nd day (dive #4), something just clicked, and got my air usage pretty much in line with the rest of the guys my size. I still laugh when I think of the look on his face when he asked for my air pressure during that dive, and I had plenty left for once...