Low pressure tank question

[rjack321]

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

I have actually been asked here. And some of the BC charters do check pressures coming and going.

I recommend converting whatever they say into cubic feet using the formula provided in post 2.

Then when they say X psi you answer back, "you bet I will be back with Y cubic feet" If they get confused just walk them though how you arrived at reasonable number for you. Which will vary depending on your depth assumptions, see other posts on "rock bottom".

As you can imagine, 750psi in a hp80 is a lot less than 750 psi in an AL80 and even less than 750psi in a lp95.

 

[AfterDark]

OP IMO you should come up with what you think is an adequate reserve. If the DM happens to check and your reserve is lower than was requested say, " I'm sorry. I'll do better next time".

I read this entire thread and have seen several references to 20cuft being something of a universal unofficial reserve to end a dive with. So this leads me to think that the 19cuft pony bottle I tote for emergencies is about right despite what others may think.

 

[kelemvor]

I read this entire thread and have seen several references to 20cuft being something of a universal unofficial reserve to end a dive with. So this leads me to think that the 19cuft pony bottle I tote for emergencies is about right despite what other may think.

You should probably switch to an LP121 for your pony. You know, for safety!

---------- Post added June 17th, 2015 at 09:25 AM ----------

Actually yes, and every time in the Keys and a few times, though not as stringently in Cozumel. They did a gauge check and wrote them down and then checked them coming out. Nanny diving to the max. One was on a Vandie dive and the entire group, guided (ughhh) were on 100 cf tanks but for me on an aluminum 80. The dive turned and I was back on the boat still with 1500 psi :/ . They were all, including the guide, at around 500, on the deck. N

I haven't had that experience in the keys. I've dove with a couple different Rainbow Reef captains, Silent World, and Flakeys. None of those asked me about my gas. If that had happened to me, I wouldn't be back on that boat. It's not the end of the world, but there's plenty of boats in the keys that will treat you with respect so why live with it?

 

[boulderjohn]

A few years ago, I sustained a bad injury that kept me out of diving for a number of months. I planned a dive trip that would include technical diving for immediately after the surgeon predicted I would be able to dive again. In setting up the first part of the trip with a dive operation in Cozumel, we decided it would be wise for me to do a day of recreational diving with a tech setup before I did real tech dives, just to be sure I could reach valves, etc. with my newly healed shoulder. It would also help me refresh skills I had not practiced for a long time. So, with the DM fully informed as to why I was joining him and two other divers in AL 80s while I was diving with double overfilled LP85s and an AL 80 stage bottle for a total of about 280 cubic feet of gas, we prepared to enter the water. The DM went over signals, including how to signal how much gas we had so he knew when to bring us to the surface.

As we dived, I practiced valve drills and switched back and forth between the stage tank and the back gas in preparation for the next day's deco dive. After a while, the DM turned and pointed to each of the other two divers, asking them how much gas they had. Then he pointed to me and asked the same thing. I responded with a "you've got to be kidding me" look. He was quite insistent, appearing almost angry in repeating the "how much gas?" signal. I pointed to each tank individually, including each of the doubles, and indicated how much gas I had in each. Of course, I still had more cubic feet of gas than the three of them had combined when they started the dive. I think he finally realized the stupidity of it, because he never asked me again.

 

[Diving Dubai]

Actually yes, and every time in the Keys and a few times, though not as stringently in Cozumel. They did a gauge check and wrote them down and then checked them coming out. Nanny diving to the max. One was on a Vandie dive and the entire group, guided (ughhh) were on 100 cf tanks but for me on an aluminum 80. The dive turned and I was back on the boat still with 1500 psi :/ . They were all, including the guide, at around 500, on the deck. N

Maybe a different culture but I don't consider this Nanny diving in the least.

We do it every dive and we dive unguided.

Our Stats Record Gas in, Mix, Tank size and Gas out. We have a rule that you're on the boat with 50bar (although we are lenient with that. Most people come up with twice that after an hour dive depending on what depth they've been hanging out at.

The reason we and I presume some others record that info, is if you're overdue the boat has some idea of your gas time.

Fundamentally the reason people dive different sized cylinders is that they have differing air consumptions? My wife dives a 12l, I dive a 15l we end up within 10bar (150psi) of each other - in theory we would both suck our tanks dry around the same time. The fact that I have 150l more gas than her is irrelevant (okay over simplified I know but you get my point).

Not wishing to go by the "house rules of the boat" reminds me of an old UK pub landlord I knew. When he rang the bell and called TIME there would on occasion be some smart a$$ would would shout out that he was 5 mins early etc.

The Landlords response was always the same: "If you want to call time by your watch..Buy your own @#$$@# PUB!"

Kinda the same with Dive boats no?

 

[AfterDark]

Maybe a different culture but I don't consider this Nanny diving in the least.

We do it every dive and we dive unguided.

Our Stats Record Gas in, Mix, Tank size and Gas out. We have a rule that you're on the boat with 50bar (although we are lenient with that. Most people come up with twice that after an hour dive depending on what depth they've been hanging out at.

The reason we and I presume some others record that info, is if you're overdue the boat has some idea of your gas time.

Fundamentally the reason people dive different sized cylinders is that they have differing air consumptions? My wife dives a 12l, I dive a 15l we end up within 10bar (150psi) of each other - in theory we would both suck our tanks dry around the same time. The fact that I have 150l more gas than her is irrelevant (okay over simplified I know but you get my point).

Not wishing to go by the "house rules of the boat" reminds me of an old UK pub landlord I knew. When he rang the bell and called TIME there would on occasion be some smart a$$ would would shout out that he was 5 mins early etc.

The Landlords response was always the same: "If you want to call time by your watch..Buy your own @#$$@# PUB!"

Kinda the same with Dive boats no?

Or go to a different pub with a slower clock.

 

[Phil_C]

Or go to a different pub with a slower clock.

Nah, sooner they lock the door the sooner you can get down to some serious drinking, 'lockins' used to be all rage when I was younger .