Oh no! My 72s are too small for tech classes! :(

[boulderjohn]

1/3rds of a single is dumb

People who have not had the training tend to think of the word "thirds" is that it is some sort of absolute, not realizing that not all thirds are created equal. Most importantly, they need to realize that the value of the gas reserve created by diving thirds is measured in time--time to solve whatever problem may arise during the dive. It may be a buddy losing gas, but it could be any of a number of problems that arise. Whether it is a siltout, a lost line, and entanglement, or whatever, 1/3 of a single tank gives you precious little time to solve the problem and get back to your exit.

 

[elan]

I could understand that requirement. Diving say to 150 ft the min gas to 70ft will calculate to approx 65-70cuf. 20 min bottom time will require 80-90cuf so those carrying LP72 will be limiting others. Which is not fair

 

[elan]

My plus-hydro'd 72 (PST/U.S. Divers with first hydro 1970, IIRC) often comes back filled to 3,000 psig. I think this is because scuba shops in SW MO and NW AR have their fill stations set up to automatically fill Al 80's.

rx7diver

They often are I get the same here. Especially when I bring my LP together with stages. They just hook them up to the same station with the regulator set to 3100 I never complain.

 

[rob.mwpropane]

Idk, I'm not at all an expert, I don't claim to be. I asked around a lot about filling LP tanks when I got mine. I liked the idea of being able to move into some type of training with rule of 3rds, so I opted to fill my tanks to 3300. Easy to divide by 3 for 3rds, but not so overly full that it becomes scary (to me), plus my LP85's filled to 3300 closely match my HP100's filled to 3900 cf wise.

PLUS....LP's filled to 3300 can be filled right along with and in the same group as my AL tanks to simplify things a bit.

Just my 2 cents....reminds me, I need to hydro some 72's

 

[rx7diver]

... We rented a LP Scubapro 75 (?) there. I was still a relatively new scuba diver. This was the first time I was ever asked, "How much air [how many psig?] do you want in it?" The very first time, I actually thought the employee was making fun of this obviously brand new cave-diving student.

I just checked my 1988 Scubapro catalog. The cylinder was a 75.8 cu ft @ 2,640 (= 2,400 + 10%) psig, ~26" length, ~6.8" O.D. I can't remember what pressure the employee filled the tank to, but I do remember being so surprised it was significantly greater than 3,000 psig (fill pressure of an Al 80), that I mentioned it to my cavern/cave instructor.

Oh, and I also remember the employee asking me what regulators I was using for my classes, before he commenced filling. "Scubapro Mk 10's + G250/Balanced Adjustable" was met with immediate approval.

rx7diver

 

[jtsfour]

Or 4,250...

Does anybody actually do that? I’d think anything north of 3800 is starting to approach sketchy territory.


That reminds me. The shop I used to buy my LP85s neglected to tell me to “double up” my burst disks. When they pulled them (4 months later) the copper was stretched to half spheres….

Moral of the story is don’t use 4000PSI Burst Disks if you are going to cave fill tanks.

 

[Marie13]

The most I can get around here is 3000 in my 85s. Couple of places will fill to 3200 so they cool to 3000. Not sure what I’ll fill them to when I’m down in cave country at Christmas.

 

[Manatee Diver]

Does anybody actually do that? I’d think anything north of 3800 is starting to approach sketchy territory.


That reminds me. The shop I used to buy my LP85s neglected to tell me to “double up” my burst disks. When they pulled them (4 months later) the copper was stretched to half spheres….

Moral of the story is don’t use 4000PSI Burst Disks if you are going to cave fill tanks.

Not on purpose. Once you get close to or over 4K Z factor means you aren't gaining much.

In that case, it was a cool rainy day in Florida, and the banks were just topped off to 4K. I filled my tanks to 4K cold, and proceeded to drive across the country to Nevada. Where in the desert sun it warmed up a bit.

For me the goal is to have about 3,700-3,800 when I am putting my tanks on in the water, so I have a 1,200psi third after predive checks and drysuit stretching. That gives me more options to dive reduced thirds of 1,100psi or 1,000psi when needed or safety.

 

[jtsfour]

Not on purpose. Once you get close to or over 4K Z factor means you aren't gaining much.

In that case, it was a cool rainy day in Florida, and the banks were just topped off to 4K. I filled my tanks to 4K cold, and proceeded to drive across the country to Nevada. Where in the desert sun it warmed up a bit.

For me the goal is to have about 3,700-3,800 when I am putting my tanks on in the water, so I have a 1,200psi third after predive checks and drysuit stretching. That gives me more options to dive reduced thirds of 1,100psi or 1,000psi when needed or safety.

What is Z factor? That’s the first time I have heard that term.

Does anybody know what the failure pressure of a LP tank is? Has anybody ever exploded one to test?

 

[Manatee Diver]

What is Z factor? That’s the first time I have heard that term.

Does anybody know what the failure pressure of a LP tank is? Has anybody ever exploded one to test?

Z factor is a variable that we use to compensate from the fact that real world gases don't act like ideal gasses.

The Z factor changes, gets worse, as you reach higher pressures as things like intermolecular forces come into a greater amount of play. You know all the stuff they mention in Gen Chem, and promptly forget about after the final?

Cliff notes version you get less and less gas per psi as you increase the pressure. There are long charts on what Z factor to use for a given gas, pressure, and temperature.