Math Question

[SkimFisher]

It's one of those personal preference things I guess.

I make the suggestion based on what I've been told about yoke valves and high pressures. I've never seen or experienced a blown o-ring, but have been *told* that once you get into HP tanks your chances of having one go on a yoke valve are much higher. I certainly wouldn't take the chance if I was going to overfill an HP tank either.

But it could all be conjecture too. I tend to listen to the old-timers (not implying that others don't).

 

[meesier42]

keep in mind, end of dive @500psi is a good rule of thumb to maintain the tank and the performance of your regulator. You should always plan to not use that last 500psi. the math above from Street is just plain wrong. a AL80 has 64.5% of the air that an AA120 (HP or LP). 77.4/120=0.645 compression factors have no play in Boyles Law/Ideal Gas Law.

the real question is how much gas can you use and wear should your turn values be, I am only discussing recreational diving environments, not technical that has much more complex gas management rules.
well there is the easy rule of thumb, the rule of thirds. first 1/3 of take is going out, second third is coming back, the last third is ascent which should easily leave 500psi in the tank. Even an ascent from 130' will only take 2.5 minutes plus 3-5 more for the "safety stop". Unless your a very heavy breather that shouldn't be 500psi or your using a small 67cf tank.

notice I said 1/3'rds. So it doesn't matter what tank you have. If its a 3000psi tank (e.g. AL80) then its really easy- turn back at 2000psi, start surfacing by 1000psi. If your on an LP72, or LP104, your fill pressure is 2640, so you'll turn at ~1800 and start ascent by 900psi. If you have an AL100, HP100, HP120, HP130 etc.. your fill pressure is ~3300 (3442 actually for the steels) your turn pressure should be about 2200 and start surfacing by 1100.

trying to figure math on the boat is a PITA, just remember rule of thirds and you should be fine.


as for yoke o-rings. well I have seen an o-ring extrude, it was really loud. a small bit of it pressed through and as soon as a small bit is weakened it burst through that spot. Now the issue with yokes IMO is that different manufacturers make different sized o-ring seats. I am sure that we have all seen valves that our regs fit nicely on, some that were really large, and I remember one that was too small for my reg to seat onto. The standards were terrible. I believe they have improved the consistency of these now, at least in the US with Sherwood and Thermo being really the only shops making them. I know with the Thermo "Pro" valve that is convertible, if you examine the yoke adapter plug you'll see that the ring is much more captured than a true yoke valve, which I am guessing further reduces the likelyhood of issues. but for me, its all DIN. I keep a thread on yoke adapter, but really only use it for travel, and I have a second yoke only reg for that.

 

[ps1mhd]

I have lp120s and they are big. I have switch
to lp95s and fill them to 3000 or more
(burst disk replaced) You said "purchase some
LP steel tanks" are you setting up doubles or
just getting a spare? If you are going big because
of a high SAC it will get better the more you dive.
Like others have said Its the CF not the pressure
that you have to match up. Remember you have
turn the dive when the first person in the team
reaches there turn press. If your buddy has a Al. 80
you will be coming up with alot of air left.
Also sometime its hard to find shops that will fill
HP tanks all the way.


Mike

 

[DA Aquamaster]

I am looking to purchase some LP steel tanks and with the lower service pressure how do you figure out a good turn pressure? Obviously 1000 psi in an AL80 is not the same as 1000psi in a LP120.

If you are using an LP 120 then you know that you have 120 cu ft at a fill pressure of 2640 psi (the rated pressure for LP steel tanks is figured with a 10% overfill.)

Divide 120 by 2640 and then multiply the result by 100. (120/2640)*100= 4.5

This is the tank factor and indicates how many cu ft you have per 100 psi.

So...once you decide how much gas you want for ascent and for a reserve for you and your buddy, you can easily figure the turn pressure.

Lets' say you want a 30 cu ft reserve:

30/4.5= 6.6, or 660 psi in an LP 120.

You can guestimate the math in your head fairly easily as well by rounding up to a tank factor of 5 or down to a tank factor of 4. Using 5, you get 600 psi (30/5= 6), and using 4 you get roughly 700 psi (4*7=28) Either way, it is probably as accurate as your SPG is anyway.

An LP 120 is a large tank and you may want to pick something in the LP 95 to HP 100 or HP 130 range. The thing to consider is that if you lose all your gas, your buddy needs to have enough gas to get you to the surface and if they have a samll tank, they still need to turn the dive with enough gs to get you to the surface, so the benefits of a large tank in team oriented diving is minimal if any tank on the team is substantially smaller.

 

[StreetDoctor]

keep in mind, end of dive @500psi is a good rule of thumb to maintain the tank and the performance of your regulator. You should always plan to not use that last 500psi. the math above from Street is just plain wrong. a AL80 has 64.5% of the air that an AA120 (HP or LP). 77.4/120=0.645 compression factors have no play in Boyles Law/Ideal Gas Law.

the real question is how much gas can you use and wear should your turn values be, I am only discussing recreational diving environments, not technical that has much more complex gas management rules.
well there is the easy rule of thumb, the rule of thirds. first 1/3 of take is going out, second third is coming back, the last third is ascent which should easily leave 500psi in the tank. Even an ascent from 130' will only take 2.5 minutes plus 3-5 more for the "safety stop". Unless your a very heavy breather that shouldn't be 500psi or your using a small 67cf tank.

notice I said 1/3'rds. So it doesn't matter what tank you have. If its a 3000psi tank (e.g. AL80) then its really easy- turn back at 2000psi, start surfacing by 1000psi. If your on an LP72, or LP104, your fill pressure is 2640, so you'll turn at ~1800 and start ascent by 900psi. If you have an AL100, HP100, HP120, HP130 etc.. your fill pressure is ~3300 (3442 actually for the steels) your turn pressure should be about 2200 and start surfacing by 1100.

trying to figure math on the boat is a PITA, just remember rule of thirds and you should be fine.

LOL, using 1/3's on a recreational open water dive is not only stupid but it's a waste of gas. I won't even get into how you broke down your 1/3's either, I guess if I was your buddy I'd be screwed at the end of the dive. You can't just randomly pick a number and decide on that as a turn pressure.

Open water rec dives are usually all use or 1/2 use. The "500 rule" is also a load of #$@#$, there are depths well within recreational range where if you have 500 psi you AND your buddy will not get to the surface together. What you need to do is plan minimum gas needed to get you AND a buddy to the surface while making all required stops. This can be done by using a 33/ft a min ascent rate for time, a stressed SAC rate of 2, and your ATA's, not by randomly picking turn pressure numbers

I should add that an ascent from 130 with a WELL trained team that encountered a problem will take longer than 2.5 minutes let alone the average recreational diver...

Identifying the problem and getting your air share figured out, ascent rate of 33'/min, deep stop, safety stop, I think you're looking at closer to 10 minutes for the average diver.

10minutes x SAC of 2 x avg ata of about 3 = 60 cubic feet required Minimum gas... that's a bit more than 1/3's in an AL80, more like 2400psi. Thanks for playing though.

Vice versa if I'm doing a shallow multi-level reef dive in bonaire there's no way in hell I'm surfacing with 1000 psi left in my al80.

 

[markyd]

DA Aquadiver (and others), why do you complicate the math by introducing "tank factors"? That just makes it MUCH more difficult and gives more numbers to try to memorize.

In your example, you have a 120cuft tank, and you say you want 30cuft reserve. Okay, so divide 120 by 30 and you get 4, so your reserve is 1/4 of your air. So divide your pressure by 4 and there you have 660psi.

marky-d

 

[scubapeople59]

Street Doctor is right on. Perhaps the poster should get a copy of this months Divers Alert Mag. where they go into this and spell every step of the calculation out. Good article.