dive plan

one diver has three lp72's (full at 2475; 2250 with plus rating) and the other diver has a set of doubled 119's (full at 3442). The first diver wants to use a third of one 72 and drop it, switch to back gas (doubled 72's), or should he/she carry it all the way?

how much usable gas for thirds for both divers, what is team turn pressure? how and why? no deco, no depth concerns. we can assume all tanks are full at start of dive, but that really makes no difference, right?

a third independent stage tank for one diver (while other uses doubles) has me questioning my math....

just curious, don't try this at home!

-skip

Dude.....I'm not wanting to offend you and maybe I have your intentions wrong, but if you have to ask here if it's okay.....DON'T DO IT!!!

I just don't want to hear of someone getting hurt. Ya' know what I mean? 8)

Dive the stage to half + 200 and keep your reserve in your back gas.

Angie I disagree with you on that.Posting senarios for opinions is harmless.He may not even be planning on doing that dive,but may be curious as to others opinions.

Okay - but listen - don't forget to gas match/turn allowing for having only the smallest volume available. Especially if Mr./Ms. big tanks has a higher SAC rate. It ain't worth a 50-100' more when the poo hits the fan.

Stairman -

You are probably right. Just that sometimes I see people post stuff that sounds like they are trying to learn something really new for them online. Please correct me if I'm wrong (my memory isn't always the best) but isn't Skip still working on/from his Intro level. I really didn't want to insult Skip - I just was concerned that he was trying to do something that would get him hurt.

Skip -

Maybe try a PM to some folks first for something like what you just posted. I'm sure that just about anybody would be happy to confirm your calculations.

Angie

PS. I didn't run the numbers, I just reacted to one of several posts I've seen of late suggesting what I mentioned in 2nd paragraph. :-)

Quote:

---

Originally Posted by skip

one diver has three lp72's (full at 2475; 2250 with plus rating) and the other diver has a set of doubled 119's (full at 3442). The first diver wants to use a third of one 72 and drop it, switch to back gas (doubled 72's), or should he/she carry it all the way?

how much usable gas for thirds for both divers, what is team turn pressure? how and why? no deco, no depth concerns. we can assume all tanks are full at start of dive, but that really makes no difference, right?

a third independent stage tank for one diver (while other uses doubles) has me questioning my math....

just curious, don't try this at home!

-skip

---

See AW's post.

I dive sidemount to much... :roll:


Of course if you are not trained to FULL cave, stages are beyond your training level and should be waited on until your skills catch up to your desires. Either way, this was a valid question. Hope I was helpful.

Didn't mean to evoke an emotional response from anyone. Not trying to dive beyond my training. Just thought a thread with some math, gas calculations, turn pressures, cf to psi conversions, and all would be fun...was sitting here building a chart for various configurations and tank sizes and thought it would be fun for others too.

By the way, nothing here beyond my training or experience and nothing I need others to figure out (well, maybe some help with social skills and how to get a smilie added ;-), as you can see i'm limited to keyboarding smilies - oh, and other things I'm not thinking of right now).

It does strike me that many on this forum seems very concerned about level of training of posters/readers; almost a knee-jerk reaction. Can the forum be held liable? I can just see the courtroom, "I read it on the internet your honor, so thought it must surely be safe for anyone to do!"

I respectfully withdraw the question.

-skip

We could just add certification level at the bottom in your signature block or add an extra option to the form for cert level.

Ben

Quote:

---

Originally Posted by skip

I respectfully withdraw the question.

-skip

---

Skip...

Do not take some of the "slightly" over zealous responses on here about your level of training as "emotional response".

You'll find that in this online community there are a vast amount of personalities and opinions along with a vast amount of expierence and knowledge.

Some were just concerned about someone asking questions that seemed to be "beyond" their level of certification and with some of the latest deaths that concern has been heightened.

As you can see, some of the responses did answer your question and you should take that for what its worth.

Feel free to continue to ask questions on this online forum because the more you ask, the more you can learn.

Welcome to the forum. :-D

I was thinking that one of the interesting aspects to a set of doubles and a stage for one diver and dissimilar tank volumes for the two divers was using the third cylinder to enter, drop it on thirds, continue on back gas to thirds. the diver with big doubles stays on his/her doubles the whole trip in (to thirds too). Now if both divers need to get out on the cylinders of the one with smaller doubles and the stage, can they do it? What is volume in both sets at turn (assume full failure in big doubles right at thirds)? What is volume in smaller doubles and is it enough to get both divers back to the stage? What is volume in stage? anyway, it was supposed to be an interesting hypothetical designed to play with the math.....

ooop, mike's here, time to go diving.

-skip

Diver 1's turn pressure would be 1700 psi on stage and doubles.

Diver 2's turn pressure would be 2400 psi on doubles.

Both would use 72 cf for penetration and another 72 cf for exit. This would leave Diver 1 with 72 cf for an emergency and Diver 2 with 94 cf (roughly).

In theory only, it appears Diver 1 could get both out of the cave alive even if Diver 1 dropped his stage early and then Diver 2 had a catastrophic gas loss at max penetration. In reality, however, 1 or both divers would most likely die.

Here's why - Diver 1 has 144 cf available in his doubles. If he uses 1/3 for penetration (after dropping stage), then that leaves only 96 cf in backgas. Diver 1 needs another 48 cf to get back to his stage. That leaves 48 cf for Diver 2. That last 48 cf is sufficient to get Diver 2 back to Diver 1's stage. But then Diver 1's doubles are completely empty. And all they have is a stage bottle with 48 cf left (which is enough to get them both out, theoretically). BUT UNLESS they were both VERY good at traditional buddy breathing, you'd have 2 divers fighting over one stage bottle. And you'd have 1 if not 2 fatalities. So many other variables not taken into account here (exit flow, stressed SAC rates, faster exit swimming rates, fumbling at stage pickup).

Best option would be to carry the stage the entire dive. Ditch the stage on exit when it goes empty, and run slick for the door sharing 7 ft hose.

Very good point, Alan. So you think it's safer to breathe the stage empty as your third and leave your BG as your exit/reserve? I suppose it's really no more dangerous then the traditional dive, but breathing the stage and BG down a third leaves 3 sources of gas instead of two.

There are plenty of people who dive 85s and will sling a stage to match up with 104s.

That is why I would opt to breathe the stage down (without dropping it unless empty and an emergency is at hand) and recalculate the reserves in my back gas. With the reserve in your back mounted cylinders you maintain the redundancy of having that backup reg drawing from your emergency third without having to play musical cylinders in a #### storm.

Sounds like a little gas matching leaves them equal or so close that 100 psi off either's thirds is basically correct. (However at least one reference indicates that this steel cylinder held 71.2 cubic feet at 2475 and became know as the standard 72 so perhaps holds less gas then assumed.)

Whether it comes as 3x79 or 2x119 the amount of air used is the same and weigh's the same. Dropping the empty 72 will cause very little difference from a buoyancy point of view since the tank is probably very close to neutral when empty. Buoyancy wise either diver is in about the same situation and should have the same concerns about becoming buoyant as their tanks are near empty.

Whether he drops the 3rd tank or not he's got reserve air for both me and himself and if he's more comfortable swimming without the stage bottle, has manifolded tanks and a long hose, etc. then he's in the same situation at that point as the guy with 119's.

However...

A diver breathing the 72 empty and dropping it is of slight concern to me because it is possible to get water in the tank and even the regulator. If he is the type of diver that typically mistreats his equipment, dives ancient and outdated 72 cuft tanks likely banded together with the wrong sized bands and who knows what sort of manifold... dives an unusual combination of tanks that are somewhat suspect in current condition (and I may have my doubts as to his skills and experience). Unless I know more about this guy and his equipment I may have other concerns about diving with him then whether he is dropping the stage or not.

Not breathing the stage empty and keeping it close does ballance out the air reserves better then dropping an empty but I don't see the dropped stage as any more of a problem then the 119's diving with a manifold and no stage and the same air reserves and dive plan - they are in nearly the exact same situation once the empty stage is dropped.

quote: "If he is the type of diver that typically mistreats his equipment, dives ancient and outdated 72 cuft tanks likely banded together with the wrong sized bands and who knows what sort of manifold... dives an unusual combination of tanks that are somewhat suspect in current condition (and I may have my doubts as to his skills and experience). Unless I know more about this guy and his equipment I may have other concerns about diving with him then whether he is dropping the stage or not. "

Hey! I resemble that remark! Dang. you young guys always think anything more than 10 years old is ancient and outdated. a set of 72's is nice for us old skinny guys who love to squeeze into slits where only side-mounters dare to go in mudholes typically reserved for dairy cows. we don't get much deeper than 40 feet and sip air like a princess. and what's that about bands? we use rope to hold em together, so's we can just cut and run ifin we have ta. And whut manifold? I like the good ole reliable j-valves so we know when it's time to look for the way out. I her'd some folks once talk about lines or reels or sumpin like dat; must be some new-fangled thang for them girlee-divers what'ens fraid they might get lost in the dark.

-skip

Quote:

---

Originally Posted by OneBrightGator

So you think it's safer to breathe the stage empty as your third and leave your BG as your exit/reserve?

---

Well, I'm still advocating breathing stage and doubles down to thirds. But carrying the stage throughout the dive since the other diver is not stage diving, to avoid the above scenario. In the event of that emergency, Diver 1 could donate his 7 ft hose and return to his stage until the remaining 2/3 is gone. Then dump the empty tank and continue out on his reserve backgas reg.

But you could do as you're thinking and just breathe the stage to zero for the penetration portion, if you wanted to deal with a completely empty tank from max penetration when there is no emergency. Guess you could do it either way and still have resolved the original problem.

I don't like the idea of a completely empty tank or near empty. Seems best to leave 2/3's and drop it. It's back there waiting and you know where it is. Two second stages on it might help with a fight over the gas scenario. As long as you made sure it was dropped with adequate gas for both divers to exit from that point, both divers know it and should be relieved. If divers are air sharing already, it's really nice to know and to find a tank on the way out with enough gas for both (and only one needs it).

I'm thinking of the probability of failure of 5 tanks versus 4. If at any part of the dive, you have fewer possibilities, it seems you are not as safe?

-skip

Quote:

---

Originally Posted by skip

I don't like the idea of a completely empty tank or near empty.

---

If you do any real stage diving (where both are staging and you drop at 1/2 plus 200), then a near-empty stage is a norm.

Quote:

---

Originally Posted by skip

If divers are air sharing already, it's really nice to know and to find a tank on the way out with enough gas for both (and only one needs it).

---

As I stated above, if Diver 2 is sharing off Diver 1's gas from max penetration, they will both need that stage bottle by the time they reach it.

More than one way to skin a cat . . . have Diver 2 drop a safety stage along with Diver 1's partially empty stage.

reading this makes my head hurt

Quote:

---

Originally Posted by skip

one diver has three lp72's (full at 2475; 2250 with plus rating) and the other diver has a set of doubled 119's (full at 3442). The first diver wants to use a third of one 72 and drop it, switch to back gas (doubled 72's), or should he/she carry it all the way?

how much usable gas for thirds for both divers, what is team turn pressure? how and why? no deco, no depth concerns. we can assume all tanks are full at start of dive, but that really makes no difference, right?

a third independent stage tank for one diver (while other uses doubles) has me questioning my math....

just curious, don't try this at home!

-skip

---

Diver 1
72's baseline 2.7 cu/ft for every 100 psi x 2 = 5.4 for back gas
psi @ 2475

72 baseline 2.7 cu/ft for every 100 psi = 2.7 for stage
psi @ 2250

Useable gas:

Back gas
2475/3=825 round down to 800
(800/100) x 5.4 = 43.2 cu/ft

Stage
2250/3=750 round down to 700
(700/100) x 2.7 = 18.9 cu/ft

Total 62.1 cu/ft


Diver 2
119's baseline 3.45 cu/ft for every 100 psi x 2 = 6.91 for
psi @ 3442

Useable gas:

Back gas
3442/3=1147 round down to 1100
(1100/100) x 6.91 = 76.01 cu/ft

Total 76.01 cu/ft

Therefore Diver 1 is the controlling gas volume.

To convert 62.01 cu/ft for Diver 2's 119:

(62.01/6.91) x 100 = 897.39 psi

So Diver 2 w/ 119's will use no more than 897 psi or you can round down to 800 psi for safety in order to gas match with Diver 1.

Diver 1 turn pressure:

Back gas 1600 psi
Stage 1500 psi

Please feel free to double check my math. I was using the NACD Cavern/Cave student workbook as a reference.

Hope this helps.

Tony Flaris



Sent from my iPad using Tapatalk

Why not spend $10, rent a set of tanks that match, and eliminate all the confusion?

Quote:

---

Originally Posted by jj1987

Why not spend $10, rent a set of tanks that match, and eliminate all the confusion?

---

I had fun doing the math. But you definitely have an easy solution.


Sent from my iPad using Tapatalk

Given the 4 year old posts I suspect this dive happened a long time ago...

Quote:

---

Originally Posted by JahJahwarrior

Given the 4 year old posts I suspect this dive happened a long time ago...

---

Gas matching maybe a lost science but still a lot of fun.

Thanks for the heads up.


Sent from my iPad using Tapatalk

Quote:

---

Originally Posted by tflaris

Diver 1
72's baseline 2.7 cu/ft for every 100 psi x 2 = 5.4 for back gas
psi @ 2475

72 baseline 2.7 cu/ft for every 100 psi = 2.7 for stage
psi @ 2250

Please feel free to double check my math. I was using the NACD Cavern/Cave student workbook as a reference.

---

Hey Tony, you might want to recheck your baseline calculations, these are the OLD LP72's we're talking about (rated pressure is 2250/2475, not 2400/2640 and their baseline's a bit higher than that. Hope you're having a great Thanksgiving, bud...... :)

Safe diving,

Rich

He was thrown off when the 72s were referred to as "LP-72." The "LP" designation implies 2400 wp.

A better nomenclature would be "old style 72s."

Quote:

---

Originally Posted by Rich

Hey Tony, you might want to recheck your baseline calculations, these are the OLD LP72's we're talking about (rated pressure is 2250/2475, not 2400/2640 and their baseline's a bit higher than that. Hope you're having a great Thanksgiving, bud...... :)

Safe diving,

Rich

---

You are correct the baseline for the 72's would be lower. Thanks for catching that. I have never owned any 72's so I wasn't aware of their rated pressure or plus pressures.

Safe Diving.

Sent from my iPhone using Tapatalk

Yes, the LP 72 designation is confusing. There are:
1) "Steel 72s" generally regarded as 71.2 cu ft at 2475 psi (64.7 cu ft at the 2250 psi service pressure),
2) MP 72s (PST and Faber 72s) that hold 72 cu ft at 3300 psi, and
3) aluminum 72s that hold 72 cu ft at 3000 psi.
4) IIRC, Faber made a 2400 psi service pressure 75.8 cu ft tank in the 1980s that then held 68.9 cu ft at 2400 psi.

But there is no 2400 service pressure tank that holds an actual 72 cu ft at 2640 psi.

On the Steel 72 the tank factor would be 2.8, and the same is true with the Faber 75.8.
On an MP 72 it would be 2.2, and
On an AL 72 it would be 2.4

In comparison on the mythical 2400psi service pressure LP 72 it would be 2.7 so the difference betwen it and a steel 72 and Faber 75.8 is minor.

----------

As for using a steel 72 as a stage, it really depends on the steel 72. I have probably owned 20 of them over the last quarter century and I noted that the buoyancy traits varied a lot depending on who made them and when. I am generally happy with those made by norris industries and those were the ones I have kept and they are a pound or so positive with valve when empty. Some of the others I have owned were boat anchors and were still a couple pounds negative with valve when empty and when full were a bit too negative to carry well as a stage.

-----

The numbers are what they are, but it's easy to get wrapped around the statistical axel and miss the big picture in terms of where that reserve gas is distributed and located. There are realistically only three approaches.

1) You carry the entire stage as a reserve gas, which means you'd be breathing the back gas right down to nothing (although in practice I exit quicker than I entire by design so I never fully use the 2nd "third" so there would probably be 200-500 psi left over. Something to consider though is that many regs don't breathe well when the tank pressure drops below the IP, so there is 8.6 cu ft of gas in the second "third" that is potentially not usable in a set of double steel 72s.

In the event the guy with the 119s loses all his gas at max penetration, this idea still does not completley as it potentially leaves both divers OOA and trying to be on the stage in the last portion of the exit. To keep it real, you'd need to reduce the penetration gas used by the guy with the 72s to reflect the non useable gas in his back gas to help reduce the probability that both divers would need to share the same stage. And obviously you still need to gas match and adjust the tuirn pressure to ensure that the contents of the steel 72 are enough to get the 119 diver out from max penetration.

2) You use the steel 72 as part of the "thirds" gas plan along with "thirds" in the back gas. The problem here is that if it is part of the total plan, you need to carefully consider where it may be needed. Dropping it when you hit thirds will leave it too close to the exit if you really need it in a OOA situation. You'll need to carry it in so that the full 2/3rds left in it is useful (twice as far as you actually were breathing off it). In the event the guy with the 119's loses all his gas at max penetration, the remaining gas in the steel 72s will get you back to a stage left at thirds, but you've only got one second stage on it, so the "extra" third will be problematic to access with two divers needing to buddy breathe the tank at the same time.

3) You can manage the steel 72 stage at 1/2 plus 200 with all of the reserve in the back gas. In this case you are dropping the stage when you get off it, and in that case it is going to theoretically have enough gas to get you out, but again gas matching to turn the dive appropriately is vital.

Personally, none of the plans are appealing and it makes sense to either just incldue the doubles in the plan and /or just get a bigger set of doubles for the guy with the steel 72s.

I think steel 72 are great for shallow, tight caves when diving sidemount, but I don't think they are a great when bakc mounting in a two person team with a team mate with significantly larger tanks. Adding a stage to make up the difference in gas just creates a false sense of security and a temptation to get into situations that won;t really work all that well in the event an OOA emergency really develops.