Definiton of cylinder working pressure

[dieseldude]

I'd appreciate some clarification of cylinder working pressure in terms of calcualting SAC.
Is this always & forever the max working pressure of the cylinder OR is it the available pressure you started the dive with.
It makes a large difference in the final SAC #-
Also if you do use the actual available psi then do you change the cyl capacity based on the reduced amount of air?

I have never had a tank filled to capacity, in fact my regular LDS does good to manage 3000 psi & get the mix right. I'm doing calcualtions based on available air at the start & it just doesn't pencil out in my head. I breathe a little heavy but not terrible.

Anything I should know when calculating doubles othe than cylinder capacity x 2?

Thanks,
MJII

[lamont]

working/service/rated pressure is the same.

typically what you are doing is figuring out the fraction of the tank used in psi and using that to compute the fraction of the tank used in cu ft. you need to use rated capacity at rated pressure to do that. for 2000 psi in a luxfer aluminum 80 (actually 77.4 cu ft):

2000 psi / 3000 psi * 77.4 cu ft = 51.6 cu ft

the 77.4 and 3000 psi come from the tank specs, and it doesn't matter how much it got filled to. if it was filled to 3100 psi and you used 2000 psi then you wound up with 1100 psi. doesn't matter if it was filled to 2900 psi, though, if you use 2000 psi of it, then its still 51.6 cu ft for that 2000 psi you burned.

[Paulicarp]

I'm not qualified to properly answer this question as I am only starting to learn this myself, but I wanted to respond right away to point you in the right direction and warn you that you should probably get the kinks out of this before you dive. Your life depends on it!!

I'd appreciate some clarification of cylinder working pressure in terms of calcualting SAC.

cylinder working pressure is helpful in knowing the tank factor or tank constant, that is, the number of cubic feet of gas a given cylinder holds at a particular psi. As psi in the tank changes, so does the amout of cubic feet of gas. Working pressure of the clyinder really has nothing to do with calculating your SAC directly. It only helps you identify the tank factor that you need to know in order to calculate your SAC. Once you know that "tank factor" for the tank you're using, working pressure is not relevant to calculating your SAC.

Is this always & forever the max working pressure of the cylinder

In a sense, yes. it is the PSI that that tank is rated to be filled to, that number never changes regardless of how many PSI are actually in the tank.

OR is it the available pressure you started the dive with.

NO!! you start the dive with what you start the dive with! that's what the guage is for. Tanks may be filled to any given PSI, and you are always responsible to know what pressure and what gas you start dive with in your tank!!

It makes a large difference in the final SAC #-

yes, it would. If you don't use the right numbers, you will likely get the wrong results. I strongly recommend that you read this article, if you haven't already http://www.nwgratefuldiver.com/articles/gas.html and if you have a hard time understanding this paper, please talk to a qualified instructor to learn how to calculate this properly.

Again, those of you who are experienced with this stuff, please correct me if I'm wrong, I'm just learning this stuff and I hate to be the blind leading the blind.

[dieseldude]

Thanks for both your answers. I don't actually think I'm blind here or I wouldn't have been asking the question in the 1st place. I did get the answer I was looking for & my #'s are better than what I had pencilled out previously though I would still like to see my sac considerably lower. In my case I bought a used set of doubles(120's) & have no clue what brand they are as I can't find it on the bottle-I do see 3500 stamped in it & will assume this is my working pressure- I found a chart for xs scuba tanks that tells me a 120 actual capacity is 120.6 . I was using 240 even for cylinder capacity which should give me a small measure of conservatism(ok real small)
I have begun Tech training so I'm starting to see the value in these computations but have to be honest, at a rec level for the casual out & back at my favorite sight, all this computation is alot of work. I've never had an issue with using the rule of 3rds & like most of you I have pushed this limit a little knowing that I was in close proximity to me exit point & at a safe depth to allow for multiple exit strategies if i did catastrophically loose air. My strategy with a dive buddy is a little more conservative than when I'm solo given theres only 1 guy to rescue.
Of course Deco stops put a whole new twist in things & thus my interest in computing sac. I don't know how many of you have been asked to swim as fast as you can against a rock that doesn't move for as long as you can but it definitely is an enlightening experience for how much air a person can suck when severely stressed. I belive it's cheap entertainment for the instructors too, but hey, for the little money they make they deserve a good laugh.
Thanks for the help & the link to Bob's page-Together with what i've learned already it makes good sense & reads well.
MJII

[Sounder]

As good as the information on this board is, this sounds like a good discussion to have, in person, with your tech instructor.

On a side note, who are you taking your technical training through?

[Pez7378]

Also if you do use the actual available psi then do you change the cyl capacity based on the reduced amount of air?

No. The capacity of the cylinder will never change.

Anything I should know when calculating doubles othe than cylinder capacity x 2??

Tank factor. Without getting into all the calculations, Your tank factor for 120's is 6. This is useful information if you know how to use it.

I agree with Sounder, consult with your instructor also. He/She may have a very simple way to answer your questions.

[CaptnJack]

Tank factor. Without getting into all the calculations, Your tank factor for 120's is 6. This is useful information if you know how to use it.

Not quite...
Tank factor of double hp120s is 7 cf/100psi
240/34.42 = 6.972 rounds to 7

Tank factor of double lp120s (lordy does anyone actually own these??) is 9 cf/100 psi
240/26.40 = 9.09

[dieseldude]

To respond to the question, My training is through TDI & my instructor is always available for quesitons & comments & frankly gets pummelled several times a week with questions. I was looking for some different verbage that might make it stick in my head-
For what it's worth, there was a discussion not so long ago on "these boards" about me carrying a spare air bottle. While on these boards people like Mr.Sounder did an excellent job of making me feel stupid. My instructor took me under water & made me do some sac work(not in a tech class)& SHOWED me the bottle did not have enough air by letting me see for myself how much air I would need in a panic situation.
I am a senior master technician & if there is one thing that I can carry over from my work to diving, it is that you can tell anybody what to do, teaching them to think & figure it out for themselves is the real challenge. I don't take anything from anyone without investigating it until it makes solid sense in my own mind.
Again thanks for the answers I got-they helped me get where I need to be
MJII

[Pez7378]

Tank factor. Without getting into all the calculations, Your tank factor for 120's is 6. This is useful information if you know how to use it.

Not quite...
Tank factor of double hp120s is 7 cf/100psi
240/34.42 = 6.972 rounds to 7

Tank factor of double lp120s (lordy does anyone actually own these??) is 9 cf/100 psi
240/26.40 = 9.09

Thanks Richard, I was going off of these numbers. ](*,)

I bought a used set of doubles(120's) & have no clue what brand they are as I can't find it on the bottle-I do see 3500 stamped in it & will assume this is my working pressure

But for some reason, I get a different set of numbers on my phone calculator that seems to round down? Stupid Math.

[Pez7378]

I was looking for some different verbage that might make it stick in my head-

I need to do this also. I've listened to so many variations of deco procedures.

I need a RONCO ECCR. That way I can just "Set it and forget it!"

[Paulicarp]

While on these boards people like Mr.Sounder did an excellent job of making me feel stupid.

Yeah, me too - he really knows his stuff. but every time I get taken to task, I learn more and that's what really matters. Plus, when I start to feel stupid, I can just compare myself with PEZ and don't feel so bad

I am a senior master technician & if there is one thing that I can carry over from my work to diving, it is that you can tell anybody what to do, teaching them to think & figure it out for themselves is the real challenge. I don't take anything from anyone without investigating it until it makes solid sense in my own mind.

I couldn't possibly agree with you more. Diving has inherent hazzards associated with it, and learning to be a "thinking" diver is a critical step to safely enjoying what I'm told is some of the best and most challenging diving in the world.

Again thanks for the answers I got-they helped me get where I need to be
MJII

Cool- glad I was able to help point you in the right direction. Didn't mean offend with the "blind" comment, it's just that I'm not one to be taking advice from, so in that sense I consider myself to be among the blind here. Even blinder than PEZ, and that's saying a lot! (My calendar says it's "bash on PEZ" day)
One more suggestion- Brian (Bdub) is doing a free workshop on Wednesday night. He will cover this stuff in detail. http://www.nwdiveclub.com/viewtopic.php?f=9&t=7575
Any chance you can make it? It would be great to see you there and meet you in person!

--Paul

[Pez7378]

Even blinder than PEZ, and that's saying a lot! (My calendar says it's "bash on PEZ" day)

Do your worst! I've got video evidence that I'm lousy at Math! And I double checked my calculator, and it's spitting out strange numbers. For instance, I know my 98's are TF 7. My phone calculates them at TF 8?

Plus, when I start to feel stupid, I can just compare myself with PEZ and don't feel so bad

But, can you dive?!?!?! no.

[spatman]

Plus, when I start to feel stupid, I can just compare myself with PEZ and don't feel so bad

But, can you dive?!?!?! no.

[Paulicarp]

StPauliGirl

[CaptnJack]

Even blinder than PEZ, and that's saying a lot! (My calendar says it's "bash on PEZ" day)

Do your worst! I've got video evidence that I'm lousy at Math! And I double checked my calculator, and it's spitting out strange numbers. For instance, I know my 98's are TF 7. My phone calculates them at TF 8?

you need a calculator??
(98 is just OMS's name for a 95 btw - Faber the manufacturer calls them 95s)
little less than a 100 divided by a little more than 25 = 3.5 (each)
precise math: 95/26.4= 3.59 cf per 100psi
x2 = 7 cf per 100 psi

Plus, when I start to feel stupid, I can just compare myself with PEZ and don't feel so bad

But, can you dive?!?!?! no.

This is internet diving, no water required.

[Sockmonkey]

This is internet diving, no water required.

I'm ready... I got my card!

PADI-Inet-Diver.JPG (5.2 KiB) Viewed 1895 times

[CaptnJack]

This is internet diving, no water required.

I'm ready... I got my card!

PADI-Inet-Diver.JPG

I'll see your card and raise you a patch!

[Pez7378]

you need a calculator??

Yes. I do. Truth is, I can't even add cream to my coffee without help.

[CaptnJack]

you need a calculator??

Yes. I do. Truth is, I can't even add cream to my coffee without help.

No patch for you!! but you can have Joe

[Paulicarp]

Can I get a cool patch for my SDI computer-diver DIR cert?

[Sounder]

Can I get a cool patch for my SDI computer-diver DIR cert?

No, but we'll "arrange" to be "in the neighborhood" when you're doing your certification dives... with HD video.

[Sockmonkey]

Can I get a cool patch for my SDI computer-diver DIR cert?

No, but we'll "arrange" to be "in the neighborhood" when you're doing your certification dives... with HD video.

You got your ass beat today didn't you.

[Sounder]

Can I get a cool patch for my SDI computer-diver DIR cert?

No, but we'll "arrange" to be "in the neighborhood" when you're doing your certification dives... with HD video.

You got your ass beat today didn't you.

Actually, today went really well!! Yes, we got our asses kicked today, but we managed it well. That water is COLD on your face without your mask for 1/2 the dive, but yeah - we really improved and I feel like we're starting to come together as a team. There is definitely a mentality can has to be taught this way - I noticed my approach to things was different today... and it worked!

I'll post a report tomorrow - came straight from Cove 2 to class tonight, then home to rinse gear later, then I have to read for tomorrow's Biz Law class, and eventually go to bed so my 8mo can wake me up (Baby Sounder is sick, again... aaah, daycare).