Calculating SAC?

[stuartv]

Perhaps it was presumptuous of me to to think you would not get it. I apologize. As someone who has gone over this 2-3 times with 5-6 divers, I can say from experience that many do not get it when they are first starting out.

A HP 80 and a LP 80 both read 800 PSI...which tank has more air or do they both have the same?

I gotcha. I understand. No worries! Like I said, I DO appreciate the time you spent writing that response.

Regarding the question of HP 80 or LP 80 with 800 psi, I would have to ask, what is the actual volume of each cylinder? I think I read that a "regular" AL80 is actually 12 liters in volume? I assume that is a LP 80? If a HP 80 is the same volume, then 800 psi in both means they have the same amount of air in them.

But the real issue with that question is that newbs like me (obviously) don't know what you mean by HP 80 and LP 80. Meaning, I/we don't know the assumptions (about cylinder volume, in this case) that are inherent in this question. You and other experienced divers probably read that question and automatically know (or assume) a specific cylinder volume for each of those. But, to me, I just know enough to know that there are many different cylinders out there, of different materials, different volumes, and rated for different pressures. So, as far as I know, there could easily be cylinders available that are 12L in volume and some rated for 2400 psi, some rated for 3000 psi, and some even rated for 4500 psi. But, if they are all 12L, then 800 psi in all of them is the same amount of air.

Honestly if you have an applied math degree with an engineering background you should be able to figure out you sac/RMV fairly easily.

I wasn't really asking about the arithmetic or formula for doing the calculation. I was asking about how one gathers the data to be used for doing the calculation. Do you REALLY just sit on the bottom at 33 feet (or whatever) and measure your pressure drop over 10 minutes to get the data for doing the calculation? Or is there some other way? That was my question. I have gotten several very good answers to that now.

Just FYI, you already have a good number of very knowledgable and experienced divers on this board just about fed up with you. It's kinda hard to get advise and help from this forum when you piss off the folks that actually have the info you need.

I probably sound like a dick right now, and if that's what it takes to get you to throttle back and think before posting...I guess being a dick isn't such a bad thing. Seriously, quit getting so pissed off every time someone tells you how to do something/why something is wrong. You come off as already having made your mind up and the hell with anyone who tells you you are wrong.

I understand most of that. I don't understand the part of about someone telling me I was wrong and I already had my mind made up (as pertaining to this thread). I don't think I said anything here where someone was telling me I was wrong. I do realize that there have been threads in the past where asking questions to try and understand what I was being told has come across as arguing with someone. I don't see that in this thread.

And I do understand that I need to not be pissing off the people who are trying to help me. And I am TRYING not to. But, sometimes it is hard when I feel like I am constantly being talked down to as if I'm a moron. I mean, seriously, if I randomly met you in a dive shop and you were looking at regs and I started explaining to you how a second stage regulator works and then paused and said to you, tomfcrist, "you're probably totally lost now", wouldn't you feel just a tiny little bit insulted and like I was being arrogant and condescending to you? I may come across as an ass sometimes, but I don't think I've been posting things that deserve me being labeled as stupid or generally uneducated. There seems to be a Culture of Superiority here on ScubaBoard and I am trying my best not to let it just completely put me off the board altogether. I really value all the information AND experience and people's willingness to share it that is here and I hope I can contribute something useful myself someday.

 

[CptTightPants21]

See post #28. 800 PSI is not the same in all of the tanks.

LP or HP is referring to the working pressure. 80, 100, etc is in reference to the volume of the tank, and the tank will be full when its PSI equals its working pressure.

 

[stuartv]

For any tank: Remaining Air is = (Remaining PSI/Working Pressure for full tank)*tank size

LP tanks will either have 2640 PSI, 3000, or 3100 PSI working pressure, HP tanks will mainly have 3442 or 3500. The smaller working pressures in the denominator means that for the same PSI and tank size, the LP will always have more air.

Can you please explain this? If I start my own manufacturing company and I make cylinders that have 12L of internal volume and I test them such that they achieve a working pressure rating of 2400 psi, and then I later take some of those exact same cylinders and test them for a higher standard and they achieve a working pressure rating of 3500 psi, and then I put 800 psi of air into one of each, how is it that one is going to have more air in it than the other.

IOW, my somewhat hazy knowledge of Physics tells me that two cylinders of the same internal volume, and being pressurized to the same pressure will contain the same amount of gas - no matter what the government or manufacturer decided to print on the side as its stated working pressure rating.

Or, to return to the point in my previous post, this only makes sense is a LP 80 and a HP 80 have different internal volumes. Which is why I responded to your earlier question by asking what the internal volume is of a LP 80 and a HP 80. If they are the same (which it sounds like they are not), then 800 psi is the same amount of gas in both.

 

[Ongrade]

There seems to be a Culture of Superiority here on ScubaBoard and I am trying my best not to let it just completely put me off the board altogether. I really value all the information AND experience and people's willingness to share it that is here and I hope I can contribute something useful myself someday.

This has been a recurring theme around here. Probably quite a few people who could offer something useful or are in need of good advice have left or just lurk because of it. I only have a couple posts because I don't feel like weeding through the smarmy, I'm smarter than you posts I'd receive if I asked a question.

 

[redacted]

Can you please explain this? If I start my own manufacturing company and I make cylinders that have 12L of internal volume and I test them such that they achieve a working pressure rating of 2400 psi, and then I later take some of those exact same cylinders and test them for a higher standard and they achieve a working pressure rating of 3500 psi, and then I put 800 psi of air into one of each, how is it that one is going to have more air in it than the other.

IOW, my somewhat hazy knowledge of Physics tells me that two cylinders of the same internal volume, and being pressurized to the same pressure will contain the same amount of gas - no matter what the government or manufacturer decided to print on the side as its stated working pressure rating.

Or, to return to the point in my previous post, this only makes sense is a LP 80 and a HP 80 have different internal volumes. Which is why I responded to your earlier question by asking what the internal volume is of a LP 80 and a HP 80. If they are the same (which it sounds like they are not), then 800 psi is the same amount of gas in both.

the internal volume of an LP x is always greater than the internal volume of an HP x. And for any given pressure, the LP x will always have more gas than the HP x (because it has more volume).

 

[Lorenzoid]

Can you please explain this? If I start my own manufacturing company and I make cylinders that have 12L of internal volume and I test them such that they achieve a working pressure rating of 2400 psi, and then I later take some of those exact same cylinders and test them for a higher standard and they achieve a working pressure rating of 3500 psi, and then I put 800 psi of air into one of each, how is it that one is going to have more air in it than the other.

IOW, my somewhat hazy knowledge of Physics tells me that two cylinders of the same internal volume, and being pressurized to the same pressure will contain the same amount of gas - no matter what the government or manufacturer decided to print on the side as its stated working pressure rating.

Or, to return to the point in my previous post, this only makes sense is a LP 80 and a HP 80 have different internal volumes. Which is why I responded to your earlier question by asking what the internal volume is of a LP 80 and a HP 80. If they are the same (which it sounds like they are not), then 800 psi is the same amount of gas in both.

I'm not sure this addresses your question, but the "working pressure rating" of a cylinder isn't entirely about a test result. It's, by definition, the pressure at which a cylinder having a given internal volume (aka "water volume") contains a stated volume of gas.

I'm not familiar with LP 80s and HP 80s, but for the HP 80 to contain 80 cf of gas at some "high pressure" and the LP 80 to contain 80 cf of gas at some "low pressure," it follows that they must have different internal volumes (aka "water volume").

 

[Edward3c]

...
Or, to return to the point in my previous post, this only makes sense is a LP 80 and a HP 80 have different internal volumes. Which is why I responded to your earlier question by asking what the internal volume is of a LP 80 and a HP 80. If they are the same (which it sounds like they are not), then 800 psi is the same amount of gas in both.

For someone who's supposed to understand math, your not coming across to bright.


Using information from post 20:
LP 80 (78cuft) = 2,209Lt divide by working pressure (207bar) = 10.7Lt
HP 80 (82cuft) = 2,322Lt divide by working pressure (232bar) = 10.0Lt


Therefore at 800PSI (55bar) the
* LP will hold 588.5Lt
* HP will hold 550Lt

 

[Lorenzoid]

Using information from post 20:
LP 80 (78cuft) = 2,209Lt divide by working pressure (207bar) = 10.7Lt
HP 80 (82cuft) = 2,322Lt divide by working pressure (232bar) = 10.0Lt

Therefore at 800PSI (55bar) the
* LP will hold 588.5Lt
* HP will hold 550Lt

What can be confusing is that a tank commonly referred to as an "HP 80" does not actually hold 80 cf but rather 82 cf, and a tank commonly referred to as an "LP 80" does not hold 80 cf but rather 78 cf. In a class I took, my instructor asked us for the sake of simplicity to disregard the fact that a LP 80 actually holds only 78 cf and just use 80 cf in the computations. With that simplification made for classroom purposes, it was easy to grasp the underlying principle.

 

[uncfnp]

Ok. I am no math major (in fact I hate it) but here goes...

A HP (High Pressure though some will argue this is not true) is 3442 psi at rated pressure and about 82 cf.
A LP (Low Pressure) tank is filled at 2400 psi and about 77 cf

More pressure = more gas at a given volume. And overfill (a fill over rated pressure) equals even more gas. More pressure in a given volume will = a greater drop in cf per psi.

So, one must calculate the cf per psi for the tank.

A reference for various size and pressure tanks...

http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

A great primer on scuba cylinders...

https://www.divegearexpress.com/library/tanks.shtml#pressure

 

[EFX]

For someone who's supposed to understand math, your not coming across to bright.

Using information from post 20:
LP 80 (78cuft) = 2,209Lt divide by working pressure (207bar) = 10.7Lt
HP 80 (82cuft) = 2,322Lt divide by working pressure (232bar) = 10.0Lt

Therefore at 800PSI (55bar) the
* LP will hold 588.5Lt
* HP will hold 550Lt

Stop throwing stones you, who lives in a glass house.
First, your working pressures in bar are wrong.
Second, your wrong answers of 10.7 and 10.0 are in the wrong units.

LP80: 800 psi / 2400 psi * 78 ft3 = 26.0 ft3 = 736 L
HP80: 800 psi / 3450 psi * 82 ft3 = 19.0 ft3 = 538 L

For any tank: Remaining Air is = (Remaining PSI/Working Pressure for full tank)*tank size

CptTightPants21, the above quoted information is too obtuse to be of any help to new divers in this forum. The tank size could be: (1) outside volume, (2) inside liquid volume, or (3) the gas volume at 1 ATA. Everyone on SB, please give more information and be careful of your units so those of us who aren't so "bright" stand some chance of getting it. Sheeesh!!!!