SAC Rate

[BradMM]

Still an interesting discussion but I found my mistake. Thx

 

[Alekseolsen]

I do feel sorry for you imperial guys when reading your formulas!

I struggle to follow the imperial calculations, but do remember that the compressability if air reduces with pressure, so a 10L bottle at 300 bar contains roughly 2700 liters, not 3000.

Could that explain your difference?

 

[NAM001]

Interestingly, on the same page, the metric version of the formula does not include working pressure.

(bars used x cylinder capacity) / {[(depth/10) + 1] x minutes}

that is a bit different because the term bar is tied to both depth and the volumn in a workable format. If imperial used our capacity in say cubic inches it would be some what the same. but we dont measure consumption in cubic inches. metric volumn is the same measurment used in consumption measuring. liters to liters ours is cubic inches to cubic feet. metric is so much better to do these calc's than imperial is. metric is gage bar times tank physical volumn say 15 liters. if you use 50 bar you used about 750 liters of gass. the same in imperial would be 750 psi * 10/28 equals ltrs. For those that can visualize it. take a s 80 .... 77 cuft per 3000 psi easier is divide by 3 and say 1000 psi per (about) 25 cuft. that makes 100 psi about 2.5 cu ft. You can do it any way you want to. Just make it easy so being narced a bit dont make it too hard. if your gage is marked in hundred psi tics then it would be 100 psi = 3000/77. what ever you do make the tank factor the same on all of your tanks. IE psi per cuft or cu ft per 100 psi. Standard is i believe,,,,, psi per cu ft so a s 80 should have a tank factor of (77 cu ft tank) 3000/77.4....

 

[NAM001]

Last first... yes, I used 3400 and not the actual capacity of 3442. Thanks for pointing that out.

I never considered the fact that my pressure gauge reading on one tank will not reflect the actual gas consumed based on the different tank being used. I just recently bought the HP80 so I'm still learning but that appears to be where things got off track.

Thanks a bunch, that will keep me busy for a while!

tank actual volumn 80 vs 77.4 is different also on the 2 tanks.

 

[tbone1004]

@BradMM I don't think you used the actual water capacity or you would have found the difference right there.
Faber HP80's are 10.2L water capacity
AL80's are 11.1L water capacity

That is fairly substantial when it comes to SAC calculations so I suspect you were using an incorrect value in the formula

 

[Storker]

Could that explain your difference?

Probably not. Compressibility doesn't matter much until you pass 240 bar, and 'murrican tanks aren't hydro'd for more than some 240-ish bar.

 

[Kmart921]

In the future, whenever doing conversions of any sort, look at the units when doing your calculations and everything makes a whole lot more sense.

For example... my average SAC using an AL80 is around 14 PSI/min. An AL80 tank contains 77.4 cubic feet of gas at 3000 PSI working pressure. So to convert from SAC (psi/min) to RMV (cuft/min) you multiply 14 PSI/min x 77.4cuft/3000PSI, the PSI cancels out and you get 0.36cuft/min. You can repeat this calculation for any tank configuration you like and you should arrive at the same value for RMV.

 

[atdotde]

Probably not. Compressibility doesn't matter much until you pass 240 bar, and 'murrican tanks aren't hydro'd for more than some 240-ish bar.

I thought so, too but in particular for gas consumption it does: When real gas corrections matter – The Theoretical Diver

BTW, imperial tank sizes are also often not actual sizes but marketing numbers somewhat related to the actual content.

 

[Storker]

imperial tank sizes are also often not actual sizes but marketing numbers somewhat related to the actual content.

One of the many reasons I prefer the Euro standard for designating tank size: water volume and service (fill) pressure

 

[NothingClever]

I’m so not looking forward to Imperial measurements when I return to the US.