Petrel help for the mathematically handicapped...

[cmint]

Ok,

This is driving me bonkers trying to figure out...

I have a dive that I downloaded into the dive log software for iphone from a petrel. If I put my tanks in, the app gives me a "consumption rate" of .77 cf/min

Every RMV calculator on the internet I can find comes up with .88 cf/min.

Does anyone have a clue what math the app is doing to come up with .77?

[twaldo]

I LOOKED at the jpg, but could you clarify? How much gas did you use in PSI ? What are you putting in for your rated pressure of cylinders or what is the Petrel using for rated pressure of cylinders? it is showing a set of doubles but the numbers you are giving me are for a single tank.

The way I am reading the screen shot is that you started with 3600 psi and used 2200 psi leaving 1400 psi in tanks......

[jj1987]

Ok,

This is driving me bonkers trying to figure out...

I have a dive that I downloaded into the dive log software for iphone from a petrel. If I put my tanks in, the app gives me a "consumption rate" of .77 cf/min

Every RMV calculator on the internet I can find comes up with .88 cf/min.

Does anyone have a clue what math the app is doing to come up with .77?

Attachment 12854

Gas used (In cu ft) / ATA / minutes = sac

We know 85cuft was used, we know .77 was the sac, we know the dive was 51min. The only difference is more than likely the depth used.

85/x/51 = .77
-- multiply both sides by 51--
85/x = 42.35
-- multiply both sides by x for simplicity--
42.35*x = 85
-- divide both sides by 42.35--
x = 2.007 ATAs

So it's assuming average depth of 1.007*33 = 33.231ft

I'm guessing you meant that every SAC calculator is giving you .68 when you use 47ft for 51min using 85cu ft of gas.

[cmint]

Gas used (In cu ft) / ATA / minutes = sac

We know 85cuft was used, we know .77 was the sac, we know the dive was 51min. The only difference is more than likely the depth used.

85/x/51 = .77
-- multiply both sides by 51--
85/x = 42.35
-- multiply both sides by x for simplicity--
42.35*x = 85
-- divide both sides by 42.35--
x = 2.007 ATAs

So it's assuming average depth of 1.007*33 = 33.231ft

I'm guessing you meant that every SAC calculator is giving you .68 when you use 47ft for 51min using 85cu ft of gas.

It's two LP 85's, wouldn't that be 170 cubic feet? and gas used was 1400 psi ..

[cmint]

I LOOKED at the jpg, but could you clarify? How much gas did you use in PSI ? What are you putting in for your rated pressure of cylinders or what is the Petrel using for rated pressure of cylinders? it is showing a set of doubles but the numbers you are giving me are for a single tank.

The way I am reading the screen shot is that you started with 3600 psi and used 2200 psi leaving 1400 psi in tanks......

Two Lp 85's... Started with 3600 in each, ended with 2200 in each. Used 1400 psi. I used 2640 for the rated pressure and checked the box that said "doubles"

[Pavão]

It's two LP 85's, wouldn't that be 170 cubic feet?

At 2640 PSI, yes!

[twaldo]

So... I did a little research and this is what I found... NOT my numbers, link below.
June 19th, 2014, 03:36 PM
A little data crunching:

PDC: average depth reported: 61'
Desktop: average depth reported: 59.3'
Xcel: (1) average function applied to all depth data: 59.344'
(2) average function omitting zero values at end: 60.475'
(3) average function omitting first value and zero values at end: 60.528'

http://www.scubaboard.com/forums/arc.../t-485482.html


The entire thread was interesting.... seems Petrel and Shearwater software report different average depths...

Tim

[cmint]

OK to me, and it seems every other rmv calculator out there I get this:

1400 pSI divided by 51 minutes = 27.45 psi per minute

27.45 x tank factor of double 85's (0.064393939393) = 1.7676136363636

33 feet just happens to be two atmospheres

1.7676136363636 / 2 = .883806818


......

so how they got .77 is beyond my abilities....

[jj1987]

It's two LP 85's, wouldn't that be 170 cubic feet? and gas used was 1400 psi ..

Ah okay, I assumed 85 was what you used and the tank size just wasn't in view from that page.

Either way, swap out your variables and the formula I provided works the same way.

[cmint]

Got it! I had to go deeper in the settings and change the working pressure. it's working now and coming up with the same thing I got.