questions re. SAC rate calc's

[DaleC]

I've been using Lamonts rock bottom charts for turn pressures etc... while rec diving and I thought I'd set up an excel program to calculate my sac rates and want to run it by someone to make sure it's correct. I know an accurate SAC rate takes into account the SAC for each depth and stops etc... but I am using my Avg. depth just to get a rough SAC for rock bottom calculations (hopefully that's alright).

Here are my fields in MS Excel:
A. Date
B. Dive #
C. Max depth
D. Avg. depth
E. Start PSI
F. End PSI
G. PSI used [= E-F]
H. Tank Size.
I. Dive duration in minutes
J. PSI to Cu ft. used [=Gx77.4/3000]
K. Cu ft./min. used [=J/I]
L. Avg. Atmospheres [=(D+33)/33]
M. SAC rate [=K/L]

I am not sure about J and L.
J: This is assuming a tank fill of 3000 PSI but will other fills (say 2800psi or 3200psi) effect the final SAC rate dramatically?
L: Is this formula correct? I sometimes see this and sometimes (D+33)/33+1

Here's an example of what I get:

C. Max. Depth: 70'
D. Avg. Depth: 34'
E. Start PSI: 2900
F. End PSI: 600
G. PSI used 2300
H. Tank Size: Al 80 (77.4 cu ft.)
I. Time: 38 min.
J. PSI to cu ft. used: 59.34
K. cu ft./min. used: 1.56
L. Avg. atmospheres: 2.03
M. SAC rate: 0.769

So when I look at J. I wonder if the formula should be [Gx77.4/E] not [Gx77.4/3000]
and when I look at L. I see an avg. atmospheres of 2.03 but I wonder if it should actually be 3.03 (accounting for the extra surface Ata)?

I like to ask the good questions

 

[Bubbletrubble]

1. Perhaps you should modify the enterable tank data to be "Tank Capacity" (in Cu. ft.) and "Tank Service Pressure" (in PSI). This would make it more useful for people who dive different tanks. For example, for an AL80, Tank Capacity = 77.4 and Tank Service Pressure = 3000.

2. Your calculation of Avg. Atmospheres should be [=D/33+1] with D representing "Avg. depth."

I would change the fields to be:
A. Date
B. Dive #
C. Max depth
D. Avg. depth
E. Dive duration in minutes
F. Start PSI
G. End PSI
H. Tank Capacity in Cu. ft.
I. Tank Service Pressure PSI
J. PSI used [= F-G]
K. Cu. ft. used [=JxH/I]
L. Avg. Atmospheres [=(D/33+1)]
M. RMV in Cu. ft./minute [=K/L/E]

I placed all of the fields that require User values in the first block A.-I.
"Calculation" fields are J.-M.

Hope this helps...

P.S. I did this very quickly so please check the arithmetic.

 

[DaleC]

Thanks BT,
I see that both your and my avg. ata formulas get the same result but by comparing the two I now get where the surface atmosphere is accounted for.

I agree that the user input and formula fields should be grouped that way. It makes data entry easier.

About the tank service pressure. For an AL 80 would it be a constant (3000) or would it be the psi that the tank is holding at the beginning of the dive. ie.) if I enter with a tank holding 2500 psi do I enter 2500 or 3000? I'm still fuzzy on how this value effects the formula.

 

[pengwe]

Dale,

At J: Where you have 77.4/3000 that means that that particular tank would hold 77.4 cu. ft of air at 3000 psi. It is given for that tank, and doesn't change. The pressure of your actual fill affects how much air you actually have, but it doesn't change how much air you would have, if the tank was filled to 3000psi.


3000 psi = 77.4 cu ft of air (for that tank)
1 psi = 77.4/3000 cu ft
Q: If I used 2200 psi, how many cu ft did I use?
A: 2200*(77.4/3000)

Different sized tanks will hold different amounts of air at 3000 psi. It's a given value that you need to know about each tank.

At L: (D+33)/33 = (D/33) + (33/33) = (D/33) + 1. If the equations are written on a single line without brackets, it gets confusing, but they are all the same.

 

[DaleC]

Thanks Pengwe,

Just to make sure I have it right in my head.

If I'm using bubbletrubbles corrected format: [I.] is a constant value for an Al80 (3000) no matter the actual psi in the tank at the start of the dive; but changes for a different tank size (ie ST130)?

Sorry if I'm being a bore about this but once I get it I get it - it's just getting it that's the hard part; like the Avg. ata formula... I can actually see how they all work that way now.

 

[TSandM]

Each tank has a working pressure at which it holds its rated volume. An Al80 is 77.4 cubic feet at 3000 psi. That tells you that each psi in the tank is 77.4/3000 cubic feet. It doesn't matter whether that's the first psi or the 3000th, it's the same volume. So if you use 1000 psi, you're using 77.4/3 cubic feet (25.8 cubic feet) no matter whether you went from 3000 to 2000, or 2000 to 1000. It's the same.

A different tank will have a different ratio of volume to pressure. 100 psi in an Al80 is about 2.5 cubic feet. 100 psi in my LP95s is more like 3.5 cubic feet. You have to know the working pressure and the rated volume to put the numbers into a SAC rate calculation.

 

[DaleC]

Thanks Lynne,
I guess I just don't see how the values in the formula relate to each other.

For example in (D+33)/33 I see the depth plus 33 (for the surface atmosphere) divided by an atmospere to give the avg. atmosphere.

But in JxH/I I don't quite understand the relationship between psi used, tank volume and tank service pressure. I keep thinking that the various starting psi pressures should be taken into account some how, that [I.] should be a variable. Maybe I'm just chasing a ghost though.

 

[AndyNZ]

Life is a little easier if you go metric. A 12 litre tanks holds 12 litres of gas at atmospheric pressure, 24 liters at 2 bar etc.

The imperial meaurement of tanks is a combination of working pressure and tank volume - so when an 80 cu ft tank is filled to the correct working pressure, the volume of gas in the tank expanded to atmospheric pressure fills 80 cu ft.

Unfortunately, a "nominal" 80 cu ft tank rarely holds 80 cu ft of gas - hence the confusion between gas used (in psi), tank volume and working pressure.

I'm probably on a hiding to nothing to convert everyone to metric, but....


So, I dive two 12 litre tanks totalling 24 litres internal volume. If in 10 minutes at the surface I breathe 5 bar, that's a total of 24 * 5 = 120 litres, or 12 litres per minute.

If I breathe 20 bar at 10m in 10 minute, I just need to adjust it back to the surface but essentially it's the same calculation.... so total gas used is 24 * 20 = 480 litres, or 48 litres per minute. Ambient pressure at 10m is 2 ATM, so divide by 2 to give my surface rate of 24 litres per minute.

 

[pengwe]

Thanks Pengwe,

Just to make sure I have it right in my head.

If I'm using bubbletrubbles corrected format: [I.] is a constant value for an Al80 (3000) no matter the actual psi in the tank at the start of the dive; but changes for a different tank size (ie ST130)?

Sorry if I'm being a bore about this but once I get it I get it - it's just getting it that's the hard part; like the Avg. ata formula... I can actually see how they all work that way now.

Yep, you got it. H and I are constant for any given tank. The manufacturer tells you what they are. Different tanks have different values. E.g. for an ST130, H=130, I= 3442.

It is a measurement of volume. How big is this tank? It's big enough to hold 77.4 cu ft of air at 3000psi. If you have a 1-gallon jug, and pour a pint of water in it, it's still a 1-gallon jug. If you have a "77.4 cu ft of air @ 3000 psi" tank, it doesn't change size just because it's only three-quarters full.

Here's some questions for you:
If you fill your tank to 3000 psi, how much air is in it?
If you fill the tank to 2000 psi, how much air is in it?
If you fill it to 2800psi, how much air is in it?
Then you breathe it down to 700psi, how much air is left?
How much air did you use?

 

[NJMike]

I created an Excel spreadsheet to compute the rock bottom turn pressures. It requires that you input the SAC rate, though, as well as tank volume, tank pressure, and the max depth.

I'll be happy to share that if anyone wants it.