Quaack
Contributor
JudeT-F:
You could possibly call or email the dive op you went with to see what they rented you. Chances are it was an Al 80.
Dave
Another Oceanlog 2.0.2 Question (SAC Rate)
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You could possibly call or email the dive op you went with to see what they rented you. Chances are it was an Al 80.
Dave
The Kraken
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Quaack:
Kinda-sorta. An aluminum 80, at rated pressure, contains only 77.4 cubic feet of gas.
But theoretically, yes . . .
Now as to Quaack's question . . .
You could pretty much safely assume that it's an aluminum 80 with a working pressure of 3000 psi and a nominal capacity of 77.4 cubic feet.
the K
Most of my starting pressures were slightly over 3000psi; I assume, then, that assuming the working pressure is only 3000 would cause some mathmatical issues. Should I just take my starting pressure (as recorded by the computer) and round up to the nearest hundred?
jtf
jtf
The Kraken
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If the correct formula has been entered into the program code correctly, you should be able to enter your exact starting pressure and the software should calculate your SAC correctly.
It's important that the correct pressures be entered into the formula, otherwise it will return an erroneous SAC.
For example, if you understate your beginning pressure, your SAC rate will be over stated. Conversely, if you overstate your beginning pressure, your SAC rate will be understated.
the K
It's important that the correct pressures be entered into the formula, otherwise it will return an erroneous SAC.
For example, if you understate your beginning pressure, your SAC rate will be over stated. Conversely, if you overstate your beginning pressure, your SAC rate will be understated.
the K
Some samples:
Standard AL 80 - 77.4CF @ 3000 psi = 38.76 PSI/CF
"Super" 80 - 80 CF @ 3000 psi = 37.5 PSI/CF
E7-80 - 80 CF @ 3442 psi = 43.03 PSI/CF
So... if your SAC Rate is 25 psi/min,
For the standard 80 your SCR would be .65 CF/M (RMV 18.4 LPM)
For the "Super" 80 it would be .67 CF/M (RMV 19 LPM)
and for the E7-80 it would be .58 CF/M (RMV 16.4 LPM)
- or -
to look at it in the other direction...
If your SCR is .65 CF/M
You will use (per ATA) 25 psi/min from a standard 80,
24 psi/min from a super 80,
or 28 psi/min from an E7-80
And that's why working pressure and tank capacity both count
Rick
Standard AL 80 - 77.4CF @ 3000 psi = 38.76 PSI/CF
"Super" 80 - 80 CF @ 3000 psi = 37.5 PSI/CF
E7-80 - 80 CF @ 3442 psi = 43.03 PSI/CF
So... if your SAC Rate is 25 psi/min,
For the standard 80 your SCR would be .65 CF/M (RMV 18.4 LPM)
For the "Super" 80 it would be .67 CF/M (RMV 19 LPM)
and for the E7-80 it would be .58 CF/M (RMV 16.4 LPM)
- or -
to look at it in the other direction...
If your SCR is .65 CF/M
You will use (per ATA) 25 psi/min from a standard 80,
24 psi/min from a super 80,
or 28 psi/min from an E7-80
And that's why working pressure and tank capacity both count
Rick
Kraken,
Given that my starting pressure is what the computer recorded in the first place (I used the wireless-integrated computer), then, if I'm understanding you correctly, I'm just copying the starting psi as recorded into the working pressure field.
Well, I tried it, and here's what I got back:
Info recorded by computer
Elapsed dive time: 67 minutes
Start pressure: 3040
End pressure: 750
Info I entered in myself
Working pressure: 3040
Cylinder size: 80 (cubic feet)
This returned a SAC Rate of: 0.603 cubic feet/minute...
Does that sound about right?
jtf
Given that my starting pressure is what the computer recorded in the first place (I used the wireless-integrated computer), then, if I'm understanding you correctly, I'm just copying the starting psi as recorded into the working pressure field.
Well, I tried it, and here's what I got back:
Info recorded by computer
Elapsed dive time: 67 minutes
Start pressure: 3040
End pressure: 750
Info I entered in myself
Working pressure: 3040
Cylinder size: 80 (cubic feet)
This returned a SAC Rate of: 0.603 cubic feet/minute...
Does that sound about right?
jtf
The Kraken
He Who Glows in the Dark Waters (ADVISOR)
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Depends . . .
If it's an aluminum 80, the working pressure would more than likely be 3000 psi and your volume should be 77.4 cubic feet.
What did it show as your average depth of dive?
From the numbers you give, it would seem that your average depth of dive was ~15'.
Using 15' as average depth of dive and the data you supply, I come up with a SAC rate of 0.618.
Changing the working pressure to 3000 psi and the tank volume to 77.4 cf and leaving the other data the same, I come up with a SAC rate of 0.606.
the K
If it's an aluminum 80, the working pressure would more than likely be 3000 psi and your volume should be 77.4 cubic feet.
What did it show as your average depth of dive?
From the numbers you give, it would seem that your average depth of dive was ~15'.
Using 15' as average depth of dive and the data you supply, I come up with a SAC rate of 0.618.
Changing the working pressure to 3000 psi and the tank volume to 77.4 cf and leaving the other data the same, I come up with a SAC rate of 0.606.
the K
I don't know of any 80 CF cylinder that has a working pressure of 3040 psi. What's stamped on the neck/shoulder of the cylinder?JudeT-F:
Rick
We might be talking past each other...
I have no idea what the _actual_ working psi of the tanks were - they were rentals and it wasn't something I took note of (I'm a new, recreational diver). I used my VT Pro w/ wireless transmitter, so my only indicating of my starting/ending psi is what the computer recorded.
In a previous post Kraken suggested entering my working pressure as the starting pressure in order to derive psi. Since the computer recorded my starting pressure as 3040, that's what I entered as the max pressure. Presumably that's not the _actual_ max pressure of the tank, but I was just following the suggestion. (It seems to me that, if the computer has 3040 listed as the starting pressure, I shouldn't put 3000 as the working pressure - at least intuitively, it doesn't make sense to have the starting pressure be higher than the pressure the tank is capable of, no?)
The tanks were, as I recall, aluminum.
But it seems the math is right; Kraken, you called it... The avg. depth for that dive is listed as 16.17' So, I guess it seems more or less correct. Given that I don't care what my _actual_ SAC is but rather want to begin to compare one dive to the next to get a feel for how it changes, then as long as I have a consistent system (I'll use the stating pressure as the working pressure, and 77.4 as the cylinder size unless I know it to be differet), I should be able to begin to learn about my air consumption, at least in a relative way.
jtf
I have no idea what the _actual_ working psi of the tanks were - they were rentals and it wasn't something I took note of (I'm a new, recreational diver). I used my VT Pro w/ wireless transmitter, so my only indicating of my starting/ending psi is what the computer recorded.
In a previous post Kraken suggested entering my working pressure as the starting pressure in order to derive psi. Since the computer recorded my starting pressure as 3040, that's what I entered as the max pressure. Presumably that's not the _actual_ max pressure of the tank, but I was just following the suggestion. (It seems to me that, if the computer has 3040 listed as the starting pressure, I shouldn't put 3000 as the working pressure - at least intuitively, it doesn't make sense to have the starting pressure be higher than the pressure the tank is capable of, no?)
The tanks were, as I recall, aluminum.
But it seems the math is right; Kraken, you called it... The avg. depth for that dive is listed as 16.17' So, I guess it seems more or less correct. Given that I don't care what my _actual_ SAC is but rather want to begin to compare one dive to the next to get a feel for how it changes, then as long as I have a consistent system (I'll use the stating pressure as the working pressure, and 77.4 as the cylinder size unless I know it to be differet), I should be able to begin to learn about my air consumption, at least in a relative way.
jtf
The Kraken
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The reason you need to add the working pressure of the tank to the equation is that it acommodates for an over fill.
Simply put, if your starting pressure was 3040 and you put your working pressure in as 3040 and the tank volume at 80 cubic feet, then the calculation would assume that you had 80 cubic feet of gas in the tank.
However, if the working pressure of the tank is 3000 psi, then you had a very slight overfill, which means that you had MORE than 80 cubic feet of gas in the tank. This would result in an erroneous reduction of your gas usage.
I'll expand.
Suppose you have a 100 cf tank at 3000 psi. That equates to 30 psi per cubic foot.
Now suppose your starting pressure was 3900 psi (I know, a bit exaggerated).
This would mean that you had an additional 30 cubic feet of gas in the tank (130 cf).
If you used your starting pressure as your working pressure, then the formula would determine that you had used only 100 cf of gas during your dive when, in actuality, you had used 130 cubic feet.
the K
Simply put, if your starting pressure was 3040 and you put your working pressure in as 3040 and the tank volume at 80 cubic feet, then the calculation would assume that you had 80 cubic feet of gas in the tank.
However, if the working pressure of the tank is 3000 psi, then you had a very slight overfill, which means that you had MORE than 80 cubic feet of gas in the tank. This would result in an erroneous reduction of your gas usage.
I'll expand.
Suppose you have a 100 cf tank at 3000 psi. That equates to 30 psi per cubic foot.
Now suppose your starting pressure was 3900 psi (I know, a bit exaggerated).
This would mean that you had an additional 30 cubic feet of gas in the tank (130 cf).
If you used your starting pressure as your working pressure, then the formula would determine that you had used only 100 cf of gas during your dive when, in actuality, you had used 130 cubic feet.
the K
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