Software - Working Pressure (SAC)?

[ChrisEdwards]

I have a Pro Plus 2 and am using version 2.1.4 bult 7/16/07.

I notice the SAC rate only calculates when a Working Pressure is entered manually. What is Working Pressure?

Please excuse my noobie understanding, but I would have thought SAC would be calculated using the starting and ending pressures.

Should I enter my Starting Pressure into the Working Pressure?

Thanks

 

[MacGyverDiver]

I'm not familiar with this particular software package... but working pressure is (normally) defined to be the rated working pressure of the tank (e.g., 3000 PSI for a typical 80 cu ft aluminum tank).

 

[The Kraken]

The working pressure of the tank.

Al 80's are usually 3000 psi. HP steels are usually 3442 psi.

the K

 

[ChrisEdwards]

That's what I figured but why should that be used to calculate the SAC?

Am I off base here?

 

[Charlie99]

It has to do with how tanks are measured in the imperial system.

An AL80, for example, when pressurized to 3000psi holds roughly 80 cubic feet (actually 77.4) of air when allowed to expand to 1ata. When you enter 77.4 capacity at 3000psi, the program can calulate the internal volume and figure out how much gas you have used for a given amount of pressure.

If you have a tank that hold 80 cu feet with a pressure of only 2000psi, then the same amount of pressure difference between beginning and end of the dive corresponds to a greater amount of air used.

To say the above another way, take an example of two tanks each with 80 cubic feet capacity. One has to be filled to 3000psi to contain 80 cubic feet. The other has 80 cubic feet when filled to 2000. If you use 1000psi during the dive, for one tank you have only used 1/3 of 80 cu ft, with the other than you have used 1/2 of 80 cu ft.

=============

In the metric system, it is a non-issue, since they refer to tanks by the internal volume rather than the total rated capacity at rated service pressure.

 

[ChrisEdwards]

Thanks, that cleared it up completely.

So if I want to calculate an accurate SAC, I have to note the Working Pressure on the tank I'm diving with,and I guess most of them will be stamped 3000, if it's an AL80. So if I have a tank that has a WP of 3000psi, but the tank has 2900psi, when I gear up for a dive, I've actually got less than 77.4cu/ft.


Now I understand, I'll start at a baseline of 3000 for WP for past dives.

 

[The Kraken]

2900psi/3000psi = .9666666666666 of total working pressure capacity

.96666666666 X 77.4 cu. ft. = 74.82 cu. ft.

the K