SAC equation for diving program..NEED HELP PLEASE :)

[gregmontalbano]

OK, so I guess I'll say hey to everyone, and start off with a pretty easy question, I'm doing a project for school, Java class (a computer programming class) and need some help with the diving aspect. Although the teacher will never know the difference, that the math is wrong...still I think this could be useful. So far I have (from the SSI Log book pages-

SAC(its SCR really, same thing?) = (PSI / time) * 33 / Depth + 33

So is this right, all I need? I did some more hunting and found quite a few equations, such as -

SCR = (P2/P1) x Vf/Aa/Tm
Where
P1 = pressure in psi at beginning of test dive
P2 = pressure in psi at end of test dive
Vf = actual volume of air in tank in cu. ft.
Aa = pressure in absolute atmospheres
Tm = time of dive in minutes

as well as others. So now that I have the geeky coding part done... I need some help with the tech. diving part! I'll be posting a EXE (application for windows) as well as the code as soon as it's done. Thanks everyone!

P.s-I'm almost done with a very basic dive table program-more to come on that soon.

 

[gregmontalbano]

Also, What is the result (SCR) In, as far as units go?

 

[AaronR103]

The two equations are the same thing, just expressed in different terms (this first is PSI/min, the second is volume/min) , as are SAC and SCR (Surface Air Consumption vs Surface Consuption Rate).

A

 

[vondo]

OK, so I guess I'll say hey to everyone, and start off with a pretty easy question, I'm doing a project for school, Java class (a computer programming class) and need some help with the diving aspect. Although the teacher will never know the difference, that the math is wrong...still I think this could be useful. So far I have (from the SSI Log book pages-

SAC(its SCR really, same thing?) = (PSI / time) * 33 / Depth + 33

So is this right, all I need? I did some more hunting and found quite a few equations, such as -

SCR = (P2/P1) x Vf/Aa/Tm
Where
P1 = pressure in psi at beginning of test dive
P2 = pressure in psi at end of test dive
Vf = actual volume of air in tank in cu. ft.
Aa = pressure in absolute atmospheres
Tm = time of dive in minutes

as well as others. So now that I have the geeky coding part done... I need some help with the tech. diving part! I'll be posting a EXE (application for windows) as well as the code as soon as it's done. Thanks everyone!

P.s-I'm almost done with a very basic dive table program-more to come on that soon.

You want volume/minute, not PSI/minute unless you assume everyone always uses the same size tank.

Volume used = Tank Volume * (Start - End Pressure)/Rated Pressure, e. g. 80 cu ft*(2900-700)/3000 = 59 cu ft

Average pressure on you: (Average Depth/33 + 1) Atmospheres (eg. 66/33 + 1 = 3)

Consumption/minute: Volume/Avg Pressure/Dive Time (eg 59 cu ft. / 3 ATM / 59 min = 0.33 cu ft / min at 1 ATM)

This is similar to your first set of calculations, just make sure you do 33 / (Depth + 33)

 

[vondo]

The two equations are the same thing, just expressed in different terms (this first is PSI/min, the second is volume/min) , as are SAC and SCR (Surface Air Consumption vs Surface Consuption Rate).

A

The volume/minute assumes the tank is filled to its nominal pressure, which may or may not be the case.

 

[Rick Murchison]

Volume used = Tank Volume * (Start - End Pressure)/Rated Pressure, e. g. 80 cu ft*(2900-700)/3000 = 59 cu ft

I assume you're thinking AL 80 here...
Either change 80 to 77.4, or 3000 to 3100. If you want to be accurate, that is (57 vice 59 CF in this case)
Rick

 

[DBailey]

The volume/minute assumes the tank is filled to its nominal pressure, which may or may not be the case.

I was wondering about this.

If you rearrange the "Volume Used" equation you can get it to read:

[start psi - end psi] * [rated tank volume / rated tank pressure]


For a given tank, the value of [rated tank volume / rated tank pressure] would always be the same in this equation. It would be a constant in that equation. For a typical AL 80, the value would be 0.0258 CF/psi = [77.4/3000].

I know that as pressure decreases that volume decreases (within the tank), but is it linear?

If it is a true linear relationship, whether or not you received a "rated fill" doesn't matter as the value is constant. If it is not linear, then your fill volume and pressure matter.

 

[vondo]

I was wondering about this.

If you rearrange the "Volume Used" equation you can get it to read:

[start psi - end psi] * [rated tank volume / rated tank pressure]


For a given tank, the value of [rated tank volume / rated tank pressure] would always be the same in this equation. It would be a constant in that equation. For a typical AL 80, the value would be 0.0258 CF/psi = [77.4/3000].

I know that as pressure decreases that volume decreases (within the tank), but is it linear?

If it is a true linear relationship, whether or not you received a "rated fill" doesn't matter as the value is constant. If it is not linear, then your fill volume and pressure matter.

Basically, yes. The ideal gas equation is PV=nRT. Assuming Volume (V) and Temperature (T) don't change, P is directly related to n (the amount of gas). I think T inside the tank does drop somewhat as gas is released. But T is in Kelvins, so while it might drop a few degrees out of ~300 (room temperature in K), the effect is small.

 

[gregmontalbano]

Wow, just wanted to thank everyone for there response. I'm almost done with the program, i'll be posting the .java/.class files as soon as there done in case anyone wants to try it out. A simple executable file will be coming soon...It would be done now, but ya'll have inspired me to go above and beyond and do some more gas planning features. Thanks!