SAC Math Questions

[JasonH20]

Just to make sure I understand this correctly...

For example, if I have a SAC of .5, would a 6CF pony bottle last me 12min @ surface, 6min @ 33FSW, or 4min @ 66FSW?

Also for a 80CF 3000psi tank, does 700 psi leave me approx 18.7CF air?

Lastly, what is the common rule of thumb on SPG accuracy? I think my instructor mentioned something like +/- 200? Also, will the first stage operate all the way down to 0 psi, or will it stop at some point before 0? I guess my question here is, if the primary stops at say 200 and the SPG is +/- 200, I can see why you don't want to run your pressure too low!

PS, I wonder what percentage of rec divers carry a pony bottle? And what size?

Thanks,
Jason

 

[wedivebc]

Just to make sure I understand this correctly...

For example, if I have a SAC of .5, would a 6CF pony bottle last me 12min @ surface, 6min @ 33FSW, or 4min @ 66FSW?

Also for a 80CF 3000psi tank, does 700 psi leave me approx 18.7CF air?

Lastly, what is the common rule of thumb on SPG accuracy? I think my instructor mentioned something like +/- 200? Also, will the first stage operate all the way down to 0 psi, or will it stop at some point before 0? I guess my question here is, if the primary stops at say 200 and the SPG is +/- 200, I can see why you don't want to run your pressure too low!

PS, I wonder what percentage of rec divers carry a pony bottle? And what size?

Thanks,
Jason

Thats a lot of questions for 1 post Jason. You math is correct for the sac rate.
SPG accuracy gets worse at the lower end of the scale so it may be right on at 1500psi but could be 200 off and the bottom of the tank. That is why most instructors recommend 500psi left in the tank. Rule of thirds is probably better for more demanding dives.
There are probably more threads on pony bottles on SB than anything else so I'll leave that one alone.

 

[jonnythan]

You're correct.. except Al 80's only hold about 77.4cf, so 700 psi in one is more like 18cf than 18.7cf.

 

[matt_unique]

You're correct.. except Al 80's only hold about 77.4cf, so 700 psi in one is more like 18cf than 18.7cf.

Depending upon the fill pressure of course....

 

[jonnythan]

Depending upon the fill pressure of course....

You can pump em up to 15,000 psi if you want, 700 psi in them is still 18 cf

 

[ChrisA]

Lastly, what is the common rule of thumb on SPG accuracy? I think my instructor mentioned something like +/- 200? Also, will the first stage operate all the way down to 0 psi, or will it stop at some point before 0?

Someone suggested to me to try this experiment at home. Do it just before you leave to go to the dive shop to get a fill so as not to be storing an empty tank.

Simply breath a tank down to the point where you can't suck any more air out and watch the SPG. Note the SPG reading.

 

[MXGratefulDiver]

You can pump em up to 15,000 psi if you want, 700 psi in them is still 18 cf

I took it he was referring to "working pressure" (i.e. 3,000 or 3,300), as opposed to actual fill pressure.

If he meant fill pressure, you're right.

... Bob (Grateful Diver)

 

[jonnythan]

I took it he was referring to "working pressure" (i.e. 3,000 or 3,300), as opposed to actual fill pressure.

If he meant fill pressure, you're right.

... Bob (Grateful Diver)

I know he meant that you can get more than 77.4 psi into one, but contrary to popular belief (as you no doubt know) AL 80's hold 77.4cf at 3000 psi

 

[knotical]

Lastly, what is the common rule of thumb on SPG accuracy? I think my instructor mentioned something like +/- 200? Also, will the first stage operate all the way down to 0 psi, or will it stop at some point before 0? I guess my question here is, if the primary stops at say 200 and the SPG is +/- 200, I can see why you don't want to run your pressure too low!

Consider that the intermediate pressure (in the hose between your first and second stages) is around 140psi. It gets real hard to suck air below that, so you want the air pressure in your cylinder to stay comfortably higher. I think that’s why some instructors say there’s about a 200psi inaccuracy at the low end. It’s a short-hand way of saying don’t go so low. As an aside, that 140psi is above ambient. Therefore, if you really mess up and run out of air at depth, keep your regulator in your mouth, because you can get another breath or two while ascending. I did this once in the days before submersible pressure gauges.