Diver's air consumption calculations?

[Seuss]

I am studing for the PADI DMC physics test and have run into something I don't understand. From the workbook:

A diver has an air consumption rate of 25 psig per minute at the surface. If all factors, but depth remain unchanged, what will his consumption rate be at 132 fsw?

The given answer is 125 psig/min (5 ata * 25 psig per minute).

What I don't understand is why we can use psig rather than psia in this case?

 

[DA Aquamaster]

Because;

1) psig is the unit you are starting with,

2) the base rate is figured at 1 atm

3) psig is the unit you will have access to in the water, and

4) it is a waste of time and effort to use psia and would just add one more area for an error to be introduced.

Keep it simple whenever possible.

 

[Charlie99]

The consumption rate in psia is the same as in psig anyway ---- the consumption rate is a difference between 2 pressures and the psig to psia offset gets subtracted out.

 

[Seuss]

I finally figured out my misunderstanding... I was treating a volume as a pressure... a mental mixup. Volumes are not adjusted to absolutes the way pressures and temperatures are. At 132 fsw, the pressure is 5 ata. The volume of air, 25 psig, is simply multipled against the pressure following Boyles' law, resulting in 125 psig per minute of air consumption at depth.

Thanks for the help everybody!

 

[Scuba_Steve]

Don't worry, you'll get a chance to use the volume thingy when it comes to figuring out how much (volume) of gas it takes to lift XXX via a liftbag

Fun stuff, but I must say, I'd need a refresher on that part.......Basically bring lots, and add until you get the desired direction going on.

That's answer "D" on the multiple guess exam.......Not saying it's the one they're looking for...........

Steve