This somewhat relates to pisa and psig. Suppose I have a 100 in^3 air cylinder full of air with a psig of 0. Now I push the rod in halfway, the cylinder now has 50 in^3 of air and assuming the temperature doesn't change the pressure should be double, but 2 * 0 is still 0. Obviously the atmospheric pressure (14.7 pis) comes in to play here. So the psia should be twice the normal atmospheric pressure or 29.4 psi but the gage will read what the atmospheric pressure is 14.7 right? What about if we start with a pisg of 100 then psia is 114.7 so when we double the psia is 229.4 but the psig is 214.7? And it is psig that is what determines how much force is required to hold an air cylinder at a certain point right?

So am I right here or am I completely screwed up?