# What are the differences between Gauge and Absolute pressure?

I am aware that Absolute pressure is zero-referenced to a perfect vacuum and Gauge pressure is zero-referenced to ambient air pressure. So, Absolute pressure is equal to Gauge pressure plus atmospheric pressure.

However, the Part B of this problem, confused me on what the absolute pressure under piston B is, which comes from the fact that the system is open on both piston A and B. In this case, is the (Weight of block)/(Area of piston), Gage or Absolute pressure?

It's not really possible to give an answer at all since the described situation is not static. If the mass is increased to $150\ kg$, it will launch piston 1 rapidly in the air. Furthermore, in a static situation, the pressure below B is/must be the same as below piston A or anywhere else in the fluid. The whole question gives me the idea that the creator of the test doesn't really know what he's talking about.
I think he just wants to hear what the relative pressure in the fluid would be, if the mass on piston B is $150\ kg$, and piston A is frozen. Then you would come to $3316\ kgf/m^2$ or $32.1\ kPa$ relative, assuming a gravitational acceleration of $9.81\ m/s^2$.