Mechanical 1

Here's some basic idea of pressure.

Absolute pressure = gauge pressure + atmosphere pressure

Hence, gauge pressure measurement is measurement of the different of the total pressure and the atmosphere. U-tube is an instrument that measuring the difference of 2 pressure.

Bourdon can be use to measure atmosphere, but it has to be reset. Mostly, it measure the pressure wherever you put the instrument.

Reference:
http://en.wikipedia.org/wiki/Pressure_measurement