Measuring Pressure
Barometers are used for measuring pressure. You can make a simple barometer but a proper one is needed to make correct observations.

The simplest and cheapest is a basic aneroid barometer which has a partial vacuum in a special metal box. This expands and contracts as air pressure changes, moving a pointer around a dial. Measurements are given in millibars (mb) or in hectopascals (hPa), identical in value.You must adjust your barometer to sea-level and it is best to put it inside a screen.

Mercury Barometers
Mercury barometers give the measurement of air pressure in inches or millimetres of mercury. An aneroid barograph uses a pen instead of a pointer and marks a roll of paper against a revolving drum. These provide a continuous record of air pressure.