Overview of the different pressure types

When measuring pressure, a currently prevailing pressure is compared to a reference pressure. This reference pressure can be, for instance, the respective atmospheric air pressure, the pressure in a vacuum, or another prevailing pressure. Consequently, these types of pressure describe the relationship between the pressure being measured and their reference pressure.

Positive pressure difference in relation to the ambient pressure.
For instance, to check the tyre pressure.

Negative pressure difference in relation to the ambient pressure.
For instance, to measure medical suction machines.

Pressure in relation to empty space (vacuum).
For example, for meteorological air pressure measurement.

Pressure difference between any two pressures.
For example, for the control of ventilation systems.