relativepressure

Relative pressure is the pressure difference between a measured pressure and a chosen reference pressure. It expresses how much pressure exists in a system relative to a baseline and can be positive, negative, or zero depending on the reference.

In formal terms, relative pressure p_rel is defined as p_rel = p_abs − p_ref, where p_abs is the absolute pressure measured with respect to a perfect vacuum, and p_ref is the reference or baseline pressure. When the reference is the ambient atmospheric pressure, p_rel corresponds to gauge pressure: it is positive when the system pressure exceeds atmospheric pressure and negative when it is below it. If the reference is vacuum, p_rel equals the absolute pressure.

Common terminology reflects these relationships. Absolute pressure is always relative to vacuum; gauge pressure is typically the pressure above atmospheric pressure; and differential or relative pressure can be used more generally with any chosen p_ref. The concept is central to many instruments, such as pressure transducers, manometers, and barometers, which effectively report p_abs, p_rel, or p_ref depending on design and calibration.

Units are the same as for pressure, most commonly pascals (Pa) or kilopascals (kPa), with psi used in some regions. Relative pressure is widely applied in engineering and science to quantify pressure loads, flow dynamics, HVAC systems, and meteorological measurements, where the baseline reference is essential for meaningful comparison.