There are many types of pressure units used depending on the country, industry and application.
For example pascals (Pa) and inches of water (inH2O) are used for defining low air pressures, in applications such as measuring air flow through ventilation ducts on HVAC or building management systems.
All pressure units are directly proportional to each other, and pressure values can be defined using whole units with a unique name (e.g. bar or kPa), combinations of force and area units (e.g. kgf/cm² or lbs/in²) or height of a column of liquid such as (e.g. mH2O or inHg).
Pressure units which represent the height of a liquid are useful for measuring depth and liquid level, however due to the changing density of fluids at different temperatures, they are not recommended for precise measurements.