Units of Measurement for Pressure
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N/m2). The SI adopted the name pascal as the unit of pressure in 1971. When indicated, the zero reference is stated in parenthesis following the unit, for example 101 kPa (abs). Atmospheric pressures are often stated using hectopascal (hPa), kilopascal (kPa), millibar (mbar) or atmospheres (atm).
It is worth noting that pound per square inch (psi) is still used in many situations. For example oil well pressure is usually stated in psi, and more commonly in the UK it is often used for tyre pressure. A letter is often appended to the psi unit to indicate the measurement's zero reference; psia for absolute, psig for gauge, psid for differential.
Because pressure was once commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g. inches of water). Manometric measurement is the subject of pressure head calculations. The most common choices for a manometer's fluid are mercury (Hg) and water. Both have advantages, for example water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The abbreviation "W.C." or the words "water column" are often printed on gauges and measurements that use water for the manometer. Natural gas pipeline pressures are often stated in inches of water, expressed as "inches W.C."
In vacuum systems, the units torr (millimeter of mercury), micron (micrometer of mercury), and inch of mercury (inHg) are most commonly used. Torr and micron usually indicates an absolute pressure, while inHg usually indicates a gauge pressure.
Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. So measurements in "millimetres of mercury" or "inches of mercury" can be converted to SI units as long as attention is paid to the local factors of fluid density and gravity. Temperature fluctuations change the value of fluid density, while location can affect gravity.