What is pressure? Pressure is the force that is perpendicular to the surface divided by the area it is effecting. So pressure equals force per area, or p = F / A. There are a large number of different pressure units used around the world and this can be sometimes very confusing. The engineering unit for pressure, according to SI system, is Pascal (Pa), being a force of one Newton per one square meter area, 1 Pa = 1 N / m2. Since Pascal is a very small unit, it is most often used with coefficients, such as hecto, kilo and mega. A large number of different pressure unitgs are being used around the world. For more information on pressure and different pressure units and their background, please see the blog post Pressure units and pressure unit conversion.
Several different pressure types exist including gauge, absolute, vacuum, differential and barometric. The main difference of these pressure types is the reference point against which the measured pressure is being compared. Pressure gauges also are available for all of these pressure types. Also, compound gauges are available, including a combined scale for both positive gauge pressure and vacuum (negative gauge) pressure. For more detailed information on different pressure types, please see post Pressure calibration basics – pressure types.