Pressure measurement because atmospheric pressure is always changing and the reference in this case is fixed at 1 bar. To produce an absolute pressure sensor, the manufacturer seals a high vacuum behind the sensing diaphragm. If the process-pressure connection of an absolute-pressure transmitter is open to the air, it will read the actual barometric pressure. The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m or kg·m·s). This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N·m. When indicated, the zero reference is
Bar (unit) numerically equivalent to millibars; for the same reason, the hectopascal is now the standard unit used to express barometric pressures in aviation in most countries. For example, the weather office of Environment Canada uses kilopascals and hectopascals on their weather maps. In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms. In fresh water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the water surface in metres. Specifically, an increase of 1 decibar occurs for every 1.019716 m increase