With a wide variety of pressure sensors available, it can be challenging to know which one is best suited to a specific task. When it comes to measuring air pressure, specifically for applications such as barometric measurements for weather or in altimeters, an absolute pressure sensor is the device of choice. However, your possible application usage isn’t limited just to air or gas.
The absolute measurement is made possible by measuring the target pressure relative to the known pressure of an absolute vacuum (see diagram below). This can be compared with measuring temperature in Kelvin, where the lowest possible temperature is 0 °K.
By using a vacuum as the reference against which everything is measured, all measurements will deliver a value larger than the absolute minimum as defined by the reference. This is essential to accurate measurement, since Boyle’s Law states that the pressure of a gas is inversely proportional to its volume at a constant temperature. Thus, anything other than a perfect vacuum will result in an absolute pressure sensor whose measurement varies with temperature.
A perfect vacuum is, however, highly challenging to achieve, especially if the sensor is to remain within an acceptable price range. Thus, absolute sensors typically have to make do with an approximate vacuum, typically in the range of 35 micro bar (0.0005 PSI).