To select the right pressure sensor for a specific application besides the pressure range first of all the type of pressure measurement has to be considered. Pressure sensors measure a certain pressure in comparison to a reference pressure and can be divided into absolute, gage and differential devices. These terms will be explained on the basis of the piezoresistive pressure sensors of First Sensor.
Figure 1: Comparison of absolute, gage and differential pressure
Absolute pressure is referred to the vacuum of free space (zero pressure). In practice absolute piezoresistive pressure sensors measure the pressure relative to a high vacuum reference sealed behind its sensing diaphragm. The vacuum has to be negligible compared to the pressure to be measured. First Sensor's absolute pressure sensors offer ranges from 1 bar or even 700 mbar as well as barometric pressure ranges.
Figure 2: Principle of an absolute pressure sensor (piezoresistive technology)
Gage pressure is measured relative to the ambient atmospheric pressure. The average atmospheric pressure at sea level is 1013.25 mbar. Changes of the atmospheric pressure due to weather conditions or altitude directly influence the output of a gage pressure sensor. A gage pressure higher than ambient pressure is referred to as positive pressure. If the measured pressure is below atmospheric pressure it is called negative or vacuum gage pressure. In general a vacuum is a volume of space that is essentially empty of matter. According to its quality vacuum is divided into different ranges such as e.g. low, high and ultra high vacuum.
Gage pressure sensors only offer one pressure port. The ambient air pressure is directed through a vent hole or a vent tube to the back side of the sensing element and thus compensated.
Figure 3: Principle of a gage pressure sensor (piezoresistive technology)
Differential pressure is the difference between any two process pressures p1 and p2. Therefore, differential pressure sensors must offer two separate pressure ports with tube or threaded connections. First Sensor's amplified pressure sensors are able to measure positive and negative pressure differences, i.e. p1>p2 and p1<p2. These sensors are called bidirectional differential pressure sensors with ranges of e.g. -1...+1 bar or -2.5...+2.5 mbar. In contrast, unidirectional differential pressure sensors only operate in the positive range (p1>p2) e.g. from 0...1 bar or 0...2.5 mbar and the higher pressure has to be applied to the pressure port defined as "high pressure".
Figure 4: Principle of a differential pressure sensor (piezoresistive technology)
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