Types of pressure and how to make a pressure gauge
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Types of pressure
The various types of pressure differ only in respect of their reference point.
• Absolute pressure - The most unequivocal reference point is zero pressure. This is the pressure of empty space in the universe. When a pressure is based on this reference point, it is called absolute pressure. To distinguish it from other types of pressures, it is accompanied by the suffix ‘abs’ (from the Latin: absolutus = independent, separate from).
• Atmospheric air pressure - The most vital pressure for life on earth is atmospheric pressure pam (amb = ambiens, surrounding). It is produced by the weight of the atmosphere surrounding the earth up to an altitude of 500 km. Atmospheric pressure decreases continuously up to this altitude until it practically reaches the absolute pressure pabs = 0. Atmospheric pressure undergoes climatic changes, as shown by the daily weather report. At sea level, pamb has an average value of 1 013,55 millibar (mbar) *). In high or low pressure weather zones it can fluctuate by around ±5%.
• Pressure difference, differential pressure - The difference between two pressures p1 and p2 is referred to as the pressure difference ∆p = p – p2. If the pressure difference between two pressures is itself the measurement quantity, it is called the differential pressure p1.2.
• Atmospheric pressure difference, overpressure - The pressure needing to be measured most often in the technical field is the atmospheric pressure difference pe (e = exceeds, more than). It is the difference between an absolute pressure pabs and the respective (absolute) atmospheric pressure.
pe = pabs - pamb
It is referred to simply as overpressure.
• Positive overpressure - This term is used if the absolute pressure is higher than the atmospheric pressure.
• Negative overpressure - This term is used for the opposite case. Previously the term ‘vacuum’ was used for overpressures with a negative sign. This should now be avoided to rule out the misunderstandings that may arise from the sign. The term can still be used, however, to designate a condition, eg ‘vacuum chamber’ or ‘vacuum in the suction line’. Similarly, composite words with the term overpressure can suppress the prefix ‘over’ if the corresponding dimension is clearly an overpressure, eg blood pressure or tyre pressure.
The symbol indices abs, amb and e, define the reference point of the corresponding pressure. They are to be used only in conjunction with the symbol p. Their combination with the name of a unit, eg bare or paabs is unacceptable.
*) Standard pressure according to DIN 1343
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