There are four types of pressure measurements:
- Measurement of absolute pressure: measurements are made with respect to absolute pressure, for example as with atmospheric pressure.
- Measurement of relative or effective pressure: measurements are made with respect to ambient pressure, generally atmospheric pressure.
- Measurement of differential pressure: this is the difference between two pressures.
- Measurement of vacuum pressure: this is referred to ambient pressure, usually atmospheric pressure.
One can also distinguish pressure measurements according to their behaviour with time:
Static or quasi-static pressure, which varies only slowly with time.
Examples are atmospheric pressure, level measurements and certain pressures in industrial processes such as in the petrochemical, food and pulp and paper industries.
Fluctuating pressure, which can be random or cyclic. Examples are the pressure inside a gun barrel, in a plastic injection mould, in an internal combustion engine cylinder, etc.
PRESSURE GAUGE – Mechanical apparatus indicating the value of the local pressure
3. Window gasket
8. Blow out disc
10. Connection gasket
PRINCIPLE OF OPERATION:
The tube is displaced in proportion to the applied pressure, which causes the pointer to move by means of a mechanically amplifying linkage.
Components of a metallic pressure gauge:
BOURDON tube* or «C» tube: The sensing element consists of a metallic tube of various crosssections shaped in the form of a «C». One end is fixed and connected to the pressure circuit and the other is sealed and free to move. When pressure is applied to the tube the free end is forced outwards. (Measuring range from around 400 mbar to 60 bar). * invented by E. Bourdon in 1849
The sensing element consists of two thin corrugated sections laser welded together. The capsule thus formed acts as a cavity that slightly deforms with variations in pressure. It is suitable for measuring very low pressures. (Measuring range from 0 to about 600 mbar).
Helicoil tubes (or pig tails) :
The coil is cylindrical. It has the same characteristics as a spiral tube but with greater resistance to fatigue due to better stress distribution. (Measuring range from about 60 to 3000 bar).
The sensing element consists of two flanges connected to a flexible cylinder. Pressure applied to the interior of the chamber causes it to deform by an amount proportional to the pressure. They are mainly used in differential pressure gauges and pressure switches.
These use the same principle as the «C» tube but with the deflection of the free end of the tube amplified due to the spiral shape.
They are mainly used in gas expansion thermometers.
Accuracy is the maximum allowable error expressed as a percentage of the measurement span P.
Accuracy does not apply in a region P/10 1O from the zero point for pressure gauges and vacuum gauges; for compound gauges it does not apply in a region P/20 on either side of the zero point. It applies to a calibration carried out on a new equipment, at ambient temperatures between + 18°C and + 28°C before shipping. It includes repeatability, hysteresis (reversibility).
The accuracy class of an instrument is defined conventionally as the greatest value of the maximum allowable error over the specified measurement range.