2.3 Pressure Measurement

Most liquid and all gaseous materials in the process industries are contained within closed vessels. For the safety of plant personnel and protection of the vessel, pressure in the vessel is controlled. In addition, pressured is controlled because it influences key process operations like vapor-liquid equilibrium, chemical reaction rate, and fluid flow.

The following pressure sensors are based on mechanical principles, i.e., deformation based on force.

Bourdon : A bourbon tube is a curved, hollow tube with the process pressure applied to the fluid in the tube. The pressure in the tube causes the tube to deform or uncoil. The pressure can be determined from the mechanical displacement of the pointer connected to the Bourdon tube. Typical shapes for the tube are “C” (normally for local display), spiral and helical.

Bellows : A bellows is a closed vessel with sides that can expand and contract, like an accordion. The position of the bellows without pressure can be determined by the bellows itself or a spring. The pressure is applied to the face of the bellows, and its deformation and its position depend upon the pressure.

Diaphragm : A diaphragm is typically constructed of two flexible disks, and when a pressure is applied to one face of the diaphragm, the position of the disk face changes due to deformation. The position can be related to pressure.

The following pressure sensors are based on electrical principles; some convert a deformation to a change in electrical property, others a force to an electrical property.

Capacitive or inductance : The movement associated with one of the mechanical sensors already described can be used to influence an electrical property such as capacitance affecting a measured signal. For example, under changing pressure a diaphragm causes a change in capacitance or inductance.

Resistive, strain gauge : The electrical resistance of a metal wire depends on the strain applied to the wire. Deflection of the diaphragm due to the applied pressure causes strain in the wire, and the electrical resistance can be measured and related to pressure.

Piezoelectric : A piezoelectric material, such as quartz, generates a voltage output when pressure is applied on it. Force can be applied by the diaphragm to a quartz crystal disk that is deflected by process pressure.

For an overview of pressure sensors and transducers, select these buttons to be directed to sites on the WWW.

For an overview of piezoelectric pressure sensors, select this button to be directed to a site on the WWW.

For an overview of load cell sensors to measure weight, select this button to be directed to a site on the WWW.

Table 3. Pressure sensors

Sensor	Limits of Application	Accuracy	Dynamics	Advantages	Disadvantages
bourdon, "C"	up to 100 MPa	1-5% of full span	-	-low cost with reasonable accuracy -wide limits of application	-hysteresis -affected by shock and vibration
spiral	up to 100 MPa	0.5% of full span	-
helical	up to 100 MPa	0.5-1% of full span	-
bellows	typically vacuum to 500 kPa	0.5% of full span	-	-low cost -differential pressure	-smaller pressure range of application -temperature compensation needed
diaphragm	up to 60 kPa	0.5-1.5% of full span	-	-very small span possible	-usually limited to low pressures (i.e. below 8 kPa)
capacitance/ inductance	up to 30 kPa	0.2% of full span	-	-	-
resistive/strain gauge	up to 100 MPa	0.1-1% of full span	fast	-large range of pressures	-
piezoelectric	-	0.5% of full span	very fast	-fast dynamics	-sensitive to temperature changes