DIFFERENT TYPES OF PRESSURE INSTRUMENTS ON PROCESS CONTROL

DIFFERENT TYPES OF PRESSURE INSTRUMENTS ON PROCESS CONTROL

Pressure:

Pressure is nothing but a force applied on the unit area is called (Kg/cm² )

Classification of pressure instruments:

1.   Mechanical type pressure gauges and switches

·         Bourdon tube

·         Bellows

·         Diaphragm

·         Capsule

2.   Electronics type

·         Capacitive type

·         Piezoresistive type

·         Piezoelectric type

3.   Liquid column elements:

·         Barometer

·         Manometer –u tube

Mechanical type pressure gauges and switches

Bourdon tube type pressure gauge:

The Bourdon pressure gauge operates on the principle of deflection when pressurized, a compressed tube tends to straighten or regain its circular form in cross-section. Then Bourdon tube becomes in C shape, helical, and spiral shapes , which connected with a pinion arrangement to turn the needle.,

Bellows type pressure gauge:

Action of Bellows type pressure monitoring devices are bending and tension activates the elastic member the tension should be least. And design criteria for a diaphragm and bowels are more are less same. But following two characters – “maximum stroke and maximum allowable pressure” May decide diameter and stroke length can be increased by increasing the folds or convolutions.

Diaphragm pressure Gauge and switches:

Diaphragm type pressure switch is a form of switch this closes an electrical contact when a certain set pressure has been reached on its input. This switch may be designed to make contact either on high pressure or low pressure. Another type of pressure switch detects mechanical force such as pressure contains a capsule, bellows, Bourdon tube, diaphragm or piston.

Electronics type

Capacitive type pressure transduces:

The capacitive transducer construction is similar to the variable capacitance, that value will change while comprises of two parallel metal plates (Anode and cathode) that are separated by dielectric (Air).  And distance between the two plates is fixed in normal capacitors, but in variable capacitance transducers the distance between the two plates is variable with respect to applied pressure. Change in capacitance value is directly proportional to the applied pressure this will measure by a Wheatstone bridge.

The capacitance C between the two plates of capacitive transducers is given by:

C = ε_o x ε_r x A/ d

Where C is the capacitance of the capacitor or the variable capacitance transducer

ε_o - is the absolute permittivity

ε_r - is the relative permittivity

The product of ε_o & ε_r is also called as the dielectric constant of the capacitive transducer.

A is the area of the plates

D is the distance between the plates

Piezoresistive type pressure Transducer:

Piezoresistive Transducer are made by Piezoresistive crystals and resistance of crystal will change while apply the pressure   

The sensing material in a piezoresistive pressure sensor is a diaphragm formed on a silicon substrate, which bends with applied pressure. A deformation occurs in the crystal lattice of the diaphragm because of that bending. This deformation causes a change in the band structure of the piezoresistors that are placed on the diaphragm, leading to a change in the resistivity of the material. This change can be an increase or a decrease according to the orientation of the resistors.

Piezoelectric type pressure transducer:

Principle of a piezoelectric transducer is that produces electric charges, when force applied on the quartz crystal surface.  The charge thus produced can be called as piezoelectricity which is directly proportional to the applied pressure.