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
= εo x εr x A/ d
Where
C is the capacitance of the capacitor or the variable capacitance transducer
εo -
is the absolute permittivity
εr -
is the relative permittivity
The
product of εo & εr 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.
