Pressure Sensors

1. Khalid Nawaz
CE25M11-15
Instrumentation And Process Control

2. Pressure Sensors
IN ANY GIVEN PLANT, THE NUMBER OF PRESSURE
GAUGES USED IS PROBABLY LARGER THAN ALL
OTHER INSTRUMENTS PUT TOGETHER
TOPIC
OF
PRESENTATION

3. Importance Of Pressure
Pressure can be used to measure other variables like level , flow
rate, temperature .
Pressure play major role in determining boiling point of liquids.
Climate changes can be predicted by change in pressure of
atmosphere
But most important application of pressure being a chemical
engineer is in plant site.

4. Absolute And gauge Pressure
Pressure is commonly quoted as being Absolute or Gauge
Easiest way of thinking
Some Fluid = Some Pressure = Some absolute pressure
No Fluid = No Pressure = Zero absolute pressure
Whereas
Fluid Pressure+ Atmospheric Pressure = Some Gauge Pressure
No Fluid + Atmospheric Pressure = Zero Gauge Pressure
Which follows
Gauge Pressure – Atmospheric Pressure = Pressure due to
fluid itself = Absolute fluid pressure

5. Pressure Measurement
1. Mechanical Methods
2. Electrical Methods

6. Different Method
1. Elastic pressure transducers
2. Manometer method
3. Pressure measurement by measuring vacuum
4. Electric pressure transducers
5. Pressure measurement by balancing forces produced on a
known area by a measured force

7. Elastic Pressure Transducer
1. Bourdon tube pressure gauge
2. Diaphragm pressure transducers
3. Bellows
Uses flexible element as sensor. As pressure changed
,the flexible element moved, and this motion was
used to rotate a pointer in front of dial.

8. Bourdon tubes are generally are of
three types;
1. C-type
2. Helical type
3. Spiral type
Bourdon Tube Pressure Gauge

9. Diaphragm are popular because they required less space
and the motion they produce is sufficient for operating
electronic transducers
Diaphragm and Bellows Pressure Gauge

10.  More sensitive than bourdon
type gauge.
 Used to measure low
pressures
 Brass, phosphor bronze,
stainless steel, beryllium copper
etc.
Bellows

11. Types of Electric Pressure Transducers
Strain gauge pressure transducers
Capacitive pressure transducers
Potentiometer pressure transducers
Resonant Wire pressure transducers
Piezoelectric pressure transducers

12. A strain gauge is a type of resistance pressure transducer
whose electrical resistance changes when it is stretched or
compressed
The wire filament is attached to a structure under strain and
the resistance in the strained wire is measured
Strain Gauge Pressure Transducer

13. C=ε0 εr A/d
Where,
C = the capacitance of a capacitor in farad
A = area of each plate in m2
d = distance between two plates in m
εr= dielectric constant
ε0 = 8.854*10^-12 farad/m2
Thus, capacitance can be varied by changing
distance between the plates, area of the plate or
value of the dielectric medium between the plates.
Any change in these factors cause change in
capacitance.
In capacitive transducers, pressure is utilized to vary any of the above mentioned
factors which will cause change in capacitance and that is a measureable by any
suitable electric bridge circuit and is proportional to the pressure.
Capacitive Pressure Transducer

14. Capacitive Pressure Transducer
Originally developed for use in low vacuum research
1) Wide range ability from high vacuum in the micron range to
10,000 psig
2) Differential pressure as low as 0.01 inch can be readable
3) Accurate within 0.1 % of reading or 0.01 % of full scale
4) More Corrosion resistant

15. When a pressure , force or acceleration is applied to a
quartz crystal , a charge is developed across the crystal that is
proportional to the force applied
Piezoelectric Pressure Transducer

16. -- Signals generated by crystals decays rapidly so unsuitable for static
force or pressure measurements
-- measure rapidly changing pressure resulting from blasts, explosions
or pulsation pressures
-- Range : 5,000 to 10,000 psi
-- Rugged construction, small size and high speed
Piezoelectric Pressure Transducer

17. Factors Affecting The Selection Of
Pressure Sensors.
Several factors influence the suitability of a particular pressure sensor
for a given process:
1. The characteristics of the substances being used or formed during
the process,
2. The environmental conditions of the system,
3. The pressure range of the process,
4. And the level of precision and sensitivity required in
measurements made.

18. Selection Criteria
 Process:
if process fluid is corrosive then we can not use bourdon ,or
manometer type pressure sensor.
 pressure range:
Different pressure sensors work optimally in different pressure
ranges.
So there is need to select such pressure sensor which can work with
process requirement pressure ranges.
 sensitivity:
Different process require different accuracy and precision. we have
to select according to our requirement.