Khalid Nawaz

CE25M11-15

Instrumentation And Process Control

Pressure SensorsIN ANY GIVEN PLANT, THE NUMBER OF PRESSURE GAUGES USED IS PROBABLY LARGER THAN ALL OTHER INSTRUMENTS PUT TOGETHER

TOPIC

OFPRESENTATION

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.

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

Pressure Measurement

1. Mechanical Methods

2. Electrical Methods

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 aknown area by a measured force

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.

Bourdon tubes are generally are of three types;

1. C-type

2. Helical type

3. Spiral type

Bourdon Tube Pressure Gauge

Diaphragm are popular because they required less space

and the motion they produce is sufficient for operating

electronic transducers

Diaphragm and Bellows Pressure Gauge

More sensitive than bourdon type gauge.

Used to measure low pressures

Brass, phosphor bronze,

stainless steel, beryllium copper etc.

Bellows

Types of Electric Pressure Transducers

Strain gauge pressure transducers

Capacitive pressure transducers

Potentiometer pressure transducers

Resonant Wire pressure transducers

Piezoelectric pressure transducers

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

C=ε0 εr A/dWhere,

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

Capacitive Pressure Transducer

Originally developed for use in low vacuum research

1) Wide range ability from high vacuum in the micron range to10,000 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

When a pressure , force or acceleration is applied to aquartz crystal , a charge is developed across the crystal that isproportional to the force applied

Piezoelectric Pressure Transducer

-- Signals generated by crystals decays rapidly so unsuitable for staticforce or pressure measurements

-- measure rapidly changing pressure resulting from blasts, explosionsor pulsation pressures

-- Range : 5,000 to 10,000 psi

-- Rugged construction, small size and high speed

Piezoelectric Pressure Transducer

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.

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.