Pressure Transducers

Pressure Transducers

Pressure sensor acts as a transducer; it generates a signal as a function of the pressure imposed typically on gases or liquids. 
Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area
o (pounds per square inch-psi)
Pressure sensors are used for control, monitoring and measurement of variables such as fluid/gas flow, speed, water level, and altitude.
Pressure sensors – Units
The basic SI unit of pressure is the Pascal, which is N/m2. 
o Another common unit used in vacuum is Torr (Torricelli): pressure exerted by a 1mm-Hg of mercury or 1/760 of an atmosphere. 
o Atmosphere Pressure: force per unit area exerted against a surface by the weight of air above that surface.
o 1 atm = 101.325 kPa = 760 Torr
o 1 atm= 14.696 psi
1 psi=6,894.76Pa 
Another commonly used unit is bar, which is 100K pascal.
o Bar : atmospheric pressure on earth at sea level.

Pressure sensors Units: Relationship

Pressure Measurement Scales
Five basic scales used to measure pressure:
o Atmosphere pressure (low vacuums measurements)
o Gauge pressure
o Absolute pressure (high vacuums measurements)
o Differential pressure
o Vacuum pressur

Atmosphere pressure
Atmospheric pressure is the air pressure exerted upon the earth. 
Atmospheric pressure is approximately 14.7 psi at sea level and decreases as height increases (earth level).
For measuring low vacuum below 1 atm, atmospheric pressure is taken as “ZERO”, with unit in-Hg (inch of Hg) to indicate the vacuum (1 atm = 29.92 in-Hg) without the negative sign. 
Compound vacuum pressure gauge can measure pressures both above and below one atmosphere.
Gauge Pressure Scale
Gauge pressure scales use atmospheric pressure as a reference point and extends in the positive direction.
If the sensing element is exposed to the atmosphere, it registers zero pressure.
Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. 
o Negative signs are usually omitted.
Gauge pressure is measured in psig (pounds per square inch, gage).
o Examples: Tire pressure, blood pressure

Absolute Pressure Scale
Absolute pressure is always used to measure high vacuums, where Torr and bar are the most commonly used units.
Absolute pressure is referenced to absolute zero (no fluid), or lack of pressure.
Absolute pressures are always indicated by positive numbers.
If the sensing element is exposed to the atmosphere, it will register 14.7 psia (pounds per square inch, absolute).
Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure + atmospheric pressure.
Examples: atmospheric pressures, vacuum pressures, altimeter pressures (Altitude).

Differential Pressure Scale
Differential pressure is used to express the difference between two measured pressures.
It is determined by subtracting the lower reading from the higher reading.
Differential pressures are commonly used in industrial process systems.
Vacuum Pressure Scale
The vacuum scale ranges from absolute zero pressure to atmospheric pressure (as a reference point). 
The absolute zero pressure point (total vacuum) represents a total lack of pressure

A vacuum gage will read zero when measuring atmospheric pressure and 29.92 in Hg when measuring a complete vacuum. 

Comparison of Pressure Scales

Every process control industrial applications requires accurate pressure measurement and control.
Every process control industrial applications requires accurate pressure measurement and control.
Both electronic and non-electronic instruments are used for pressure measurement.
Instruments are often classed by whether they make direct or indirect measurements.
As pressure changed ,the flexible element moved, and this motion can be used to rotate a pointer in front of dial.
Two broad categories of pressure measurement.
o Mechanical Pressure Transducers
o Electrical Pressure Transducers
Mechanical (Elastic) pressure transducers utlizes flexible element as a sensor. 
Mechanical (Elastic) pressure transducers utlizes flexible element as a sensor. 
Pressure sensor measured in form of : 
o Liquid Column Gauges
o Manometer
o Bourdon tube pressure gauge
o Diaphragm (Capsule) pressure transducers
o Bellows
o Spring and Piston.
Liquid Column Gauges
Measuring pressure is possible by monitoring the height of a liquid in a column.
These gauges are very accurate and may be used as calibration tools for other instruments
A barometer is an example of a liquid column gauge.
Barometer is a instrument used in meteorology to measure atmospheric pressure by using water, air, or mercury. 
Manometer is the liquid column device used to measure pressure. 
A simple manometer consists of a U shaped tube of glass filled with some liquid. 
o Typically the liquid is mercury because of its high density

Each column is exposed to a different pressure source.
Read the rise of liquid in one column and the drop in the other, and add them together.

Bourdon Tube Pressure Gauge
Most common device around us is the pressure gauage which utilizes a bourdon tube as its sensing elements.
The tube serves as the primary detector-transducer, changing pressure into near linear displacement.
A bourdon tube is a curved, hollow tube with the process pressure applied to the fluid in the tube. 
Pressure in the tube causes, tube to deform or uncoil.
Bourdon tubes are generally are of three types. 
o C-type
o Helical type
o Spiral type

Bourdon Tube Construction Mechanism 

Bourdon tube is a hollow tube with an elliptical cross section.
When a pressure difference exists b/w the inside and outside, tube tends to straighten out and the end moves.
Movement is usually coupled to a needle on a dial to make a gauge.
It can also be connected to a secondary device such as an air nozzle to control air pressure
o This type can be used for measuring pressure difference.
o Air pressure of Vehicle Tire 

Essentials of a Bourdon-tube pressure gage
1. Pressure causes the section of the flattened tube tends toward a more circular form 
2. This causes the free end A to move outward 
3.The resulting motion is transmitted by link B to sector gear C 
4. Then to pinion D and cause the indicator hand to move over the scale 

Spring & Piston Type 
The pressure acts directly on the piston and compresses the spring.
The position of the piston is directly related to the pressure.
A window in the outer case allows the pressure to be indicated. 
The piston movement may be connected to a secondary device to convert movement into an electrical signal.

This type is usually used in hydraulics where the ability to withstand shock, vibration and sudden pressure changes is needed (shock proof gauge). 
Transmission requirements for remote display as electric signal transmission can be through cable or cordless.
Transmission requirements for remote display as electric signal transmission can be through cable or cordless.
Electric signals give quicker responses and high accuracy in digital measurements.
The linearity property of the electric signal produced to pressure applied favors simplicity.
They can be used for extreme pressure applications, i.e. high vacuum and pressure measurements.
EPTs are immune to hysteresis, shock and mechanical vibrations.
Pressure sensing element such as a bellow , a diaphragm or a bourdon tube
Pressure sensing element such as a bellow , a diaphragm or a bourdon tube
Primary conversion element e.g. resistance or voltage
o Mechanical Pressure Transducers to Pneumatic Pressure Transmitters to Electric Pressure Transmitters
Secondary conversion element

The conversion of pressure into an electrical signal is achieved by the physical deformation of strain gages which are bonded into the diaphragm of the pressure transducer and wired into a Wheatstone bridge configuration. 
Pressure applied to the pressure transducer produces a deflection of the diaphragm which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure. 
There are various ways of converting the mechanical movement into an electric signal. 
o Strain gauge pressure transducers 
o Capacitive pressure transducers
o Piezoelectric pressure transducers
o Piezoresistive pressure transducers
o Potentiometer pressure transducers
o Resonant wire pressure transducer
Strain Gauge Pressure Transducer
Strain gauge is a device used to measure the strain of an object. 
Mechanism:Pressure--Mechanical Motion--Strain Gage-resistance
Strain gauge pressure transducers are used for narrow pressure span and for differential pressure measurments.
Strain gauges are typically constructed of piezoelectric semiconductor material
Pressure transducer contains a diaphragm which is deformed by the pressure which can cause a strain gauge to stretch or compress. 
This deformation of the strain gauge causes the variation in length and cross sectional area due to which its resistance changes. 
Strain gauges are used as part of a Wheatstone Bridge to measure pressure.

Capacitive Pressure Transducer
Mechanism:Pressure — Diaphragm Motion– Capacitance- di-electric
The deflection of the diaphragm constitutes a capacitor in which the distance between the plates is pressure sensitive. 
Capacitive Pressure Transducer are use in low vacuum pressure applications.
C=ε0 εr A/d     Where, 
o C= capacitance of a capacitor in farad
o A = area of each plate in m2
o d = distance between two plates in m
o εr= dielectric constant 
o 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. 
Capacitive Pressure Transducer:Construction
Capacitive pressure transducer includes: 
a pair of electrically insulative elastic diaphragms disposed adjacent to each other and bonded together in a spaced apart relationship to form a sealed cavity, 
a conductive layer applied to the inside surface of each of the diaphragms
a small absolute pressure provided in the cavity. 
o This small absolute pressure cavity essentially reduces the effects of the negative temperature coefficient of the modulus of elasticity of the diaphragms. 

Piezoelectric Pressure Transducer
When pressure, force or acceleration is applied to a quartz crystal, a charge is developed across the crystal that is proportional to the force applied.
When pressure is applied to a crystal, it is elastically deformed, which results in a flow of electric charge, that indicate the pressure as applied to the crystal 
These sensors cannot detect static pressures, but are used to measure rapidly changing pressures resulting from blasts, explosions, pressure pulsations (in rocket motors, engines, compressors) or other sources of shock or vibration.
Output of Piezoelectric pressure sensors is expressed in "relative" pressure units (such as psir instead of psig). 
Maximum range of such sensors is 5,000 or 10,000 psir.
Desirable features of includes rugged construction, small size, high speed, and self-generated signal.
On the other hand, they are sensitive to temperature variations and require special cabling and amplification 

Where and How have EPTs failed?
EPTs require a constant supply of electricity for them to function. They do not come with built-in power supply.
High performance comes at a cost.
Installation of auxiliary display modules and electrical circuitry increases capital cost.
Physical properties, like temperature, which can affect electrical constants may affect the consistency of EPTs.
For this reason, temperature compensation is always required with EPTs.
Some electrical phenomena, like piezoelectric energy, have limited applicability. This limits their use in industry. 

Electricity exposes personnel to potential hazards.

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