# LECTURE#05 PRESSURE SENSORS

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LECTURE#05 PRESSURE SENSORS
AUTOMATION & ROBOTICS LECTURE#05 PRESSURE SENSORS By: Engr. Irfan Ahmed Halepoto Assistant Professor 1

PRESSURE SENSOR TRANSDUCERS
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 (pounds per square inch-psi) Pressure sensor acts as a transducer; it generates a signal as a function of the pressure imposed. A pressure sensor measures pressure, typically of gases or liquids. 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. One common unit used in vacuum is Torr, which is defined as 1 mm-Hg, or 1/760 of an atmosphere. Torr (Torricelli): pressure exerted by a 1mm of mercury (at 0C and normal atmospheric pressure) Atmosphere Pressure: force per unit area exerted against a surface by the weight of air above that surface. 1 atm = kPa = 760 torr 1 atm= psi 1 psi=6,894.76Pa Another commonly used unit is bar, which is 100K pascal. Bar : atmospheric pressure on Earth at sea level.

Pressure sensors Units: Relationship
Pascal (Pa) bar atmosphere (atm) Torr pound-force per square inch (psi) 1 Pa 1 10−5 9.8692×10−6 7.5006×10−3 145.04×10−6 1 bar 100,000 750.06 14.504 1 atm 101,325 760 14.696 1 Torr 1.3332×10−3 1.3158×10−3 19.337×10−3 1 psi 6,894.76 68.948×10−3 68.046×10−3 51.715

Pressure Measurement Scales
Five basic scales used to measure pressure: Atmosphere pressure Gauge pressure Absolute pressure Differential pressure Vacuum pressure Pressure Measurement Formulization Pressure is quoted as being Absolute or Gauge Some Fluid = Some Pressure = Some absolute pressure No Fluid = No Pressure = Zero absolute pressure Whereas Fluid Pressure+ Atmospheric Pressure= Gauge Pressure No Fluid + Atmospheric Pressure = Zero Gauge Pressure Which follows Gauge Pressure – Atmospheric Pressure = Pressure due to fluid itself = Absolute fluid pressure

Atmosphere pressure Atmospheric pressure is the air pressure exerted upon the earth. It is approximately 14.7 psi at sea level and decreases as elevation increases. For measuring a low vacuum below one atmosphere, the atmospheric pressure is often taken as “0”, and the unit in-Hg (inch of Hg) is often used to indicate the vacuum (1 atm = in-Hg) without the negative sign. Compound vacuum pressure gauge can measure pressures both above and below one atmosphere. compound vacuum- pressure gauge

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. Negative signs are usually omitted. units of measurement are recorded as psig (pounds per square inch, gage). 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 the complete 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, which is also called total vacuum, represents a total lack of pressure Most common vacuum scales use inches of mercury (in Hg) to express the value A vacuum gage will read zero when measuring atmospheric pressure and in Hg when measuring a complete vacuum.

Comparison of Pressure Scales

Pressure Measurement Methods
Pressure control is an important process in many industrial applications requiring accurate measurement and control Instruments are often classed by whether they make direct or inferred measurements Both electronic and non-electronic instruments are used in pressure measurement. As pressure changed ,the flexible element moved, and this motion was used to rotate a pointer in front of dial. Two broad categories of pressure measurement. Mechanical Pressure Transducers Electrical Pressure Transducers aaaa 12

Mechanical Pressure Transducers
Mechanical (Elastic) Pressure Transducers utlizes flexible element as a sensor. Pressure sensor measured in form of Liquid Column Gauges Manometer Bourdon tube pressure gauge Diaphragm (capsule) pressure transducers Bellows 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. A barometer is a instrument used in meteorology to measure atmospheric pressure by using water, air, or mercury.

Manometer The most common liquid column device to measure pressure is the manometer. A common simple manometer consists of a U shaped tube of glass filled with some liquid. 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.

Types of Manometers

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. C-type Helical type Spiral type

Bourdon Tube Pressure Gauge: Types
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Bourdon Tube Construction Mechanism
Bourdon tube is a hollow tube with an elliptical cross section. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves. Movement is usually coupled to a needle on a dial to make a complete gauge. It can also be connected to a secondary device such as an air nozzle to control air pressure or to a suitable transducer to convert it into an electric signal. This type can be used for measuring pressure difference. 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

Diaphragm pressure transducers
Diaphragm is constructed of two flexible disks, and when a pressure is applied to one face of the diaphragm, the position of the disk face changes due to deformation. Position can be related to pressure. Diaphragm expands when very small pressures are applied. Movement is transmitted to a pointer on a dial through a fine mechanical linkage Diaphragm are mostly made of Rubber.

Diaphragm pressure transducers: Construction
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Diaphragm Pressure Gauge
They are used to measure gauge pressures over very low ranges. Two types of diaphragm pressure gauges are: Metallic diaphragms gauge (brass or bronze) 2. Slack diaphragms gauge (Rubber) SS aaaa 23

Bellows Bellows are used to measure low pressures.
Bellows is a closed vessel with sides that can expand and contract, like an accordion. The position of the bellows without pressure can be determined by the bellows itself or a spring.  The pressure is applied to the face of the bellows, and its deformation and its position depend upon the pressure.

Bellows Constructional Elements
Bellows elastic elements are made of brass, phosphor bronze, stainless steel, beryllium-copper, or other metal suitable for the intended service of the gauge. Motion of the element (bellows) is transmitted by suitable linkage and gears to a dial pointer. Most bellows gauges are spring-loaded, as spring opposes the bellows and thus prevents full expansion of the bellows. Thus by limiting the expansion of the bellows, protects and prolongs the bellows life. Because of the elasticity in both the bellows and the spring in a spring-loaded bellows element, the relationship between the applied pressure and bellows movement is linear.

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).

Electric Pressure Transducers..... Why???
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.

Electric Pressure Transducers: Block Diagram
Pressure sensing element such as a bellow , a diaphragm or a bourdon tube Primary conversion element e.g. resistance or voltage Secondary conversion element

Electrical pressure transducers: Elements
Electrical pressure transducers consists of three elements. Mechanical Pressure Transducers to Pneumatic Pressure Transmitters to Electric Pressure Transmitters

ELECTRICAL PRESSURE TRANSDUCERS
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.

Electric Pressure Transducers: Classification
There are various ways of converting the mechanical movement into an electric signal. Following are types that directly produce an electric signal. Strain gauge pressure transducers Capacitive pressure transducers Piezoelectric pressure transducers Piezoresistive pressure transducers Potentiometer pressure transducers Resonant wire pressure transducers

Strain Gauge Pressure Transducer
Strain gauge is a device used to measure the strain of an object. Mechanism: Pressure--Mechanical Motion--Strain Gage 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.

Strain Gauge Pressure Transducer:Strcuture
Strain gauge is a passive type 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 aaaa 33

Strain Gauge Pressure Transducer:Construction & Working
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Capacitive Pressure Transducer
Mechanism: Pressure-- Diaphragm Motion-- Capacitance 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, C= 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.

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. This small absolute pressure cavity essentially reduces the effects of the negative temperature coefficient of the modulus of elasticity of the diaphragms. 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:Configuration
The sensing diaphragm and capacitor form a differential variable separation capacitor. When the two input pressures are equal the diaphragm is positioned centrally and the capacitance are equal. A difference in the two input pressure causes displacement of the sensing diaphragm and is sensed as a difference between the two capacitances

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). The 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

Piezoelectric Pressure Transducer:Construction

Piezoresistive Pressure Transducer
Piezoresistive pressure sensors operate based on the resistivity dependence of silicon under stress Like the Strain gauge, Piezoresistive sensor consists of a diaphragm onto which four pairs of silicon resistors are bonded. Unlike the construction of a strain gage sensor, here the diaphragm itself is made of silicon and the resistors are diffused into the silicon during the manufacturing process Diaphragm is completed by bonding the diaphragm to an unprocessed wafer of silicon. Silicon diaphragm is shielded from direct contact with the process materials by a fluid-filled protective diaphragm made of stainless steel or some other alloy that meets the corrosion requirements of the service. Piezoresistive pressure sensors can be used from about 3 psi to a maximum of about 14,000 psi (21 KPa to 100 MPa).

Potentiometer Pressure Transducer
The device consists of a precision potentiometer whose wiper alarm is mechanically linked to bourdon tube or bellow. The movement of wiper alarm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement using a Wheatstone bridge circuit. --- 5 – psig

Resonant Wire Pressure Transducer
Used for low differential pressure applications A wire is griped by a static member at one end and by the sensing diaphragm at the other. An oscillatory circuit causes the wire to oscillate at its resonant frequency. A change in process pressure changes the wire tension which in turn changes the resonant frequency of the wire. A digital counter circuit detects the shift. --- 5 – psig

Resonant Wire Pressure Transducer

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.

Summary of Pressure Sensor Characteristics