2PRESSURE 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.
3Pressure 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 0C 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 torr1 atm= psi1 psi=6,894.76PaAnother commonly used unit is bar, which is 100K pascal.Bar : atmospheric pressure on Earth at sea level.
4Pressure sensors Units: Relationship Pascal (Pa)baratmosphere (atm)Torrpound-force per square inch (psi)1 Pa110−59.8692×10−67.5006×10−3145.04×10−61 bar100,000750.0614.5041 atm101,32576014.6961 Torr1.3332×10−31.3158×10−319.337×10−31 psi6,894.7668.948×10−368.046×10−351.715
5Pressure Measurement Scales Five basic scales used to measure pressure:Atmosphere pressureGauge pressureAbsolute pressureDifferential pressureVacuum pressurePressure Measurement FormulizationPressure is quoted as being Absolute or GaugeSome Fluid = Some Pressure = Some absolute pressureNo Fluid = No Pressure = Zero absolute pressureWhereasFluid Pressure+ Atmospheric Pressure= Gauge PressureNo Fluid + Atmospheric Pressure = Zero Gauge PressureWhich followsGauge Pressure – Atmospheric Pressure = Pressure due to fluid itself = Absolute fluid pressure
6Atmosphere pressureAtmospheric 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
7Gauge Pressure ScaleGauge 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
8Absolute 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).
9Differential 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.
10Vacuum Pressure ScaleThe 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 pressureMost common vacuum scales use inches of mercury (in Hg) to express the valueA vacuum gage will read zero when measuring atmospheric pressure and in Hg when measuring a complete vacuum.
12Pressure Measurement Methods Pressure control is an important process in many industrial applications requiring accurate measurement and controlInstruments are often classed by whether they make direct or inferred measurementsBoth 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 TransducersElectrical Pressure Transducersaaaa12
13Mechanical Pressure Transducers Mechanical (Elastic) Pressure Transducers utlizes flexible element as a sensor.Pressure sensor measured in form ofLiquid Column GaugesManometerBourdon tube pressure gaugeDiaphragm (capsule) pressure transducersBellowsSpring and Piston.
14Liquid Column GaugesMeasuring 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 instrumentsA 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.
15ManometerThe 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 densityEach 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.
17Bourdon 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 displacementA 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-typeHelical typeSpiral type
19Bourdon 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
20Essentials of a Bourdon-tube pressure gage 1.Pressure causes the section of the flattened tube tends toward a more circular form2.This causes the free end A to move outward3.The resulting motion is transmitted by link B to sector gear C4.Then to pinion D and cause the indicator hand to move over the scale
21Diaphragm 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 linkageDiaphragm are mostly made of Rubber.
22Diaphragm pressure transducers: Construction aaaa22
23Diaphragm 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)SSaaaa23
24Bellows 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.
25Bellows 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.
26Spring & Piston TypeThe 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).
27Electric 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.
28Electric Pressure Transducers: Block Diagram Pressure sensing element such as a bellow , a diaphragm or a bourdon tubePrimary conversion element e.g. resistance or voltageSecondary conversion element
29Electrical pressure transducers: Elements Electrical pressure transducers consists of three elements.Mechanical Pressure Transducers to Pneumatic Pressure Transmitters to Electric Pressure Transmitters
30ELECTRICAL 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.
31Electric 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 transducersCapacitive pressure transducersPiezoelectric pressure transducersPiezoresistive pressure transducersPotentiometer pressure transducersResonant wire pressure transducers
32Strain Gauge Pressure Transducer Strain gauge is a device used to measure the strain of an object.Mechanism: Pressure--Mechanical Motion--Strain GageStrain gauge pressure transducers are used for narrow pressure span and for differential pressure measurments.Strain gauges are typically constructed of piezoelectric semiconductor materialPressure 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.
33Strain 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 measuredaaaa33
34Strain Gauge Pressure Transducer:Construction & Working aaaa34
35Capacitive Pressure Transducer Mechanism: Pressure-- Diaphragm Motion-- CapacitanceThe 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/dWhere,C= capacitance of a capacitor in faradA = area of each plate in m2d = distance between two plates in mεr= dielectric constantε0 = 8.854*10^-12 farad/m2Thus, 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.
36Capacitive 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 diaphragmsa 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.
37Capacitive 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
38Piezoelectric 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 crystalThese 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
40Piezoresistive Pressure Transducer Piezoresistive pressure sensors operate based on the resistivity dependence of silicon under stressLike 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 processDiaphragm 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).
41Potentiometer 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
42Resonant Wire Pressure Transducer Used for low differential pressure applicationsA 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
44Where 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.