Electric Pressure Transducer

Pressure Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Pressure = Force / Area The SI unit for pressure is the pascal (Pa)

Types Of Pressure Measurement Gage pressure (psig) Absolute pressure (psia) Sealed reference pressure (psis) Differential pressure (psid)

Pressure Measurement Methods Electric pressure transducers Elastic pressure transducers Manometer method Pressure measurement by measure vacuum Pressure measurement by balancing forces

Electric Pressure Transducers An instrument component which detects a fluid pressure and produces an electrical signals related to the pressure. Generally electrical pressure transducers consist of three elements Pressure sensing element such as bellow, a diaphragm or a bourdon tube Primary conversion element e.g. resistance or a voltage Secondary conversion element

Types of Electric Pressure Transducers Strain gauge pressure transducers Potentiometer pressure transducers Reluctance pressure transducers Piezoelectric pressure transducers Capacitive pressure transducers

Strain Gauge Pressure Transducers Strain gauge is passive type of resistance pressure transducer , whose electrical resistance changes when it is stretched or compressed. It can be attached to a pressure sensing diaphragm. They are available for wide range of measurements (from vacuum to 200000 psig)

Strain Gauge Pressure Transducers Construction and Working

Strain Gauge Pressure Transducers Advantages They are small and easy to install They have good accuracy and stability They have high output signal strength, over range capacity and simple to maintain They contain no moving parts They are readily adoptable to electronic systems and fast speed of response

Strain Gauge Pressure Transducers Disadvantages Their cost is moderate to high(could be offset by reduced installation cost) Electrical readout is necessary in these transducers They are require constant voltage supply They require temperature compensation due to problem presented by temperature variations

Potentiometric Pressure Transducers There is a potentiometer (basically a variable resistance) which is made by winding resistance wire around an insulated cylinder. A moveable electrical contact, called a wiper, slides along the cylinder, touching the wire at one point on each turn. A mechanical linkage from the pressure sensing element (such as bellows, a diaphragm) controls the position of the wiper on the potentiometer. The position of wiper determines the resistance of the potentiometer which in turn determines the pressure.

Potentiometer Pressure Transducers Working Principle An increase in pressure makes the bourdon tube straighten out partially. This motion causes the linkage to move the wiper across the winding on potentiometer as the wiper moves; it increases the resistance between terminals A and B which is equal to the pressure sensed by the bourdon tube.

Potentiometric Pressure Transducers Advantages Provide strong output so no need of additional amplifier They have high range and have high ruggedness They have simple instrumentation, have high electrical efficiency and are inexpensive Disadvantages Potentiometer pressure transducers have finite resolution, limited life and large size They have poor frequency to response and have tendency to develop noise

Reluctance Pressure Transducers Reluctance in magnetic circuit is equivalent to resistance in an electrical circuit. Whenever coupling between two magnetic coils changes, then reluctance between them also changes. So pressure sensor can be used to change the spacing between the coils by moving one part of the magnetic circuit. This motion changes the reluctance between the coils which causes the change voltage induced by one coil in other. This induced voltage can be interpreted as a change in pressure.

Reluctance Pressure Transducers Reluctance pressure transducer of several types. 1. Linear variable differential transformer (LVDT) 2. Servo pressure transducers LVDT is widely used inductive transducer that translates linear motion into an electrical signal.

Reluctance Pressure Transducers Advantages It possesses a high sensitivity, very rugged in construction and has infinite resolution It shows a low hysteresis, tolerate high degree of shock , vibration and stable Disadvantages They are sensitive to stray magnetic fields but shielding is possible Temperature effects the performance of transducers

Piezoelectric Pressure Transducers These devices utilizes the piezoelectric characteristics of certain crystalline and ceramic materials (such as quartz) to generate an electrical signals. Signals generated by crystals decays rapidly so unsuitable for static force or pressure measurements Range 5,000 to 10,000 psig

Piezoelectric Pressure Transducers Advantages The transducers no needs external power It has a good frequency response Disadvantages This type of transducers cannot measure static pressure Output of the transducers is affected by changes in temperature

Capacitive Pressure Transducers The term capacitor is defined as two metal plates are separated by a distance d. A dielectric medium is placed between the plates. When voltage or potential difference is applied to them, equal and opposite charges is getting developed on the plates. A capacitive transducers works on the principle of capacitance of parallel plate capacitor

Capacitive Pressure Transducers 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 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 Transducers Advantages These transducers have high input impedance and require small force for operation These transducers have good frequency response and less affected by stray magnetic fields Disadvantages Guard rings are necessaries so as to minimize stray electric fields They require complex circuit arrangement like bridge The performance may be affected by parameters like dust, temperature  

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