THE ENGINEERING RESOURCE
an ultimate solution for those seeking success
 

 

Electric Pressure Transducers

To obtain a full version, download a PDF copy of this report: Click here, Report 2 (Report on the same topic but from a different author)

 

Introduction:

Pressure:

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure. Pressure is an effect which occurs when a force is applied on a surface. Pressure is the amount of force acting on a unit area. The symbol of pressure is P. [5]

Types of Pressure Measurement:

Gage pressure (psig) quantifies fluid pressure relative to ambient air pressure.

Absolute pressure (psia) measures pressure relative to a vacuum.

Sealed reference pressure (psis) (bars) is measured relative to a situation pressure whose magnitude is at, or close to, standard atmospheric pressure.

Differential pressure (psid) (bard) quantifies the pressure difference between two points within a system. The measurement also must consider the magnitude of the system's line pressure. Measurements usually are taken from two dissimilar fluid inputs within the system using a transducer designed specifically for differential-pressure calculations. [5]

Pressure Transducers:

An instrument component which detects a fluid pressure and produces an electrical, mechanical, or pneumatic signal related to the pressure. [6]

Pressure Measurement Methods:

Electric pressure transducers

Elastic pressure transducers

Manometer method

Pressure measurement by measure vacuum

Pressure measurement by balancing forces [7]

 

Electric Pressure Transducers:

In general “a transducer is a device which converts one form of energy into another form of energy.” However, in the field of electrical instrumentation, a transducer is a tool that converts a physical quantity, a physical condition or mechanical output to an electrical signal. Most of method converting mechanical output into an electrical signal works equally well for the bellows, the diaphragm, the bourdon tube. In this conversion a mechanical motion is first converted into a change in electrical resistance and then change in resistance is transformed into change in electrical current or voltage. 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  [3]

 Types of Electric Pressure Transducers:

  Strain gauge pressure transducers

  Potentiometer pressure  transducers

  Reluctance pressure transducers

  Piezoelectric pressure transducers

  Capacitive pressure transducers [7]

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. [2]

Working Principle:

The strain gauge is a fine wire which changes its resistance when mechanically strained, due to physical effects. A strain gauge may be attached to the diaphragm so that when the diaphragm flexes due to the process pressure applied on it, the strain gauge stretches or compresses. This deformation of the strain gauge causes the dissimilarity in its length and cross sectional area due to which its resistance also changes, as shown in fig (a).The resistance change of a strain gauge is usually converted into voltage by linking one, two, or four similar gauges as of Wheatstone bridge (known as strain gauge bridge), and applying excitation to the bridge. The bridge output voltage is then a measure of the pressure sensed by strain gauges. [2]

 Construction and Working:

In fig. 1 (b) shows a bridge circuit with four strain gauges Rsg1, Rsg2, Rsg3 and Rsg4. Two strain gauges Rsg1 and Rsg4 are mounted so that ever-increasing pressure increases their resistance. Strain gauges Rsg2 and Rsg3 are mounted so that increasing pressure decrease their resistance. A change in temperature affects all the four strain gauges in the same way, consequential in no change in the pressure indication. [2]

(a)   Strain gauge transducer with diaphragm element

(b)   Strain gauge bridge circuit

At balance when there is no pressure, no current flows through the galvanometer G, and hence there will be no deflection in the galvanometer. As soon as pressure is applied, the strain gauge compresses or stretches hence and the bridge is unbalanced due to change in resistance of the strain gauges. Thus the current flows in the galvanometer, which is measured by the deflection of the galvanometer. These changes affect the output of bridge circuit which indicates change in measured pressure. Now this change in output voltage may be calibrated for the pressure change. [2]

Advantages:

Following are the advantages of strain gauge pressure transducers

  They are small and easy to install

  They have good accuracy

  They are available for wide range of measurements (from vacuum to 200000 psig)

  They possess good stability

  They have high output signal strength

  They have high over range capacity

   They are simple to maintain

  They contain no moving parts

  They possess good shock and vibration characteristics

  They are readily adoptable to electronic systems

  They possess fast speed of response [2]

Disadvantages:

Following are the disadvantages of strain gauge pressure transducers:

  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 [2]

Potentiometric Pressure Transducers:

In this type of pressure transducer, 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. The position of the wiper determines how much wire, and therefore, how much resistance, is between the end of the wire and the wiper. A mechanical linkage from the pressure sensing element (such as bellows, a diaphragm, and the bourdon tube) controls the position of the wiper on the potentiometer. Some potentiometer is made curved so that the wiper can pivot in a circular motion rather than moving along a straight line. The location of wiper determines the confrontation of the potentiometer which in turn determines the pressure. [2]

Construction and Working:

In fig.2  shows a diagram of a Potentiometric pressure transducer in which the sensing element is bourdon tube. An increase in pressure makes the bourdon tube relax 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.

Advantages:

Following are the advantages of Potentiometer pressure transducers:

  Provide  strong output so no need of additional speaker

  Potentiometer pressure transducers  have high range

  Potentiometer pressure transducers  have high starkness

  Potentiometer pressure transducers  have simple instrumentation

  Potentiometer pressure transducers  have high electrical efficiency

  Potentiometer pressure transducers  are in expensive

  Potentiometer pressure transducers  are best suitable for measurements in the systems with least requirements [2]

Disadvantages:

Following are the disadvantages of Potentiometer pressure transducers:

  Potentiometer pressure transducers  have finite resolution

  Potentiometer pressure transducers have limited life

  Potentiometer pressure transducers  are of large size

  Potentiometer pressure transducers  have poor frequency response

  Potentiometer pressure transducers  have tendency  to develop noise

  Potentiometer pressure transducers  have susceptibility to vibration [2]

Reluctance Pressure Transducers:

Reluctance in magnetic circuit is corresponding 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 transducer of several types. One of them is discussed below.

  Linear variable differential transformer (LVDT)

  Servo pressure transducers

Linear Variable Differential Transformer (LVDT):

It is widely used inductive transducer that translates linear motion into an electrical signal. In fig. 3 show LVDT for pressure measurement. [2]

Construction and Working:

It consists of one primary and two secondary coils. These windings are arranged concentrically next to each other. They are wound over a hollow bobbin which is usually a non-magnetic and insulating material. A ferromagnetic armature is attached to the transducers sensing shaft. A core is generally made of a high permeability alloy and has a shape of cylinder or rod. A.C excitation is supplied to a primary coil and moveable core is varies the coupling between it and two secondary coils. When the core is in centre position then coupling with secondary coil is equal. As the core moves away from the centre position then the coupling the one secondary, and hence its output voltage, increases while the coupling and the output voltage of other secondary decreases. [2]

Any change in pressure makes the bellows expand or contract. This motion moves the magnetic core inside hollow portion of bobbin. It causes the voltage of one secondary winding to increases, while simultaneously reducing voltage in other secondary winding. The difference of this voltage that appear across the output terminals of the transducers and gives the measure of physical position of the core and hence the pressure.

Advantages:

Following are of the advantages LVDT:

  It possesses a  high sensitivity 

  It is very rugged in construction

  It tolerate high degree of shock and vibration without any adverse effect

  It is stable and easy to align and maintain due to simplicity of construction, small size and light body

  The output voltage of this transducer is practically linear for the displacement of about 5 mm

  It has infinite resolution

  It shows a low hysteresis, hence repeatability is excellent under all condition  [2]

Disadvantages:

Following are of the disadvantages LVDT:

  Relatively large core displacements are required for appreciable amount of differential output

  They are sensitive to stray magnetic fields but shielding is possible

  Temperature effects the performance of transducers [2]

Piezoelectric Pressure Transducers:

These devices utilizes the piezoelectric characteristics of certain crystalline and ceramic materials (such as quartz) to generate an electrical signals.

Basic Principle:

These transducers depend upon the principle that “when pressure is applied on piezoelectric crystals (such as a quartz), an electrical charge is generated”. These are about 40 crystal-line material that, when subjected to “squeeze”, generate an electric charge. Some of piezoelectric materials are barium titanate sintered powder, crystal of quartz, tourmaline, Rochelle salts. Fig. 4 shows piezoelectric pressure transducer. [2]

Construction and Working:

It consists of diaphragm by which pressure is transmitted to the piezoelectric crystal Y1. This crystal generates electrical signal which is amplified by a charge amplifier. A second piezoelectric crystal Y2 is included to compensate for any acceleration of the device during use. This compensation is needed because rapid acceleration of transducers creates additional pressure on piezoelectric crystal. Vibration is major a source high, rapidly changing acceleration.

Signal from the compensating crystal are amplified by second charge amplifier. A different amplifier which are used to substrates pressure alone; all effects of acceleration are removed. Piezoelectric pressure transducers are used to measure very high pressure that change very rapidly. For example, the pressure inside the cylinder of gasoline engine changes very rapidly from less than atmospheric pressure to many thousands of PSI.

Similar pressure changes in compressors, rocket motors, etc. It is impossible for ordinary pressure transducers to measure such a high pressure changes over such short time periods. They do not respond fast enough. But piezoelectric materials produced electrical voltage when they or squeezed suddenly. The voltage disappears when the pressure stops changing. Piezoelectric pressure transducer may be used to measure a pressure over a range up to 0-50,000 psi. However, piezoelectric transducers cannot measure steady pressure. They respond only to changing pressures.

Advantages:

  The transducers no needs external power and is therefore self-generating (active type)

  It has a good frequency response [2]

 

Disadvantages:

Following are of the disadvantages piezoelectric pressure transducer:

  This type of transducers cannot measure static pressure

  Output of the transducers is affected by changes in temperature. Therefore, temperature-compensating devices have to be used. [1]

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. It is specified as follows: [2]

 

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

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 measurable by any suitable electric bridge circuit and is proportional to the pressure. [2]

Construction and Working:

In fig. 5 shows the construction of capacitive pressure transducer. It uses the principle of change in capacitance due to change in distance between two plates when pressure is applied to the diaphragm. A capacitive pressure transducer consist of an airtight housing in which metallic diaphragm is placed. At the inner side, fixed plate is placed. In between diaphragm and fixed plate, the dielectric medium is placed. When pressure is applied to the diaphragm, it moves towards a fixed plate resulting change in the capacitance. This capacitance is calibrated into voltage proportional to the applied pressure.

Advantages:

  As these transducers have high input impedance, the loading effect is minimum

  They require small force for operation, hence is useful for small displacement, pressure measurements and require small power

  These transducers have good frequency response

  They are less affected by stray magnetic fields [4,3]

Disadvantages:

  These transducers require good quality isolation so as protect the transducer(metal plates) from stray capacitance

  Guard rings are necessaries so as to minimize stray electric fields

  Sometime frequency response affected by loading effects from connecting links and cables

  The performance may be affected by parameters like dust, temperature, moisture content variations and so fourth

  They require complex circuit arrangement like bridge, amplifier, etc. for measurement purpose [2,3]

 

References:

1.      Bakshi U.A., Bakshi A.V.,” Electronic Instrumentation” Published by Technical Publications Pune,   1st Edition India

 

2.      Singh S. K, “Indl Inst & Control” Published by Tata McGraw-Hill, 2ND Edition (India)

 

3.      Singh, M.D., Joshi, J.G.,” Mechatronics” Published by Asoke K. Ghosh, Printice- Hall of India

 

4.      Kishore K. Lal,” Electronic Measurements and Instrumentation” Published By Dorling Kindersley (India)

 

5.      http://en.wikipedia.org/wiki/Pressure_measurement

6.      http://www.omega.com/prodinfo/pressuretransducers.html  

7.      http://www.omega.com/literature/transactions/volume3/pressure.html       

 

 

 

 

 

 

 

 

 
Home page   |   Engineering   |   O Level   |   A Level   |   Exams   |    Contact us