Measurement of Non-Electrical Quantities: Measurement of Pressure VTU B.E. (EEE) VI Sem Sensors & Transducers Module 4 Part 2

Measurement of Non-Electrical Quantities Pressure measurement — Definition of pressure — Units for pressure and pressure measuring instruments — Electrical pressure transducers — Measurement Of high pressure — Measurement of low pressure — Calibration of pressure measuring equipment;

PRESSURE MEASUREMENT PRESSURE MEASUREMENT Definition of Pressure Pressure is defined as a force Force per unit area. Pressures are exerted by gases, vapours and liquids. The instruments that we generally use, however, record pressure as the difference between the two pressures. Thus, it is the difference between the pressure exerted by a fluid of interest and the ambient atmospheric pressure. Such devices indicate the pressure either above or below that of the atmosphere. Atmospheric pressure : The atmospheric air exerts a normal pressure upon all surfaces with which it is in contact, and it is known as atmospheric pressure. The atmospheric pressure is also known as Barometric pressure. lhe atmospheric pressure at sea level (above absolute 7Tro) is called standard atmospheric pressure. Note: The local atmospheric pressure may be a little lower than these values, if the place under question is higher than sea level, and higher values if the place is lower than sea level, due to the corresponding decrease Of the column Of air standing, respectively.

PRESSURE MEASUREMENT Gauge pressure It the pressure with the help measuring instrument, in Which Atmospheric pressure is taken as datum. The atmospheric pressure on the scale is marked zero. Gauges record the above or below the local atmospheric pressure, since they measure the difference in pressure of the liquid to which they are connected and that of surrounding air . If the pressure of the liquid is below the local atmospheric pressure, then the gauge is designated as vacuum gauge and the recorded value indicates the amount by which the pressure the liquid is below local atmospheric pressure, i.e., negative pressure. (Vacuum pressure is defined as the pressure below the atmospheric pressure). Absolute pressure : It is necessary to establish an absolute pressure scale which is independent Of the changes in atmospheric pressure. A pressure of absolute zero can exist only in complete vacuum. Any measured above the absolute zero Of pressure is termed as an 'absolute pressure'. A schematic diagram showing the gauge pressure, Vacuum pressure and the absolute pressure is given in Fig.

Relationship between pressures

PRESSURE MEASUREMENT 1. Absolute pressure = Atmospheric pressure + gauge pressure Pabs=patm + Pgauge 2. Vacuum pressure = Atmospheric pressure — absolute pressure. Static pressure (p,) and total pressure (p) : Static pressure is defined as the force per unit area acting on the twill by a fluid at rest or flowing parallel to the wall in a pipeline. Total or stagnation pressure is defined as the pressure that would be obtained if the fluid stream were brought to rest isentropically. For an incompressible fluid or gas flowing at low velocities, the dynamic pressure equals where, V is the velocity of the fluid flow.

Pressure measuring instruments Pressure measuring instruments : The Pressure measurement instruments can be categorized as follows : 1. measuring low Pressure (below 1 mm of Hg) : • Manometers. • Low pressure gauges. 2. Instrument for medium and high pressures (between 1 mm of Hg to 1000 atmospheres) Bourdon tube. • Diaphragm gauges, • Bellow pressure gauge. • Dead-weight pressure gauge. 3. Instrument for measuring low vacuum ultra high vacuum (760 torr to 10-9 torr and beyond; I torr = 1 mm Of Hg) : Mcleod. Thermal conductivity. Ionization gauges. 4. Instruments for measuring very high pressures (1000 atmospheres and above) : • Bourdon tube. • Diaphragm gauges. • Electrical resistance pressure gauges. 5. Instruments for measuring varying pressure • Engine indicator. • Cathode ray oscilloscope (CRO). Fig. shows the summary of applicable range of pressure gauges.

summary of applicable range of pressure gauges

Electrical Pressure Transducers A transducer is a device for converting a signal Of one sort into the other type. In other words, a pressure gauge is a transducer for converting the distortion of a Bourdon spring into an indication through use of a calibrated dial and so on. In case of electrical transducers for measuring pressure, various transformations which are needed to convert pressure to some sort of usable electrical signal can be : The change of resistance Of an elastic material such as a wire. The change in capacitance when the distance between Capacitor plates is varied or the dielectric is changed. The conversion of a changing pressure into electrical voltage signals, Creation of a voltage by the movement of a coil within a magnetic field. Generation Of a voltage or current through the use of photoelectric effects. Torque balance system (electromagnetic).

Resistance type pressure transducers Electronic Measurements and Instrumentation The following pressure transducers will be discussed : 1. Resistance type pressure transducers. 2. Pressure voltage type. 3. Inductive type. 4. Capacitive type. 5. Carbon type. 6. Piezoelectric type. 7. Photoelectric type. 8. Electromagnetic type. Resistance type pressure transducers Resistance type devices used in pressure transducers include; l. Strain gauges. 2. Moving contacts. 1. Strain gauge : A strain gauge is simply a fine wire in the form a grid (Fig. 12.3). When the grid is distorted, the resistance (R) of the wire changes according the formula :

As the strain gauge is distorted by the elastic deformation element, its length is increased and its cross-section area is reduced. Both Of these changes increase the resistance. Since little distortion is required to change the resistance of a strain gauge through its total range, this type of transducer can be used to detect very small movement and, therefore, very small pressure changes. Since current flows in the strain gauge, some heat will be generated which must be compensated for. One method Of compensation employs a second strain gauge placed close to the measuring unit. These two are used in a bridge circuit so that the heating effect Of the two resistive elements is cancelled out, and the only resistance Change in the bridge is due to the deformation. Most often, strain gauge bridges are A.C. 2. Moving contact type transducer This type of transducer is most often used with bellows because of the force required.

Pressure-voltage transducer (potentiometer devices) One of the simplest of pressure-voltage transducer uses a potentiometer. The potentiometer is used to convert a pressure signal into an electrical voltage as shown in Fig. The potentiometer should have little friction. Levers may be used to match the required movement of the potentiometer with that of the bellows, the signal applied to potentiometer may be relatively larger of the order of several volts. By the use Of potentiometer method the signal can be sent relatively long distance. In that case a remote reading systems may employ servomechanism system. In that case the galvanometer Of the potentiometer is brought to the zero position.

Inductance-type pressure transducer Pressure-voltage transducer. consists of the following three parts : l. A coil. 2. A movable magnetic core. 3. The elastic deformation element. The element is attached to the core. When the pressure varies, the element causes the Core to move within the coil. An alternating current is passed through the ccil, and as the core moves the inductance of the coil changes. The current passing through the coil increases as the inductance decreases. This type of transducer is used in a current-sensitive coil. To get increased sensitivity the coil may be divided into two, using a centre tap (See Fig.). This actually provides two coils. As the core moves inside the coils, the inductance of one coil decreases as the inductance of the other increases. • Another type of inductance unit used for pressure measurement employs a mutual inductance unit, or linear voltage differential transducer (LVDT). In this type, three coils are wound on a single tube (See Fig.). The centre coil is connected to an A.C. source. Voltage is induced in the two outside coils. When the movable core is centered the induced voltages in each Of the outside coils are equal. Moving the core unbalances these Voltages, and the difference between them can be measured.

Reluctance-type pressure transducer : Similar to the inductance type pressure transducer is the reluctance-type. Fig. shows a reluctance-type pressure transducer. Here the electrical energy is provided by the field of a permanent magnet rather than A.C. source. For pressure measurement, the elastic deformation element moves an armature closer to or farther from the permanent magnet. The reluctance a coil wrapped around the permanent magnet varies with the movement.

Capacitive-type pressure transducer Capacitive pressure transducers consists Of two conductive plates and a dielectric. As the pressure increases. the plates move farther apart, changing the capacitance. The fluid whose pressure is being measured serves at the dielectric.

carbon pile pressure transducer is shown in Fig carbon pile pressure transducer is shown in Fig.. It translates pressure change to resistance change. As pressure change reduces the volume of the carbon pile the resistance decreases.

Piezoelectric pressure transducers Fig shows a piezoelectric Pressure transducer. As the pressure is applied to crystals to certain materials a difference in voltage across particular points of their structure occurs.

Photoelectric pressure transducers A photoelectric pressure transducer is shown in Fig. It uses a photo-tube and a light source separated by a small window, whose aperture is controlled by the force summing member of the pressure transducers.

Measurement Of High Pressures Measurement Of High Pressures Conventional pressure-measuring devices, such as strain-gauge pressure cells and Bourdon-tube gauges may be used at pressure as high as 70000 MPa. for such pressures are nearly round in section and have a high ratio of wall thickness to diameter. They are, therefore, quite stiff, and turn is For this Nason. high-pressure Bourdon tubes are Often made of a number of turns. For the measurement of very high pressure (above MPa), the "electrical gauges based on the principle of change of resistance with change of pressure are used. It is known that resistance Of fine wires changes with pressure according to the following linear relationship.

This gauge claims the following advantages : 1 This gauge claims the following advantages : 1. Commercially available with scale upto pressures of 15 MPa with accuracies of 0.1 to 0.5% when properly calibrated. 2. Good dynamic response (the resistance wire itself can respond to variations in MHz range but the overall frequency response of the pressure measurement system is limited to much lower values because of the acoustic response Of the transmitting fluid).

Measurement Of Low Pressure (Vacuum) In general, pressures below atmospheric may be called low pressures or vacuums. It is impossible to reach an absolute pressure of zero, since _positive magnitude Of absolute pressure exists at all times, even in a vacuum. The micrometer, which is one-millionth Of a meter (0.001 mm) of mercury column, is a common unit of "low pressure". Very low pressure . . any pressure below 1 mercury (or torr). ultra low pressure less than a nanometer (It-3 gm). The two basic methods Of measurement Of low pressure are : 1. Direct methods. Here, the displacement deflection caused by the pressure is measured and is correlated to the pressure. The following devices are included in this category : Spiral Bourdon tubes. • Flat and corrugated diaphragms. Capsules. • Manometers. 2. Indirect or inferential methods. In these methcxis, pressure is determined through the measurement of certain other pressure-controlled properties, including volume and thermal conductivity Devices included in this category are : • Mcleod gauge. • Thermal conductivity gauges. • Ionization gauges. • Radioactive vacuum meters. A few of these devices are discussed below. Pirani vacuum gauge In this gauge, the temperature of the wire is determined by measuring the change Of The Pirani gauge employs a single filament (in the form of four coiled wires Of tungsten Or platinum connected in parallel) enclosed in a glass tube/chamber, whose pressure is to be measured

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