Pressure Measurement Part II

INDUSTRIAL INSTRUMENTATION

PRESSURE MEASUREMENTPART - IIEr. FARUK BIN POYENDept. of AEIE, UIT, BU, BURDWAN, WB, [email protected]

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Contents:1. Elastic Pressure Transducer:C type Bourdon TubeDiaphragm Pressure TransducersBellows2. Measuring Vacuum Method:Capsule GaugeMc Leod GaugeThermal Conductivity GaugePirani GaugeThermocouple gaugeIonization GaugeKnudsen Gauge

15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN2

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Bourdon Tube

Bourdon TubeElastic type TransducerRange: 100,000 psi (700 MPa)Cross-sectional tubing when deformed in any way will tend to regain its circular form under the action of pressure. C type Bourdon tube: 27 . Commonly used materials: phosphor-bronze, silicon-bronze, beryllium-copper, inconel, and other C-Cr-Ni-Mo alloysC type, Helix type or spiral type. 15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN3

Principle: As the fluid pressure enters the bourdon tube, it tries to be reformed and because of a free tip available, this action causes the tip to travel in free space and the tube unwinds. The simultaneous actions of bending and tension due to the internal pressure make a non-linear movement of the free tip. This travel is suitable guided and amplified for the measurement of the internal pressure. 15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN4

Spiral Low range 10 100kpaC type Medium range 100 5000kpaHelical High range 5000 20000kpa

Advantages of Bourdon TubeLow costSimple constructionWide variety of rangesHigh accuracy

Disadvantages of Bourdon TubeLow spring gradientSusceptible to shock and vibrationSusceptible to hysteresis


Diaphragm

Low pressure measurement. Non metallic, metallic. Non metallic (slack diaphragm) has no elastic characteristics. Make: Polythene, neoprene, silk, synthetic material. Metallic has good spring characteristics. Materials used: phosphor-bronze, silicon-bronze, beryllium-copper, inconel, and other C-Cr-Ni-Mo alloys.Range: 50 Pa 0.1 MPa15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN6Diaphragm Pressure Transducer

Diaphragm Type Transducer


Working Principle: When a force acts against a thin stretched diaphragm, it causes a deflection of the diaphragm with its centre deflecting the most.Since the elastic limit has to be maintained, the deflection of the diaphragm must be kept in a restricted manner.This can be done by cascading many diaphragm capsules.A main capsule is designed by joining two diaphragms at the periphery.A pressure inlet line is provided at the central position.When the pressure enters the capsule, the deflection will be the sum of deflections of all the individual capsules. 15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN8

Corrugated Design and amount of deflectionDepends on following FACTORS:*Number and depth of corrugation*Number of capsules*Capsule diameter*Shell thickness*Material characteristics


AdvantagesModerate costHigh over range character isticsAdaptable to absolute and differential pressure measurementGood linearityAvailable in materials which are good corrosion resistiveSmall in sizeAdaptable to slurry services

DisadvantagesAffected by vibration and shockMaintenance is difficultLimited to relatively low pressure


Bellows Same principle as Diaphragm

Made of cascaded capsules. Multiple individual diaphragms are fastened together. One piece expansible, collapsible, axially flexible. Range: 0.2 to 1 kg/sq cm. Carbon steel. Phosper bronze, Silicon bronze, Beryllium copper, Trumpet brass. Many convolutions or folds. Thin metal into Turned into from solid stock of metal. Soldered or welded stamped annular rings. Rolled tubeBy hydraulically forming a drawn tubing. 15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN11Bellow Type Pressure Gauge

AdvantagesModerate costAble to deliver high forceAdaptable for absolute and differential pressureGood in low to moderate pressure range

DisadvantagesAmbient temperature compensation requiredUnsuitable for high pressure


Measuring Vacuum MethodMeasurement below atmospheric pressure. 10-3 - 10-9 Torr .Mechanical type Mc Lead gauge, thermal type pirani gauge & thermocouple, ionization type Hot cathode & cold cathode, radiation vacuum gauge Alphatron, quartz reference

Mc Lead Gauge

15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN14McLeod Gauge

Working PrincipleThe gauge is used to compress a small quantity of low pressure gas to produce a readable large pressure.The mcleod gauge is independent of gas composition.Bulb b of the gauge is attached to capillary aa. The mercury level in the gauge is lowered up to l1 by lowering the reservoir, thereby allowing a little process fluid to enter b. By raising the reservoir, the gas is now compressed in the capillary aa till mercury rises to the zero mark in the side tube and capillary bb. The capillary bb is required to avoid any error due to capillary.

Pirani Gauge

Fine wire of tungsten or platinum0.02 cm in diameter. Temperature range: (7-400) Celsius Heating Current: 10 100 mA. Range: 10-3 Torr to 1 Torr.15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN16Pirani Gauge

Working PrincipleWhen the pressure changes, there will be a change in current. For this, the voltage V has to be kept constant.The resistance R2 of the gauge is measured, by keeping the gauge current constant.The null balance of the bridge circuit is maintained by adjusting the voltage or current.An additional reference gauge can also be used in the adjacent arm of another pirani gauge, in the bridge circuit.

Thermocouple Vacuum Gauge

Similar to pirani gauge. Hot wire temperature measured by thermocouple. The sensitivity of such an instrument depends on the pressure and the wire current.

15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN18Thermocouple Vacuum Gauge

Working PrincipleTwo sets of thermocouples are used to measure temperatures of heater wires in the two chambers and oppose each other.When there is a difference in pressures, there occurs an unbalance which is measured by a potentiometer circuit. Instead of a single thermocouple per wire, a thermopile is often chosen to increase sensitivity.

Ionization Gauge Hot Cathode TypeA column of gas is introduced into which, a potential difference V is applied with free electron in the space. This causes the electron with a charge e to acquire a kinetic energy Ve. If the pressure range of the gas in the column goes below a certain limit, called the critical pressure, then corresponding to a voltage larger than the critical voltage Vc, the energy Ve may be high enough to initiate ionization, and positive ions will be produced when the electrons collide with the gas molecules.The value of Vc is smallest for Cesium (3.88V) and largest for helium (24.58V), among monoatomic gases or vapours. For diatomic gases like N2, H2 and so on, it is roughly about 15V. This is known as the ionization potential and at this potential the pressure is also important.At very low pressures, during the intervals of time for transit from the cathode to the plate in a vacuum chamber, more than one collision is unlikely for an electron. Then for a fixed accelerating potential V>Vc, the number of positive ions formed would vary linearly with the value of pressure. Thus, a determination of the rate of production of positive ions for a given electron current should give a measure of the pressure.Range: 10-8 to 10-3 Torr. Output current varying between 10-9 and 10-4 A.


External Type Hot Ionisation Gauge

Hot cathode type ionization gauge consists of a basic vacuum triode. The grid is at a large +ve potential wrt the cathode and the plate. The plate is at a -ve potential wrt the cathode. This method is known as the external control type ionization gauge as the +ve ion collector is external to the electron collector grid with reference to the cathode. The +ve ions available between the grid and the cathode will be drawn by the cathode, and those between the grid and the plate will be collected by the plate.

15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN21External Type Ionisation Gauge

Internal Type Hot Ionisation Gauge

Here the grid is the positive ion collector and the plate is the electron collector.It consists of a helical grid with a potential of +150 volts. This huge potential attracts the electrons and thus causes gas ionization. At -30 volts, the gas ions are attracted to the central ion collector, thus producing an ion current of 100 mA/Torr. At extreme high temperature and low pressure,High current is passed through electrodes to stop increase of pressure. Internal control type is a better option to measurepressure as low as 10-9 Torr.


Internal Type Ionisation Gauge

Ionization Gauge Cold Cathode Type

Device consists of two cathodes and a hollow anode in between.Input voltage greater than 2 Kilovolt is applied between them.A strong magnetic field is produced due to the applied voltage and thus the electrons are ejected. At pressures below 10-2 Torr, the mean free path of the gas is so large that a collision may not occur at all so that discharge is not sustained or ionization may not be initiated. Collimating magnetic field increases the path length for the electrons, enabling discharges possible at pressures down to about 10-5 Torr. Non linear. 15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN23Ionization Gauge Cold Cathode

Alphatron Vacuum Gauge: radiation gauge

Cold Cathode Ionzation Gauge. Uses alpha particle to ionize the gas. Number of ions directly proportional to gas pressure. Range: 103 to 10-3 Torr.Current flow: 10-13 and 10-9 Amperes Ions produced by the alpha particles are collected by the collector electrode 15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN24Alphatron Vacuum Gauge

Quartz Reference Vacuum Gauge

Principle similar to Bourdon tube. 2 bourdon tubes are used and a formed into a helix.When a pressure difference between the two occurs, the setup begins to rotate. This rotational deflection is picked up using an optical circuit .Quartz is that it has good spring characteristics.Range: 1 milliTorr for 100 milliTorr . Gets eroded by fluorine content 15-09-2016PRESSURE MEASUREMENT - PART II Er. FARUK BIN POYEN25Reference Quartz Vacuum Gauge

THANK YOU!!


Pressure Measurement Part II

Engineering

Transcript of Pressure Measurement Part II