Physics of Industry Instruments for.pptx

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1. Physics of Industry Instruments Prepared by, Dr. Keyur Sangani Assistant Professor EC Department, Institute of Technology Nirma University

2. Introduction to Vacuum Technology The word "vacuum" comes from the Latin " vacua ", which means "empty". However, there does not exist a totally empty space in nature. Vacuum is only a partially empty space, where some of the air and other gases have been removed from a gas containing volume. The pressure below atmospheric pressure is known as vacuum. The technology used create low pressure in any gas containing volume is known as vacuum technology. Pressure and vacuum are inversely proportional to each other i.e. if we increase pressure vacuum decreases with decrease in mean free path and vice-versa. The system used to reduce pressure or generate vacuum is known as Vacuum pump, where the system used to measure vacuum is known as Vacuum gauge.

3. Introduction to Vacuum Technology In the traditional measurement system, normal pressure is expressed in millimeters (mm) of a column of mercury (Hg) and 760 mm of Hg is equal to 1 standard atmosphere. The traditional unit of pressure is the Torr. However, the other units such as mm of Hg, Pascal, Bar, lb /in 2 are also being used to measure the pressure. The correlation among them are as follow: 1 Torr = 1 mm of Hg = 1.33 mbar = 1330 Pa 1 Pascal = 0.01 mbar = 1 N/m 2 = 1 dyne/cm 2 1 atmosphere = 10 5 Pa = 1.013 bar = 760 Torr = 14.70 lb /in 2

4. Vacuum Pumps Classification of vacuum pumps: Vacuum pumps can be classified with reference to (1) Pressure range over which they operate (2) Principle of operation There are four different types of pressure range over which they operate: High pressure or Low vacuum range (760 mm of Hg to 10 -3 Torr) Medium pressure or Medium vacuum range (10 -3 Torr to 10 -6 Torr) Low pressure or High vacuum range (10 -6 Torr to 10 -9 Torr) 4) Ultra low pressure or Ultra high vacuum range (10 -9 Torr to 10 -12 Torr)

5. Vacuum Pumps On the principle of operation, vacuum pump can classify in four different types 1) Mechanical pump: This pump operates in high pressure or low vacuum range from 760 mm of Hg to 10 -3 Torr. Principle of operation: The physical removal of gases due to cyclic motion of mechanical parts of the system. Example: Rotary pump, Roots pump, Molecular drag pump (Turbo molecular pump).

6. Vacuum Pumps 2) Ejector pump: This pump operates in medium pressure or medium vacuum range from 10 -3 Torr to 10 -6 Torr. Principle of operation: The removal of gases takes place due to transfer of momentum from one kind of molecule to other kind of molecule. Example: Diffusion pump. 3) Cryo pump: This pump operates in low pressure or high vacuum range from 10 -6 Torr to 10 -9 Torr. Principle of operation: The removal of gases takes place due to freezing effect at low temperature. Example: Cryogenic and Cryosorption In Cryogenic pump condensation of gas molecule takes place at the inner surface of the vessel which is to be evacuated this leads to reduction of pressure in the system. Where in Cryosorption pump condensation of gas molecule takes place within the sorbing material at low temperature this leads to creation of vacuum in the system. Charcoal is the best example of sorbing material. 4) Ion pump: This pump operates in ultralow pressure or ultrahigh vacuum range from 10-9 Torr to 10-12 Torr. Principle of operation: The removal of gases takes place due to ionization of gas. There are three type of ion pump exists: Evapour ion pump, Sputter ion pump and Getter ion pump. Example: Getter ion pump and Sputter ion pump

7. Vacuum Pumps In Cryogenic pump, condensation of gas molecule takes place at the inner surface of the vessel which is to be evacuated this leads to reduction of pressure in the system. In Cryosorption pump, condensation of gas molecule takes place within the sorbing material at low temperature this leads to creation of vacuum in the system. Charcoal is the best example of sorbing material.

8. Vacuum Pumps 4) Ion pump: This pump operates in ultralow pressure or ultrahigh vacuum range from 10 -9 Torr to 10 -12 Torr. Principle of operation: The removal of gases takes place due to ionization of gas. There are three type of ion pump exists: Evapour ion pump, Sputter ion pump and Getter ion pump. Example: Sputter ion pump and Getter ion pump

9. Rotary Pump This pump belongs to mechanical pump and it operates in high pressure or low vacuum range from 760 mm of Hg to 10 -3 Torr. Principle of operation: The physical removal of gases due to cyclic motion of mechanical parts of the system Construction: As shown in figure circular stator and rotor are connected at the top with each other by means of spring loaded vanes. Stator is fixed where rotor performs cyclic motion.

10. Rotary Pump The region between stator and rotor increases as rotor move away from the connected point between them which is at the top and whole system is placed in oil bath. Working: A gas vessel in which we want to create a vacuum is connected with inlet valve so molecule of gas can enter through valve in a region between stator and rotor. Due to cyclic motion of rotor gas molecule entering through inlet valve will move towards outlet and exert pressure on valve at outlet thus the gas will be thrown out into the atmosphere this is how pressure in a system decreases due to cyclic motion perform by stator and rotor.

11. Rotary Pump It is mention that whole system is placed in oil bath to reduce heat produce due to cyclic motion of rotor. Merits: The rotary pump can be used directly at atmospheric pressure because it offers a good pumping speed at this pressure. Operation of this pump is based on stator and rotor which helps in handling large amount of gas load. Demerits: Since oil is used as lubricant in this pump so there is always maximum probability of contamination from the environment in vacuum chamber. Gas is enclosed in crescent shape volume between the region of stator and rotor this may lead to condensation of gas within this volume which is never advisable.

12. Diffusion Pump This pump belongs to evapour pump and it operates in high pressure or low vacuum range from 10 -3 Torr. to 10 -6 Torr. Principle of operation: The removal of gases takes place due to transfer of momentum from one kind of molecule to other kind of molecule. Construction: As shown in figure diffusion pump consist of inlet valve, chimney, nozzle, different compressing stages, boiler containing oil and backing pump. Working: When gas molecules enter in the region of diffusion pump through inlet valve they will spread around nozzle, by heating the

13. Diffusion Pump boiler, oil molecule will transform into vapour form and will move in upward direction in chimney. Generally, silicon or hydrocarbon oil is used because of their low vapour pressure. When this oil vapour reach at top of chimney it will exert pressure on nozzle which place at the top of chimney will open up and vibrate at ultrasonic frequency as a result of this oil vapour will come out of the nozzle with high momentum in downward direction. By colliding with gas molecule oil vapour transfer their momentum to gas molecule.

14. Diffusion Pump Due to this transfer of momentum gas molecule will move towards bottom of boiler with high momentum and high velocity. During their motion towards bottom of boiler they will pass through three different compression stages as a result of this density will also increase at bottom, from there they will move towards backing pump. Generally rotary pump is being used as backing pump. Due to water cooling system oil vapour condensed on inner surface of diffusion pump will be collected at bottom of boiler and again heated up.

15. Diffusion Pump This is how system is evacuated due to momentum transfer in diffusion pump. It is important to note that diffusion pump always requires backing pump. Merits: High vacuum can be easily created in the system using diffusion pump. Maintenance of diffusion pump is very easy. Most of the application required vacuum of the order of 10 -5 to 10 -6 Torr which can be easily created using diffusion pump. Demerits: The contamination of oil takes place after a long term operation of diffusion pump so it is required to change oil time to time. Diffusion pump cannot be used directly at atmospheric pressure so backing pump is always connected along with diffusion pump.

16. Sputter-ion Pump This pump belongs to ion pump and it operates in high pressure or low vacuum range from 10 -9 Torr to 10 -12 Torr. Principle of operation: The removal of gases takes place due to ionization of gas. Construction: As shown in figure the sputter ion pump consist of anode cathode and a magnet. The anode is usually stainless steel cylinder. The cathode plates positioned on both sides of the anode tube as shown and are made up of titanium. These plates of titanium act as a gettering material. Working: As explained in classroom

17. Vacuum Gauges Classification of vacuum gauges: There are three different types of gauges used to measure the vacuum: Mechanical Gauge (760 mm of Hg to 10 -3 Torr) Working Principle : The change in length or volume of the mechanical part due to applied pressure is used to measure pressure. e.g. Bourdon Gauge, Diaphragm Gauge, Liquid manometer Gauge, Mcleod Gauge

18. Vacuum Gauges Thermal Conductivity Gauge (760 mm of Hg to 10 -4 Torr) Working Principle : The change in thermal conductivity of the gas will lead to the change in the resistance of the filament is measured externally in terms of current and this current is calibrated in terms of pressure. e.g. Thermocouple Gauge and Pirani Gauge Ion Gauge (10 -5 Torr to 10 -12 Torr) Working Principle : The ionization of gas will lead to the measurement of ion current at respective electrodes which will calibrate in terms of pressure. e.g. Hot and Cold cathode ion Gauge.

19. Pirani Gauge This gauge belongs to thermal conductivity gauge and its pressure range is 760 mm of Hg to 10 -4 Torr. Principle of operation: The change in thermal conductivity of the gas will lead to the change in the resistance of the filament is measured externally in terms of current and this current is calibrated in terms of pressure. Construction: Pirani gauge consist of four arms of resistance and one of which is connected to the vacuum system. Working: As explained in classroom.

20. Scintillation Counter This counter works on the phenomenon of scintillation. When -particles or any other such particles strike the scintillating substances like ZnS, Barium platinocyanide, calcium tungstate, a light pulse is produced. This is called scintillation. When -ray is incident on the scintillating crystal, it produces a light pulse which is directed to photocathode by means of reflective surface.   This light pulse so incident on photocathode that it ejects electrons out. With suitable voltage, these photoelectrons are made to fall on first dynode(anode).

21. Scintillation Counter Each electron incident on this dynode produces secondary electrons. The second dynode is at higher potential with respect to the first dynode. Hence, the first is cathode for second dynode. The secondary electrons emitted from the first dynode are now incident on the secondary dynode and produce the additional electrons. This process occurs on 10-14 dynodes and final electric pulse is amplified by linear amplifier. Then the amplified pulse is given to a discriminator to remove the low level noise pulse and then they are counted by the counters.

22. Fibre Optic Sensors Optical sensor is a transducer which converts any form of signal into optical signal in the measurable form. Here optical fibers are used as a guiding media. The optical sources used here are LED/Laser. The optical signal produced by the optical source and is transmitted through the transmitting fiber in the modulation zone. The optical signals are modulated based on any one of these properties, viz., Optical intensity, phase, polarization, Wavelength and spectral distribution. These modulated signals with any one of these properties are received by the receiving / fiber and is sent to the optical detector.

23. Fibre Optic Sensors Types of Sensors: Intrinsic sensors: In these type of sensors the physical parameter to be sensed directly acts on the fiber itself to produce the changes in the transmission characteristics. Examples: Temperature /Pressure Sensor( Phase and polarization sensor) and Liquid level sensor. Extrinsic Sensors: In these type of sensors separate sensing element will be used and the fiber will act as a guiding media to the sensors. Examples: Displacement sensor and Laser Doppler velocimeter sensor

24. Temperature Sensor Principle: It is based on the principle of interference between the beams emerging out from the reference fiber and the fiber kept in the measuring environment. Construction: It consists of a Laser source to emit light. A beam splitter, made of glass plate is inclined at an angle 450 with respect to the direction of the laser beam. A Reference fiber which is isolated from the environment. A Test fiber kept in the environment to be sensed. Separate lens systems are provided to split and to collect the beam.

25. Temperature Sensor Working: A monochromatic laser light is emitted from the laser source. The beam splitter divides the laser beam into two beams ( i ) main beam and (ii) splitted beam. The main beam passes through the lens L 1 and is focused onto the reference fiber which is isolated from the environment to be sensed. The beam after passing through the reference fiber then falls on the Lens L 2 . The splitted beam passes through the Lens L 3 and is focused onto the test fiber kept in the environment to be sensed.

26. Temperature Sensor The splitted beam after passing through the test fiber is made to fall on lens L 2 . The two beams after passing through the fibers, produces a path difference due to change in parameters such as pressure, temperature etc in the environment. Therefore a path difference is produced between two beams causing the interference pattern as shown in the figure. Thus the change in pressure or temperature can be accurately measured with the help of the interference pattern obtained.

27. Liquid Level Sensor

28. Carbon dioxide(CO2) LASER This is a type of gas laser and the lasing action is achieved by transitions between vibrational levels of molecules. It gives continuous output in far IR. A carbon dioxide molecule has a carbon atom at the center with two oxygen atoms attached, one at both sides. Such a molecule exhibits three independent modes of vibrations. a) Symmetric stretching mode. Bending mode c) Asymmetric stretching mode..

29. Carbon dioxide(CO2) LASER Construction: It consists of a quartz tube 5 m long and 2.5 cm in the diameter. This discharge tube is filled with gaseous mixture of CO 2 (active medium), helium and nitrogen with suitable partial pressures. The terminals of the discharge tubes are connected to a D.C power supply. The ends of the discharge tube are fitted with NaCl Brewster windows so that the laser light generated will be polarized. Two concave mirrors one fully reflecting and the other partially form an optical resonator.

30. Carbon dioxide(CO2) LASER Working: When an electric discharge occurs in the gas, the electrons collide with nitrogen molecules and they are raised to excited states. The process is given by N 2 + e* = N 2 * + e Now N 2 molecules in the excited state collide with CO 2 atoms in ground state and excite to higher electronic, vibrational and rotational levels. This process is given by N 2 * + CO 2 = CO 2 * + N 2 Since the excited level of nitrogen is very close to the E 5 level of CO 2 atom, population in E 5 level increases. As soon as population inversion is reached, any of the spontaneously emitted photon will trigger laser action in the tube.

31. Carbon dioxide(CO2) LASER There are two types of laser transition possible (1) Transition E 5 to E 4 (2) Transition E 5 to E 3 . The first transition will produce a laser beam of 10.6μm wavelength, the second transition produce a laser beam of 9.6μ m wavelength. Advantages: The construction is simple and has high efficiency. The output of this laser is continuous and give very high output of 10 kW. The output power can be increased by extending the length of the gas tube. Disadvantages: The contamination of oxygen by carbon monoxide will have some effect on laser action. The operating temperature plays an important role in determining the output power of laser. The corrosion may occur at the reflecting plates.

32. Semiconductor diode LASER It is a specially fabricated p-n junction device, which emits coherent light when operates in forward biased condition. Principle: When a p-n junction diode is forward-biased, electrons and holes cross the junction and recombine to release energy in the form of photons, which stimulates other electrons and holes to recombine and produce a laser. Construction: It consist of p-type and n-type doped layers of GaAs which forms a p-n junction and act as a active medium/layer. One side of the p-n junction is highly reflective and the other one is partially reflective which forms resonant cavity for the photons.

33. Semiconductor diode LASER The external leads provide the anode and cathode connection. Working: When a low forward-bias is applied to semiconducting diode is, electrons and holes are injected into the narrow depletion region where they recombined and release energy spontaneously in the form of photons. These emitted photons drift randomly within the depletion region and strike the reflected surface as well as they also stimulate other electrons which again recombine with the holes and produce additional photons due to avalanche effect.

34. Semiconductor diode LASER This back and forth movement of photons continues until a very intense beam of light is formed which will then emerge out from the partially reflecting surface of p-n junction with a wavelength of 900 nm. The p-n junction lasers are also called injection lasers since the laser action is generated by minority charge carriers injected across the depletion region of the junction.