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EI2251 / INDUSTRIAL INSTRUMENTATION – I
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C.MAGESH KUMAR/AIHT
YEAR / Semester: II A / IV COURSE CO-ORDINATOR: C.MAGESH KUMAR https://sites.google.com/site/mrcmageshkumar
UNIT III PRESSURE MEASUREMENT
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UNIT III PRESSURE MEASUREMENT •Units of pressure •Manometers-Different types •Elastic type pressure gauges: Bourdon tube, bellows and diaphragms •Electrical methods:
–Elastic elements with LVDT & strain gauges
•Capacitive type pressure gauge •Piezo-resistive pressure sensor •Resonator pressure sensor •Measurement of vacuum:-
–McLeod gauge, Thermal conductivity gauges –Ionization gauges:– Cold cathode and hot cathode type
•Testing and calibration of pressure gauges-Dead weight tester. 5/2/2014 3
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PRESSURE • Pressure is defined as the amount of force
applied to a surface or distributed over the surface.
• For a fluid at rest, pressure can be defined as the force exerted perpendicularly by the fluid on a unit area of any bounding surface.
• Pressure is defined as force per unit area. • It is usually more convenient to use pressure
rather than force to describe the influences upon fluid behavior.
• The standard unit for pressure is the Pascal, (Newton per square meter)
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For an object sitting on a surface, the force pressing on the surface is the weight of the object, but in different orientations it might have a different area in contact with the surface and therefore exert a different pressure
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Units of Pressure • For high pressure:
– Pascal Newton per square meter Kg m/s2
– Atmospheric pressure (1 atm) = 14.696 psi = 101.325 kPa
– PSI (Pounds per square inch)
• 1 PSI = 0.0703 Kg /cm2 1 Kg /cm2 = 14.696 psi
• For low pressure
– 1 BAR = 1.03 Kg /cm2
– 1 millibar = 14.5 x 10^-3 psi
– 1 torr = 1 mm of Hg = 19.34 x 10^-3 psi = 1000 microns
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Types of pressure
• Gauge pressure (Pg): – The indication of pressure as 0 psi (0 point indication) at surface
of liquid even though the pressure is 14.7 psi actually. – Unit: psig (pounds per square inch gauge) – Pg = Pabs - Patm
• Absolute pressure (Pabs) 14.7 psi: – Actual total pressure acting on surface of a liquid. – Unit: psia (pounds per square inch absolute)
• Vacuum or differential pressure: – Pressure below zero
• Atmospheric pressure (Patm): – Pressure due to surrounding air at earth surface
Pabs = Patm + Pg
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RELATIONSHIP BETWEEN PRESSURES
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MANOMETER • For low pressure measurements, differential
pressure
• Range 0-2 Kg / cm2
• Types: – U tube manometer
– Manometer with large seal pots
– Well type manometer
– Enlarged leg type manometer
– Inclined tube manometer
– Micro manometer
– Ring Balance Manometer
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U tube manometer
• Pressure of a fluid which is less dense & immissible with manometric fluid.
• P1 + ghρ1 = P2 + ghρm
P1 - P2 = ghρm - ghρ1
= gh (ρm - ρ1)
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Seal liquid manometer
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Manometer with large seal pots
• To increase the range
P1 - P2 = gh (ρm - ρ1)
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Well type manometer • Level variation in well is negligible
• Level of well is set to ‘0’ of scale
• P1 - P2 = ghρm (1 + α2 / α1)
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Enlarged leg type manometer
• P1 - P2 = ghρm (1 + α2 / α1) • Float is connected to pressure indicator (scale)
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Inclined tube manometer
• To measure very small pressure difference
• β is varied in order of 10’
• P1 - P2 = ghρm cos β (1 + α2 / α1)
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Micro manometer • For measurement of extremely small pressure
• Flexible
• Before applying pressure, the meniscus of inclined tube is adjusted to reference level using maginifier (P1 = P2)
• When Pressure is applied, P1 ≠ P2, so micrometer is adjusted so that P1 = P2.
• The micro meter difference gives differential pressure
• Accuracy = ±10^-2 to ± 10^-3 mm of Hg
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Micro manometer
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Ring Balance Manometer
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• Made of polythene / transparent material • Kerosene/ paraffin oil • P1 – P2 = 2WR Sin ɸ / αd
Manometeric Fluid
• Mercury, water, Kerosene, Paraffin, Ethyl alcohol
• Mercury because:
– Specific gravity of Hg is known at various temperature
– Low vapor pressure
– Non-sticky nature
– Wide temperature range (250 to 350 ‘C)
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Errors in manometer
• Temperature effect: Temperature ↑ Density↓
• Capillary effect: dia of tube is increased (over 10 mm dia), is neglected when same manometeric fluid is used
• Variable meniscus effect:
Meniscus – cresent shaped top surface of liquid
Convex – due to high surface tension of mercury
Concave - due to low surface tension of mercury
Always read the center of meniscus of liquid.
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Errors in manometer Variable meniscus effect
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Elastic type pressure gauges: Bourdon tube, bellows & diaphragms
• Bourdon tube:
– Range: 0-15 psig to 0-100,000 psig (100 Kpa to 690
KPa)
• Materials used:
– Phosphor bronze, steel, beryllium copper
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Bourdon tube
• Principle: When a fluid under pressure enters a tube, it tend to straighten due to pressure.
• Error due to friction in spindle bearing lost motion
• Type: – C type Bourdon tube – Spiral type – Twisted tube – Helical type
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Adjustments of Bourdon Tube
• Multiplication adjustment:
– Due to stress developed in bourdon tube, actual tip travel is non-linear in nature. The small linear tip movement is matched with rotational pointer movement, is called as multiplication, that can be adjusted by length of lever.
– Shorter lever gives larger rotation for the same amount of tip travel.
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Adjustments of Bourdon Tube
• Angularity:
– Linear motion is converted to circular motion with link lever & pinion arrangement.
– One to one correspondence between link lever & pinion attachment may not occur & distortion results is known as angularity
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BELLOWS ELEMENT
• For measurement of absolute pressure (normal as well as low pressure)
• More sensitive to pressure hence used for low pressure measurement.
• Range: 0 – 35 psig
• Very low pressure (40 mm Hg) can be measured by making bellow large.
• Materials (alloy which is ductile (elastic, spongy)): Brass, stainless steel, phosphor bronze, beryllium copper
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BELLOW PRESSURE ELEMENT
• Principle: When an elastic pressure sensor (Bellow) is subjected to pressure, it deflects. The deflection is proportional to applied pressure when calibrated.
– The open end is used to receive applied pressure
– The closed end will expand due to pressure which is recorded or indicated.
– Rack & pinion arrangement.
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BELLOW PRESSURE ELEMENT • Spring opposed bellow element -
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DIFFERENTIAL PRESSURE MEASUREMENT USING BELLOW ELEMENT
• 2 bellows at opposite direction
• P1 = P2, P1 > P2, P2> P1
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DIAPHRAGM • Measurement of pressure in very low range,
range 0 to 4 mm. & high precision instruments
• Flat, flexible discs or corrugated plates
• Materials: Beryillium copper, brass, monel, Titanium
• Range: 0 – 200 psig
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DIAPHRAGM - TYPES
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DIAPHRAGM • Principle: the force (applied pressure) acts
against the thin stretched diaphragm that causes a deflection, which is proportional to applied pressure.
• Diaphragm center deflects the most.
• The deflection of diaphragm is restricted to less than 1/3 of its thickness.
• Gauge types: – Metallic diaphragm gauge
– Slack diaphragm gauge
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METALLIC DIAPHRAGM GAUGE
• Portable, works in any position.
• Used mostly in aircrafts.
• the force (applied pressure) acts against the thin stretched diaphragm that causes a deflection, which is proportional to applied pressure.
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METALLIC DIAPHRAGM GAUGE
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METALLIC DIAPHRAGM GAUGE
• Corrugated diaphragm increases the strength of capsule member and gives linear deflection.
• Total amount of deflection: (d)
d = KN (P1 – P2)Dn tm K- constant of shell material & desigh
N – no. of capsules
D – capsule diameter
t – shell thickness
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SLACK DIAPHRAGM GAUGE
• A diaphragm with large change in force from small change in pressure. Similarly making the diaphragm slack (loose) rather than tight allows it to move a large distance in response to small pressure change.
• Made of rubber.
• Pressure range: 0.01 – 0.40 mm Hg
• Accuracy: 1 – 2 %
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SLACK DIAPHRAGM GAUGE
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ELECTRICAL METHODS FOR PRESSURE MEASUREMENT
• Movement of pressure sensor is converted in electrical signals
• “Mechanical motion is converted into change in electrical resistance and then change in resistance is converted into electric current or voltage”.
• Methods: – Elastic elements with strain gauges
– Diaphragm with strain gauges
– Elastic elements with LVDT 5/2/2014
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• Electrical pressure sensors consist of
– Primary sensing element (Bourdon tube, Bellow, Diaghragm)
– Primary conversion element (resistance)
– Secondary conversion element
• Elastic elements: Bourdon tube, Bellow, Diaghragm
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ELASTIC ELEMENTS WITH STRAIN GAUGES
• Principle: Due to mechanical stress (stretched or compressed), the resistance of strain gauge will change which is measured using wheatstone bridge.
• The measured output voltage is proportional to applied pressure.
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DIAPHRAGM WITH STRAIN GAUGES
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Elastic elements with LVDT
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