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EI2251 / INDUSTRIAL INSTRUMENTATION – I

5/2/2014 1 EI2251/INDUSTRIAL INSTRUMENTATION – I

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

5/2/2014 EI2251/INDUSTRIAL INSTRUMENTATION – I

C.MAGESH KUMAR/AIHT 2

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

EI2251/INDUSTRIAL INSTRUMENTATION – I C.MAGESH KUMAR/AIHT

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)



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



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



RELATIONSHIP BETWEEN PRESSURES



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



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)



Seal liquid manometer



Manometer with large seal pots

• To increase the range

P1 - P2 = gh (ρm - ρ1)



Well type manometer • Level variation in well is negligible

• Level of well is set to ‘0’ of scale

• P1 - P2 = ghρm (1 + α2 / α1)



Enlarged leg type manometer

• P1 - P2 = ghρm (1 + α2 / α1) • Float is connected to pressure indicator (scale)



Inclined tube manometer

• To measure very small pressure difference

• β is varied in order of 10’

• P1 - P2 = ghρm cos β (1 + α2 / α1)



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



Micro manometer



Ring Balance Manometer



• 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)



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.



Errors in manometer Variable meniscus effect



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



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



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.



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



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



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.



BELLOW PRESSURE ELEMENT • Spring opposed bellow element -



DIFFERENTIAL PRESSURE MEASUREMENT USING BELLOW ELEMENT

• 2 bellows at opposite direction

• P1 = P2, P1 > P2, P2> P1



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



DIAPHRAGM - TYPES



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



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.




• 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



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 %



SLACK DIAPHRAGM GAUGE



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



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.



DIAPHRAGM WITH STRAIN GAUGES



Elastic elements with LVDT



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