US06CPHY06 Instrumentation and Sensors UNIT 2 Part 2 Pressure Measurements

Pressure Measurements What is Pressure? Pressure: Force exerted by a fluid on unit surface area of a container i.e. P = F/A. Units N/m 2 = Pascal (Pa) lb/in 2 = psi atm mm of Hg Torr At sea level pressure is of: 1 atm = 760 mm of Hg = 760 Torr = 1.013 x 10 5 Pa = 14.7 psi Very High > 1000 atm Types Static Dynamic Gage Absolute Vacuum Pressure P Gage P Vac Patm P absolute Absolute /Zero Pressure Pressure Measurement : Range &Techniques Moderate Very Low < 1mm of Hg (Torr) Static OR Dynamic e.g. compressors of engine.

Moderate Pressure Measurements: Manometers : Static Pressure Other devices using elastic elements: For both Static and Dynamic Pressure Manometers: Static Pressure Measurements Scale P 1 P 2 Glass Tube h ρ Mercury/Water/Fluid P 1 - P 2 = h ρ g If P 2 is P atm then, (P 1 - P 2 ) gives gage pressure (applied).

Manometers: Pressure fluid Scale Manometer fluid Characteristics Non-Corrosive No chemical reaction with fluid whose pressure is to be measured Low viscosity for quick adjustment with pressure change Negligible surface tension and capillary effects Why manometers are modified? For ease in use To increase sensitivity Manometer fluid Types of Modified Manometers Cistern or Well Type Inclined Tube Type Movable Tube Type Micrometer Type Micro manometer with motor drive and digital read out.

Cistern or Well type Manometer Scale h P 2 Limb Area(A 2 ) P 1 Since A 1 >> A 2, Change in Change in limb level >> well level So, Change in well level is ignored and reading of only one limb (h) is recorded. Well Area (A 1 ) Well If P 1 and P 2 are absolute pressure, then force equilibrium gives P 1 A P 2 A = A hρg (since P=F/A) i.e. P 1 - P 2 = h ρ g If P 2 is P atm then, (P 1 - P 2 ) gives Gage pressure (applied).

Inclined Tube Manometer P 1 P 1 ` P 2 ` P 2 A 1 A 2 A 1 A 2 Inclined Limb h 1 h 2 l ` θ = 30 0 ` θ = 30 0 When P 1 = P 2 i.e. equilibrium When P 1 = P 2

Inclined Tube Manometer P 1 ` P 2 A 1 A 2 h 1 h 2 l ` θ = 30 0 l θ h 2

h Micrometer Type Manometer Micrometer screw head P 1 P 2 in reading of he output of manometer Increased accuracy i.e. liquid displacement. Contact between micrometer screw and liquid level can be sensed electrically for more accuracy

Movable Tube Type Manometer P 1 P 2 Reference Position h Flexible Portion Flexible Portion

Micrometer with motor-drive Type Manometer P 1 Digital Display X.YZ Pa P 2 Flexible Portion Micrometer Screw Geared Motor

Elastic Transducers: Static/Dynamic Pressure Pressure Elastic Element -Diaphragm -Bellows -Capsules -Bourdon tube -Helical tube Principle of Working: Deformation /Motion Electro Mechanic al Transduce r -Inductive -LVDT -Potentiometric -Capacitive -Piezo electric - Strain gage Output - Voltage -CRO - Recorder

Elastic Elements: Bourdon Tube Bellows Capsules Corrugated diaphragm Circular Diaphragm

Elastic Elements:

Bourdon Gage:

LVDT Type Pressure Transducer Case Primary Secondar y Voltage V 0 Changes P 2 FLUID P 1 P P 1 P V O proportional to P 1 1

Variable capacitance Type Pressure Transducer Fixed Electrode C 1 C 2 P Diaphragm Electrode d 2 d 1 ac supply Outpu t C 4 C 3 Due to pressure diaphragm displaces and Capacitance changes

Piezo-electric Type Pressure Transducer x i A B e to pressure crystal squeezes to produce voltage between A and B. Only dynamic pressures can be measured.

Resistance Strain-gages Pressure Transducer R 2 R 1 R 3 Outer radius inner radius R 4 diaphrag m Strain gage Resistor

Resistance Strain-gages Pressure Transducer R 2 R 1 ` R 3 R 4 Radia l Stres s Tangential Stress - + + - - ++ - R 4 Bridge R 1 - + + R 2 - R 3

Piston Type Strain-gages Pressure Transducer Strain gage Resistor Pisto n Sea l R 1 R 2 R 4 R 3 High Pressur e Longitudina l Strain to Member Bridge Circuit R 1 R 2

Strain-gages Transducer-For Fluid Pressure A R R 1 measures hoop 1 R 1 strain PIPE PIPE B Section AB R 1 Bridge Circuit

Elastic Element Characteristics For elastic elements used in pressure transducers Deflection due to pressure and Dynamic properties are of great importance

Elastic Element Characteristics Deflection due to pressure R Deflected diaphragm y r Pressure (p) Diaphragm of thickness t & radius R y max < t/3

Elastic Element: Dynamic Characteristics Fundamental frequency of vibration for a circular diaphragm of radius R, is

Elastic Element: Dynamic Characteristics Points to consider The properties of pressure transmitting fluid & the connecting tubing are also important. The effective mass of the moving system depends on the mass of the fluid that moves with the deflected elastic diaphragm. The damping action depends on fluid friction.

High Pressure Measurements