CRFS Study

Force, Pressure, Flow & Volume

Force & Pressure

• Mechanical

• Strain Gauge

• Piezo-resistive

• Capacitive

• Inductive

• Optical

Mechanical Gauges

• Manometers

• Force summing devices

Strain Gauge

• Wire or foil which thins when stretched

• Resistance increases when stretched

• Usually used in pairs or groups of 4

Wheatstone Bridge Circuit

• Diaphragm flex stretches some & compresses others

• Increases sensitivity & linearity

• Full or half bridge

Piezo-resistive

• Semiconductor chip

• Resistance changes when flexed

• May include amplifiers

Capacitive

• Capacitance between diaphagm & 1 or 2 fixed plates

• Requires AC

• Limited by characteristics of gap contents

Inductive

• Inductance changes produced by metal diaphragm on 2 fixed coils

• Requires AC

Optical

• Light intensity varies with pressure

Pressure

• Absolute Gauge Differential

Flow

• Fleisch pneumotach• Screen pneumotach• Pitot tube• Venturi / Orifice• Rotating vane• Mass Flow• Ultrasonic• Differentiation of volume

Reynold’s Number

• vs - mean fluid velocity,

• L - characteristic length (equal to diameter 2r if a cross-section is circular),

• μ - (absolute) dynamic fluid viscosity, • ν - kinematic fluid viscosity: ν = μ / ρ, • ρ - fluid density.

Lv sRe

Fleisch pneumotach

• Resistive

• Linear

Screen pneumotach

• Resistive

• Linear

Pitot tube

• Bernoulli equation

P2

v Pmv2

1 2

Venturi / Orifice

• Bernoulli equation

P2

v Pmv2

1 2

Rotating vane

• Rotating air stream

• Low inertia follower

• Digital counter based

Mass Flow

• Thermal

Ultrasonic

• Doppler effect

• Accurate timing

Differentiation of volume

• Direct volume measurement

• Rate of change

Volume

• Watersealed

• Rolling seal

• Bellows

• Gasometer