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