ME 220 Measurements & Sensors ME 220 Measurements & Sensors Mechanical Measurements Applications...
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Transcript of ME 220 Measurements & Sensors ME 220 Measurements & Sensors Mechanical Measurements Applications...
ME 220 Measurements & SensorsME 220 Measurements & Sensors
Mechanical Measurements Applications
Chapters # 8, 9,10, 11 ( Figliola)
and 18 (Beckwith)
Thermometer
Thermometry based on thermal expansion
Liquid-in-glass thermometers (accuracy from ±0.2 to ±2°C)
CH. # 8 Temperature MeasurementsCH. # 8 Temperature Measurements
Bimetallic ThermometersIf you take two metals with different thermal expansion coefficients and bond them together, they will bend in one direction
Resistance Temperature Detectors RTD
R1
R2
R3 r1RRTD r3
RRTD R3 r1 r3
RR0 1 A T T0
Thermistors
RR0e 1/T 1/T0
Usually made of a semiconductor and have Much larger dR/dT (more sensitive) than RTD and has Fast Response
Thermocouple (Thermoelectric) (Thermoelectric)
Thermoelectric EffectsSeebeck effect: Generates voltages across two dissimilar materials when
a temperature difference is present.
Peltier effect: Moves heat through dissimilar materials when current is applied.
ThermocouplesThermocouples measure the difference in temperature between two points. One of those points at a known temperature.
Thermocouples in Series and in Parallel
THERMOCOUPLE TIME CONSTANT • The conservation of energy:
m cp dT / dt = h A (To – T) m : mass of thermocouple junction, Cp: specific heat of thermocouple junction h : heat transfer coefficient , A : surface area of thermocouple
T : junction temperature , To : environs temperature
θ =T – To / Ti - ToTi = initial measurement junction temperature, then the solution is
θ = e (-t / τ )
The time constant for this process is
τ = m cp /h A
Error Sources in Temperature Measurements
Conduction: Your probe can conduct heat to/from the environment to/from your desired measurement location
Radiative Temperature Radiative Temperature Measurements (Pyrometry)Measurements (Pyrometry)
Eb T 4
Temperatures greater than 500ºC
= 5.67•10-8 W/m2K4
Optical Pyrometer
CH. # 9 Pressure and Velocity MeasurementsCH. # 9 Pressure and Velocity MeasurementsDynamic Pressure = Total Pressure - Static Pressure
Use of Manometers
Pitot Tube Principles
Deadweight Testers
Elastic Pressure Transducers
Bourdon Tube Gauge
INCLINED MANOMETER
Flow velocity measurementsThermal Anemometry
Laser Doppler Anemometer (LDA)
Particle Image Velocimetry (PIV)
CH. # 10 Flow Measurements Turbine
Obstruction Flow Meter
Obstruction Meters
P1 P2
V2
2 V12
2gc
Qideal V2A2 A2
1 A2 /A1 2 1/ 2
2gc P1 P2
Rotameter or Area meter
Rotameter
CH. # 11 Strain Measurements
a dL
LL2 L1
L1
LL1
Strain Gauges
RLA
LCD2
The resistance across that conductor is
Where conductor of resistivity
If you strain this conductor axially, its length will increase while its cross sectional area will decrease. Taking the total differential of R,
dRRd R
LdL R
CD2 d CD2
1
CD2Ld dL 2L dD
D
dR
RdL
L 2dD
Dd
dR /R
dL /L1 2
dD /D
dL /L d /dL /L
dR
RdL
L 2dD
Dd
a dLL
L dD
D
La
F dR /R
dL /LdR /R
a1 2v
d /dL /L
For most strain gauges, = 0.3. If the resistivity is not a function of strain, then F only depends on poisson’s ratio, and F ~ 1.6.
Gage factor
Strain Gauge
lateral strain
axial strain
La
F dR /R
dL /LdR /R
a1 2v
d /dL /L
1
F
RR
F and R are supplied by the manufacturer, and we measure ∆R.
Strain Gage
Strain Gage [Gage Factor = (∆R/R)/(∆L/L)
& Young’s Modulus = (P/A) / (∆L/L) ]
Strain Gage Bridge CircuitStrain Gage Bridge Circuit
eoei
R1 /R
4 2 R1 /R
1
F
RR
eo eiF
4 2F
Wheatstone Bridge
eo eiR2
R1 R2
R4
R3 R4
make R2 = R4 = R
eo eiR
R1 R
R
R3 R
eo eiR
R1
1R
R3
1
eoei
R1
R1
1
R3
Multiple Gauge BridgeMost strain gauge measurement systems allow us to make 1, 2, 3 or all 4 legs of the bridge strain gauges.
Eo E iR1
R1 R2
R3
R3 R4
Say that unstrained, all of these have the same value. If they are then strained, the resultant change is Eo is
dEo EoRii1
4
dRi
Eo
Multiple Gauges•All gauges have the same nominal resistance (generally true)
•All gauges have matched gauge factors
EoE i
F
41 2 4 3
Eo
Force Measurements
Torque & Power MeasurementsTorque T = FRPower P = T
ACOUSTICS
• Acoustics is the study of Sound.
• Sound is caused by variations in Pressure transmitted through air or other materials.
• The pressure, and the resulting sound, can vary in both Amplitude and Frequency.
• Humans can detect sound over a wide range of frequencies and amplitudes.
What is Sound?• Sound is a propagating disturbance in a fluid or in
a solid. The disturbance travels as a longitudinal wave.
• Airborne sound Sound in air is called airborne sound generated by
a vibrating surface or a turbulent fluid stream. • Structure borne sound Sound in solids is generally called structure borne
sound.• Sound: is measured by a microphone and has
Amplitude and Frequency
SOUND WAVESrapid pressure variation
cycles of compressions and rarefactions
SOUND WAVES• Sound energy is transmitted through air as a pressure wave.• Frequency : The frequency of a sound (cycles / sec.) hertz (Hz). f = 1/T (Hz) The range for human hearing is from 20 to 20.000 Hz. • Wavelength :The distance between analogous points of two
successive waves. λ = c / f where c = speed of sound (m/s)
f = frequency (Hz)
Frequency (Hz) 63 125 250 500 1K 2K 4K 8K Wavelength (m) 5,46 2,75 1,38 0,69 0,34 0,17 0,085 0,043
Frequency Independent of sound-pressure level.
The speed of sound in air = 344 m/s fn (Temp)
The speed of sound in water = 1000 m/s
The speed of sound in solid = 3000 m/s
Speed of Sound and Wavelength
Sound Waves
Pure Tone and Noise
Human Ear
External, Middle and Inner Ear
Middle &Inner ear
Hearing
Noise to Outside
• From Machines such as Airplanes, Pumps, Compressors and generators.
• From Air conditioning such as condensers, Chillers, Ventilation Opening, Louvers
• Nose Control by: Relocation, Use of Vibration Damping, Use of Attenuator and Use of Enclosure.
Typical Noise Sources
The Decibel (dB) & Sound Power LevelThe Decibel (dB) & Sound Power Level
dB = ten times the logarithm to base 10 of the ratio of two quantities.
Power Level = 10 log (w1 / w2) dB
where w1 and w2 are the two powers.
SWL = 10 log (sound power)/(ref. power)
Reference power (Watt) = 10-12 W, which is the threshold of hearing ( lowest detectable sound).
Sound Energy Decreases with (distance)2
Sound intensity• Sound intensity, power per unit area.• The intensity passing a spherical surface around source in a
free field is: I = W / A = W / 4 π r2 = p2 / ρ c (W/m2)where W = power (W)
A = area ( m2) r = radius (m) p = root mean square pressure (N/m2) ρ = density (kg/m3) c = velocity of sound (m/s)
• SOUND INTENSITY LEVEL LI = 10 log (I / I0) (dB)Where Io = reference intensity = 10-12 W/m2.
Sound pressure level (SPL)• Sound measuring device respond to sound pressure .
• Sound pressure level in decibels vary with distance from source.
• SPL = 10 log (p2 / p02) = 20 log (p / p0)
where p= rms pressure (N/m2)
& po = 20x10-6 N/m2.
For Free Field : SWL=SPL +20 log r +11 dB
Sound Pressure
The reference value used for calculating sound-pressure level is 2 ×10-5 Pa.
Note the unit of the equation
Sound Level Meter
Adding two sound pressure levels
Diff between 2-Levels Total= Larger +
0 or 1 3
2 or 3 2
4 or 9 1
10 or more 0
Total SPL =
10
SPLloglog10 i1
10
ni
1i10
Addition of two sound pressure levels Addition of two sound pressure levels
Noise criteria (NC)
1) NC relates SPL with frequency to show how SPL varies with frequency
2) The highest curve crossed by the data determines the NC rating. NC-39
A , B, and C - Weighting
A- Weighting (dBA)The ear is less sensitive with decreasing frequency.To simulate ear response use A weighting (dBA).
63 125 250 500 1kHz 2kHz 4kHz Fan Octave Band dB: 85 86 85 80 73 70 60 A-weighted : -25 -16 -9 -3 0.0 +1 +1 60 70 76 77 73 71 67 70 80 75 67 80 76
81 dBA
SOUND INSULATION• Source - Transmission Path- and Receiver
• Air-borne noise and Structure-borne noise
• Reduction of Air-borne sound:
- Relocation of source and/or receiver
- Use floating floors, Use absorbent material
- Use local insulation, and local attenuators
- Use fans with backward curved impellers
- Lined duct work and avoid crosstalk