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ME 220 Measurements & Sensors Mechanical Measurements Applications. Chapters # 8, 9,10, 11 ( Figliola) and 18 (Beckwith). CH. # 8 Temperature Measurements. Thermometer. Thermometry based on thermal expansion Liquid-in-glass thermometers (accuracy from ±0.2 to ±2°C). Bimetallic Thermometers.

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me 220 measurements sensors mechanical measurements applications

ME 220 Measurements & SensorsMechanical Measurements Applications

Chapters # 8, 9,10, 11 ( Figliola)

and 18 (Beckwith)

thermometer

CH. # 8 Temperature Measurements

Thermometer

Thermometry based on thermal expansion

Liquid-in-glass thermometers (accuracy from ±0.2 to ±2°C)

bimetallic thermometers
Bimetallic Thermometers

If you take two metals with different thermal expansion coefficients and bond them together, they will bend in one direction

thermistors
Thermistors

Usually made of a semiconductor and have Much larger dR/dT (more sensitive) than RTD and has Fast Response

thermoelectric effects
Thermoelectric Effects

Seebeck effect: Generates voltages across two dissimilar materials when

a temperature difference is present.

Peltier effect: Moves heat through dissimilar materials when current is applied.

thermocouples
Thermocouples

Thermocouples measure the difference in temperature between two points. One of those points at a known temperature.

thermocouple time constant
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 - To

Ti = 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
Error Sources in Temperature Measurements

Conduction: Your probe can conduct heat to/from the environment to/from your desired measurement location

radiative temperature measurements pyrometry
Radiative Temperature Measurements (Pyrometry)

Temperatures greater than 500ºC

s = 5.67•10-8 W/m2K4

ch 9 pressure and velocity measurements
CH. # 9 Pressure and Velocity Measurements

Dynamic Pressure = Total Pressure - Static Pressure

Use of Manometers

strain gauges
Strain Gauges

The resistance across that conductor is

Where r = 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,

slide30

Gage factor

For most strain gauges, n = 0.3. If the resistivity is not a function of strain, then F only depends on poisson’s ratio, and F ~ 1.6.

strain gauge
Strain Gauge

F and R are supplied by the manufacturer, and we measure ∆R.

wheatstone bridge
Wheatstone Bridge

make R2 = R4 = R

multiple gauge bridge
Multiple Gauge Bridge

Most strain gauge measurement systems allow us to make 1, 2, 3 or all 4 legs of the bridge strain gauges.

Eo

Say that unstrained, all of these have the same value. If they are then strained, the resultant change is Eo is

multiple gauges
Multiple Gauges
  • All gauges have the same nominal resistance (generally true)
  • All gauges have matched gauge factors

Eo

torque power measurements
Torque & Power Measurements

Torque T = FR

Power P = wT

acoustics
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
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 waves
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

slide45

Speed of Sound and Wavelength

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

noise to outside
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.
the decibel db sound power level
The 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 intensity
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 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

slide58

Sound Pressure

The reference value used for calculating sound-pressure level is 2 ×10-5 Pa.

Note the unit of the equation

adding two sound pressure levels
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 =

slide62

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 weighting dba
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
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