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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 Â±0.2 to Â±2Â°C). Bimetallic Thermometers.
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Chapters # 8, 9,10, 11 ( Figliola)
and 18 (Beckwith)
If you take two metals with different thermal expansion coefficients and bond them together, they will bend in one direction
Usually made of a semiconductor and have Much larger dR/dT (more sensitive) than RTD and has Fast Response
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 measure the difference in temperature between two points. One of those points at a known temperature.
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
Conduction: Your probe can conduct heat to/from the environment to/from your desired measurement location
Temperatures greater than 500ºC
s = 5.67•10-8 W/m2K4
Dynamic Pressure = Total Pressure - Static Pressure
Use of Manometers
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,
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.
F and R are supplied by the manufacturer, and we measure ∆R.
make R2 = R4 = R
Most strain gauge measurement systems allow us to make 1, 2, 3 or all 4 legs of the bridge strain gauges.
Say that unstrained, all of these have the same value. If they are then strained, the resultant change is Eo is
Torque T = FR
Power P = wT
Sound in air is called airborne sound generated by a vibrating surface or a turbulent fluid stream.
Sound in solids is generally called structure borne sound.
f = 1/T (Hz) The range for human hearing is from 20 to 20.000 Hz.
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
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
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).
I = W / A = W / 4 π r2 = p2 / ρ c (W/m2)
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)
Io = reference intensity = 10-12 W/m2.
where p= rms pressure (N/m2)
& po = 20x10-6 N/m2.
For Free Field : SWL=SPL +20 log r +11 dB
The reference value used for calculating sound-pressure level is 2 ×10-5 Pa.
Note the unit of the equation
Diff between 2-Levels Total= Larger +
0 or 1 3
2 or 3 2
4 or 9 1
10 or more 0
Total SPL =
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
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
- 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