Lec2: (pressure measurement)
Some Pressure Measuring Methods:
1. Diaphragms
2. Bellows
3. Bourdon tube
4. Manometers
5. Capacitive pressure sensor
6. Strain gauge pressure sensor
7. Fiber optic pressure sensors
8. Dead-weight gauge
1)Diaphragm: Shallow cup like form which is made of thin
metal or flexible material
In figure : (A)link (B)Diaghragm (C)Pointer (D)Pressure inlet
Diaphragms shapes : flat type – corrugated type – pressure convex
capsule – Pressure nested capsule
Diaphragm Types:
1-Metallic Diaphragms : are thin circular pieces of material attached along
their circumference to a rigid plat or another diaphragm , they may be flat or
corrugated in a circular pattern
-when used with another diaphragm the two form a capsule
-materials: steel , phosphor , bronze , nickel silver
2-Non-metallic or slack Diaphragm
-measure vacuum or extremely low pressure
-made from flexible materials such: leather , rubber , plastic , polyethylene
Or other synthetic resilient materials
Advantages:
1-much faster frequency response than U-tubes
2-Accuracy up to ± 0.5% of full scale
3-Good linearity when the deflection is no larger than the order of the
diaphragm thickness
4-Useful for measurement of Absolute & Differential pressure measurement
5-made from good corrosion resistance material
Disadvantages:
1-Difficulty in maintenance
2-Shocks & vibrations affect their performance
3-When used for high pressure measurement , the diaphragm gets damaged
4-These gauges are difficult to be repaired.
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2)Bellows:
Is an elastic element fabricated as a
seamless tube of either metal or
metal alloy , produce transtional
motion of the end of the bellows.
-Can be measured by Capactive ,
inducive (LVDT) or Potentiometric
transducers.
Advantages:
1-Simple & easy to use
2-They are low cost effective and low on maintenance
3-It is able to deliver high force
4-It is adaptable for absolute & differential pressures
5-It is good to low-to moderate range ‫جيدة في النطاق من النخفض الي المعتدل‬
6-Bellow joints don’t require access.
Disadvantages:
1-It is unsuitable for high pressures.
2-The availability of construction metals is limited.
3-Bellows joints can fail easily.
4-No in place maintenance or repair can be performed – they must be
replaced if damaged.
5-Temperture compensation needed.
3)Bourdon tube:
Disadvantages:
1-They respond slowly to changes in pressure
2-They are subjected to hysteresis
3-They are sensitive to shocks & vibrations
4-Amplification is a must as the displacement of the free end of the bourdon
tube is low.
5-It cannot be used for precision measurement.
4)Manometers
Working principle of pressure transducer:
Pressure transducer(sensor), is a device converts an applied pressure into an
electrical signal (by different theories like capacitive, magnetic, piezo, etc)
5)Capacitive pressure sensor:
Pressure measurement device, which converts an applied pressure to a
current signal.
How does a capacitance pressure transducer work?
-A variable capacitance pressure transducer has a capacitive plate fixed to
unpressurized surface
-With a gap of a certain distance between the Diaphragm and the electrode.
Theory:
The capacitance is given by:
Advantages:
1-Inaccuracy 0.01 to 0.2 %
2-Linearity
3-Robust & fast response
4-Great stability and regulated output
5-Highly sensitive
Disadvantages:
1-Temperture sensitivity
2-Stray capacitance problem
3-Limited overpressure capability(Range of 80Pa to 35Mpa)
4-Cost
6)Strain gauge pressure sensor:
-Strain gauge sensing uses a metal wire to measure changes in pressure
-A change in pressure causes a change in resistance as the metal is deformed
-When force is applied the overall length of the wire tends to increase while
the cross-sectional area decreases (the electrical resistance will increase).
The gauge factor of strain gauge is defined as the
unit change in resistance per unit change in length.
Gf = (∆R/R)/( ∆L/L)
R = nominal gauge resistance,
∆R = change in resistance,
L
= length of the specimen in an unstressed condition,
∆L = change in specimen length.
It can be proved mathematically,
Gauge factor:
Gf = 1 + 2v + (∆ρ/ρ)/(∆L/L)