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PRESSURE SENSORS
Prepared by Dr. Mohamed Ahmed Awad

2. Fluid Pressure measurement
Bellows Pressure sensor
Diaphragm Pressure Sensor
Piston Pressure Sensor
Bourdon Tube Pressure sensor
Solid State Pressure sensor
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Pressure Scale
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Pressure in a fluid
To measure the pressure inside a fluid, imagine introducing a sensor in the form of a tiny evacuated cylinder, with a piston at one end supported by a spring.
This is diagram of a sensor for measuring pressure inside a fluid. The fluid exerts a force of magnitude along the axis of the piston, causing it to compress. The ratio of to the area A of the piston defines the pressure.
The compression of the spring indicates the magnitude of the force exerted along the axis of the piston by the fluid. Then, if the cross-sectional area of the piston is A, and the force acting along the piston’s axis has magnitude , the pressure P is defined to be
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5. Bellows Pressure sensor
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6. Bellows Pressure sensor
The most conventional sensitive element used in our pressure and temperature switches is a metallic bellows (bronze or stainless steel) made up of a corrugated tube fitted with two airtight rigid ends. Flexible by nature, it compresses or elongates axially under the effect of pressure and provides excellent transmission.
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Bellows Gauges
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Effective at low pressure up to 100 PSI
Made from metallic material for high pressure.
Reliable, long life
It may act directly on a valve eliminating the need for a switch
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9. Diaphragm Pressure Gage
The Diaphragm Pressure Gage uses the elastic deformation of a diaphragm (i.e. membrane) instead of a liquid level to measure the difference between an unknown pressure and a reference pressure
A typical Diaphragm pressure gage contains a capsule divided by a diaphragm, as shown in the schematic. One side of the diaphragm is open to the external targeted pressure, and the other side is connected to a known pressure, The pressure difference, mechanically deflects the diaphragm
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Diaphragm : For low pressure, bellows which are too rigid are replaced by an elastomer diaphragm which is hold in place by two metal plates. "Diaphragm" technology is also used in cases of excessively high pressure and pulsating phenomena
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11. Diaphragm Pressure Gage
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12. Diaphragm Pressure Gage
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13. Capsule To increases sensitivity
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14. Diaphragm Pressure Gage
Pros
Much faster frequency response than U tube
High Accuracy
Good linearity
Stand for higher pressure 150 PSI
Cons
Less sensitive than bellows.
More expensive than other pressure sensors
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15. Piston Pressure Sensor
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This is diagram of a sensor for measuring pressure inside a fluid.
The fluid exerts a force along the axis of the piston, causing it to compress.
The ratio of the force to the area of the piston defines the pressure.
The compression of the calibrated spring indicates the magnitude of the force exerted along the axis of the piston by the fluid.
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Pros and Cons
Pros
Can measure pressure up to 20,000 PSI
Provide reliable operation over Long life
Cons
Has lower accuracy
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Bourdon Tube
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19. Bourdon Tube Pressure sensor
It is widely used when inexpensive static pressure measurement are needed
It is the most used of all mechanical sensors.
A typical Bourdon tube contains a curved tube that is open to external pressure input on one end and is coupled mechanically to an indicating needle on the other end, as shown
When Pressure increased, the shape of the curved will be straightens.
Most direct reading dial type are Bourdon tube
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20. Bourdon Tube Pressure sensor
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21. Bourdon Tube Pressure sensor
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Bourdon Tube Gauge
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Bourdon Tubes
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24. Bourdon Tube Pressure sensor
Pros
Portable
Convenient
No Leveling required
Can stand for high pressure up to 18,000 PSI
Cons
Limited to static measurement
Accuracy may be insufficient for many applications
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25. Solid State Pressure sensor
Many pressure sensors used in medical applications are made with piezoresistive semiconductor technology and are therefore considered solid-state devices.
It produces a voltage output that is fairly linearly dependent on the applied pressure.
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Piezoresistance
A piezoresistor is basically a device which exhibits a change in resistance when it is strained.
There are two components of the piezoresistive effect in most materials - the geometric component and the resistive component
The geometric component of piezoresistivity basically comes from the fact that a strained element undergoes a change in dimension.
These changes in cross sectional area and length affect the resistance of the device.
More Information about piezo effect
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28. Piezoresistive Cantilever
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29. Cross section in Solid state
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What is a Strain Gage
A Strain gage is a sensor whose resistance varies with applied force; It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured
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Strain gauge
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1.                  Resistance
                 Increased resistance
                    heating wire resistor
                  cooling semiconductor
                stretching (making longer & thinner) wire resistor
                 ‘Strain gauge’: used in pressure transducer
                 To monitor & measure the change in resistance — use of ‘Wheatstone Bridge’ circuit
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Typical Strain gauge
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Strain gauge
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As pressure is applied to the strain gauge, the wires increase in length and decrease in diameter, increasing the resistance to flow of current through the wires of the Wheatstone Bridge
                 This change in the wires electrical resistance causes a voltage change that can be quantified to reflect the amount of pressure that changed the wires length & diameter.
                  The electrical signal can be amplified & measured; when calibrated, it is proportional to the pressure change
                 By combining strain gauge elements of which some stretch while others simultaneously compress, exaggeration of the signal within the Wheatstone bridge can be achieved
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