1. Industrial Instrumentation
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2. Pressure Sensors
“In any given plant, the number of pressure gauges used is probably larger than all other instruments put together”
Most liquid and all gaseous materials in the process industries are contained within closed vessels.  For the safety of plant personnel and protection of the vessel, pressure in the vessel is controlled.   In addition, pressured is controlled because it influences key process operations like vapor-liquid equilibrium, chemical reaction rate, and fluid flow.
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3. Pressure Measurement
Pressure = Force / Area
Pressure can be used inferentially to measure other variables such as Flow and Level
Pressure plays a major role in determining the Boiling Point of Liquids
Fluids exerts pressure on the containing vessel equally and in all directions
Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all.
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4. Easiest way of thinking
Pressure Measurement
Pressure is commonly quoted as being Absolute or Gauge
Easiest way of thinking
Some Fluid = Some Pressure = Some absolute pressure
No Fluid = No Pressure = Zero absolute pressure
Whereas
Fluid Pressure + Atmospheric Pressure = Some Gauge Pressure
No Fluid + Atmospheric Pressure = Zero Gauge Pressure
Which follows
Gauge Pressure – Atmospheric Pressure = Pressure due to fluid itself = Absolute fluid pressure
Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all.
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5. aaaa Pressure Measurement
Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all.
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6. Mechanical Methods Electrical Methods aaaa
Pressure Measurement Methods
Mechanical Methods
Electrical Methods
Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all.
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7. Elastic pressure transducers Manometer method
Pressure Measurement Methods
Elastic pressure transducers
Manometer method
Pressure measurement by measuring vacuum
Electric pressure transducers
Pressure measurement by balancing forces produced on a known area by a measured force
Instruments used to measure fluid pressure range from very simple to very complex. Some techniques require many moving parts and other none at all.
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8. Bourdon tube pressure gauge Diaphragm pressure transducers Bellows
Elastic Pressure Transducers
Bourdon tube pressure gauge
Diaphragm pressure transducers
Bellows
Uses flexible element as sensor. As pressure changed
,the flexible element moved, and this motion was
used to rotate a pointer in front of dail.
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9. Bourdon tubes are generally are of three types; C-type Helical type
Bourdon Tube Pressure Gauge
Bourdon tubes are generally are of three types;
C-type
Helical type
Spiral type
Perhaps the most common device around today is the pressure gauage which utilizes a bourdon tube as its sensing elements.
---Bourdon :  A bourdon tube is a curved, hollow tube with the process pressure applied to the fluid in the tube.  The pressure in the tube causes the tube to deform or uncoil. The pressure can be determined from the mechanical displacement of the pointer connected to the Bourdon tube.  Typical shapes for the tube are “C” (normally for local display), spiral and helical. 9
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10. aaaa Bourdon Tube Pressure Gauge
Perhaps the most common device around today is the pressure gauage which utilizes a bourdon tube as its sensing elements.
---Bourdon :  A bourdon tube is a curved, hollow tube with the process pressure applied to the fluid in the tube.  The pressure in the tube causes the tube to deform or uncoil. The pressure can be determined from the mechanical displacement of the pointer connected to the Bourdon tube.  Typical shapes for the tube are “C” (normally for local display), spiral and helical. 10
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11. Diaphragm are popular because they required less space
Diaphragm and Bellows Pressure Gauge
Diaphragm are popular because they required less space
and the motion they produce is sufficient for operating
electronic transducers
Diaphragm :  A diaphragm is typically constructed of two flexible disks, and when a pressure is applied to one face of the diaphragm, the position of the disk face changes due to deformation.   The position can be related to pressure.
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12. They are used to measure gauge pressures over very low ranges.
Diaphragm Pressure Gauge
They are used to measure gauge pressures over very low ranges.
Two types of diaphragm pressure gauges are:
Metallic diaphragms gauge
(brass or bronze)
2. Slack diaphragms gauge (Rubber)
Diaphragm :  A diaphragm is typically constructed of two flexible disks, and when a pressure is applied to one face of the diaphragm, the position of the disk face changes due to deformation.   The position can be related to pressure.
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13. aaaa Bellows More sensitive than bourdon type gauge.
Used to measure low pressures
Brass, phosphor bronze,
stainless steel, beryllium copper etc.
Bellows :  A bellows is a closed vessel with sides that can expand and contract, like an accordion.  The position of the bellows without pressure can be determined by the bellows itself or a spring.  The pressure is applied to the face of the bellows, and its deformation and its position depend upon the pressure.
A bellows elastic element is a convoluted unit that expands and contracts axially with changes in pressure. The pressure to be measured can be applied to either the outside or the inside of the bellows; in practice, most bellows measuring.
Simple Bellows Elements
Bellows elastic elements are made of brass, phosphor bronze, stainless steel, beryllium-copper, or other metal suitable for the intended service of the gauge. Motion of the element (bellows) is transmitted by suitable linkage and gears to a dial pointer. Most bellows gauges are spring-loaded—that is, a spring opposes the bellows and thus prevents full expansion of the bellows. Limiting the expansion of the bellows in this way protects the bellows and prolongs its life. Because of the elasticity in both the bellows and the spring in a spring-loaded bellows element, the relationship between the applied pressure and bellows movement is linear.
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14. aaaa Electric Pressure Transducers Mechanical Pressure Transducers to
Pneumatic Pressure Transmitters
Electric Pressure Transmitters
Electrical pressure transducers consists of three
elements
---Pressure transducers used in industry today take on a rather typical appearance.  A round, tubular stainless steel body with a pipe fitting on one end and a cable coming out of the other end.
Stainless is used because of its high strength and resistance to corrosion.
What is inside?
Starting at the pipe threaded end, the opening or port has a stainless steel diaphragm inside that protects the sensor element from the media being measured (i.e. water).
As we continue our journey through the transducer we come to the other side of the diaphragm where one side of the sensor element is. The actual element is a strain gauge; that is, a resistive element whose resistance changes with the amount of strain placed on it. This variable resistor forms one leg of a bridge circuit. The other side of the strain element is the reference port that the measuring port is compared to. All transducers have two sides; sometimes the other side has its own pressure connection and the device is called a differential pressure transducer.
  The two voltage out points from the bridge circuit are fed to an amplifier that changes the very small voltage into a 0-5V signal or most commonly to a 4-20 mA signal. This signal is fed out the cable (sometimes along with a vent tube) which finishes our voyage.    
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15. Why Electrical Pressure Transducers?
Electric Pressure Transducers
Why Electrical Pressure Transducers?
Transmission requirements for remote display as electric signal transmission can be through cable or cordless.
Electric signals give quicker responses and high accuracy in digital measurements.
The linearity property of the electric signal produced to pressure applied favors simplicity.
They can be used for extreme pressure applications, i.e. high vacuum and pressure measurements.
EPTs are immune to hysteresis, shock and mechanical vibrations. 15
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16. aaaa Electric Pressure Transducers
Pressure sensing element such as a bellow , a diaphragm or a bourdon tube
Primary conversion element e.g. resistance or voltage
Secondary conversion element
Electrical pressure transducers consists of three
elements
---Pressure transducers used in industry today take on a rather typical appearance.  A round, tubular stainless steel body with a pipe fitting on one end and a cable coming out of the other end.
Stainless is used because of its high strength and resistance to corrosion.
What is inside?
Starting at the pipe threaded end, the opening or port has a stainless steel diaphragm inside that protects the sensor element from the media being measured (i.e. water).
As we continue our journey through the transducer we come to the other side of the diaphragm where one side of the sensor element is. The actual element is a strain gauge; that is, a resistive element whose resistance changes with the amount of strain placed on it. This variable resistor forms one leg of a bridge circuit. The other side of the strain element is the reference port that the measuring port is compared to. All transducers have two sides; sometimes the other side has its own pressure connection and the device is called a differential pressure transducer.
  The two voltage out points from the bridge circuit are fed to an amplifier that changes the very small voltage into a 0-5V signal or most commonly to a 4-20 mA signal. This signal is fed out the cable (sometimes along with a vent tube) which finishes our voyage.    
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17. Strain gauge pressure transducers Capacitive pressure transducers
Types of Electric Pressure Transducers
Strain gauge pressure transducers
Capacitive pressure transducers
Potentiometer pressure transducers
Resonant Wire pressure transducers
Piezeoelectric pressure transducers
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18. aaaa Strain Gauge Pressure Transducer
A strain gauge is a passive type resistance pressure transducer whose electrical resistance changes when it is stretched or compressed
The wire filament is attached to a structure under strain and the resistance in the strained wire is measured
A pressure transducer contains a diaphragm which is deformed by the pressure which can cause a strain gauge to stretch or compress. This deformation of the strain gauge causes the variation in length and cross sectional area due to which its resistance changes.
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19. aaaa Strain Gauge Pressure Transducer Construction and Working
A pressure transducer contains a diaphragm which is deformed by the pressure which can cause a strain gauge to stretch or compress. This deformation of the strain gauge causes the variation in length and cross sectional area due to which its resistance changes.
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20. aaaa Strain Gauge Pressure Transducer
-- Strain gauge pressure transducers are used for narrow pressure span and for differential pressure measurments
-- Available for pressure ranges as low as 3 inches of water to as high as 200,000 psig
-- Inaccuracy ranges from 0.1 % of span to 0.25 % of full scale
A pressure transducer contains a diaphragm which is deformed by the pressure which can cause a strain gauge to stretch or compress. This deformation of the strain gauge causes the variation in length and cross sectional area due to which its resistance changes.
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21. aaaa Capacitive Pressure Transducer C=ε0 εr A/d Where,
C = the capacitance of a capacitor in farad
A = area of each plate in m2
d = distance between two plates in m
εr= dielectric constant
ε0 = 8.854*10^-12 farad/m2
Thus, capacitance can be varied by changing distance between the plates, area of the plate or value of the dielectric medium between the plates. Any change in these factors cause change in capacitance.
A capacitive pressure transducer including a pair of electrically insulative elastic diaphragms disposed adjacent to each other and bonded together in a spaced apart relationship to form a sealed cavity, a conductive layer applied to the inside surface of each of the diaphragms and a small absolute pressure provided in the cavity. This small absolute pressure cavity essentially reduces the effects of the negative temperature coefficient of the modulus of elasticity of the diaphragms.
In capacitive transducers, pressure is utilized to vary any of the above mentioned factors which will cause change in capacitance and that is a measureable by any suitable electric bridge circuit and is proportional to the pressure.
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22. aaaa Capacitive Pressure Transducer
The sensing diaphragm and capacitor form a differential variable separation capacitor. When the two input pressures are equal the diaphragm is positioned centrally and the capacitance are equal. A difference in the two input pressure causes displacement of the sensing diaphragm and is sensed as a difference between the two capacitances
A capacitive pressure transducer including a pair of electrically insulative elastic diaphragms disposed adjacent to each other and bonded together in a spaced apart relationship to form a sealed cavity, a conductive layer applied to the inside surface of each of the diaphragms and a small absolute pressure provided in the cavity. This small absolute pressure cavity essentially reduces the effects of the negative temperature coefficient of the modulus of elasticity of the diaphragms.
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23. aaaa Capacitive Pressure Transducer
-- Originally developed for use in low vacuum research
-- Wide rangeability from high vacuum in the micron range to 10,000 psig
-- Differential pressure as low as 0.01 inch can be readable
-- Accurate within 0.1 % of reading or 0.01 % of full scale
-- More Corrosion resistant
A capacitive pressure transducer including a pair of electrically insulative elastic diaphragms disposed adjacent to each other and bonded together in a spaced apart relationship to form a sealed cavity, a conductive layer applied to the inside surface of each of the diaphragms and a small absolute pressure provided in the cavity. This small absolute pressure cavity essentially reduces the effects of the negative temperature coefficient of the modulus of elasticity of the diaphragms.
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24. aaaa Potentiometer Pressure Transducer
The device consists of a precision potentiometer whose wiper alarm is mechanically linked to bourdon tube or bellow. The movement of wiper alarm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement using a Wheatstone bridge circuit.
--- 5 – psig
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25. aaaa Potentiometer Pressure Transducer
-- Extremely small and installed in very tight quarters such inside the housing of 4.5 in dial pressure gauge
-- Provide strong output so no need of additional amplifier
-- Range 5 to 10,000 psig
-- Accurate within 0.5 % and 1 % of full scale
--- 5 – psig
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26. aaaa Resonant Wire Pressure Transducer
A wire is griped by a static member at one end and by the sensing diaphragm at the other. An oscillatory circuit causes the wire to oscillate at its resonant frequency. A change in process pressure changes the wire tension which in turn changes the resonant frequency of the wire. A digital counter circuit detects the shift.
--- 5 – psig
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27. aaaa Resonant Wire Pressure Transducer
-- Used for low differential pressure applications
-- Generates inherently digital signal
-- Sensitive to shock and variation
-- Range :
From Absolute pressure mm Hg
Up to Differential pressure in Water
or Gauge pressure psig
-- Accuracy 0.1 % of Calibrated Spam
--- 5 – psig
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28. aaaa Piezoelectric Pressure Transducer
When a pressure , force or acceleration is applied to a quartz crystal , a charge is developed across the crystal that is proportional to the force applied
--- 5 – psig
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29. aaaa Piezoelectric Pressure Transducer
-- Signals generated by crystals decays rapidly so unsuitable for static force or pressure measurements
-- measure rapidly changing pressure resulting from blasts, explosions or pulsation pressures
-- Range : 5,000 to 10,000 psir
-- Rugged construction, small size and high speed
--- 5 – psig
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30. INDUCTIVE/RELUCTIVE TRANSDUCERS:
High Pressure and Vacuum Measurement
-- Synthetic Diamond Manufacturing industry:
Normal reaction pressure = 100,000 psig
Some fiber and Plastic extruders operates at
10,000 psig
-- But in average plant
pressure exceeding 1000 psig are considered high
INDUCTIVE/RELUCTIVE TRANSDUCERS:
--- 5 – psig
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31. aaaa High Pressure and Vacuum Measurement High pressure designs
-- Can detect pressure up to
10,000 psig and operate up to
8000 degree F
-- The pressure of the output air
signal follows the process
pressure in inverse ratio to the
areas of the two diaphragms.
If the diaphragm area ratio is
200:1, a 1,000-psig increase in
process pressure will raise the air
output signal by 5 psig.
--- 5 – psig
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32. aaaa High Pressure and Vacuum Measurement High pressure designs
-- May include as many as
twenty coils
-- can measure pressures well
in excess of 10,000 psig
-- standard element material
is heavy-duty stainless steel
-- measurement error is around
1% of span
-- Suitable for fluctuating
pressure service
--- 5 – psig
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33. aaaa High Pressure and Vacuum Measurement High pressure designs
-- primary standards in calibrating
high-pressure sensors
-- Typical inaccuracy of an
industrial deadweight tester
is 1 part in 1,000 or 0.1%.
--- 5 – psig
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34. aaaa High Pressure and Vacuum Measurement Very High pressure
The bulk modulus cell consists of a hollow cylindrical steel probe closed at the inner end with a projecting stem on the outer end . When exposed to a process pressure, the probe is compressed, the probe tip is moved to the right by the isotropic contraction, and the stem moves further outward. This stem motion is then converted into a pressure reading.
detect pressures up to 200,000 psig with 1% to 2% full span error
--- 5 – psig
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35. High Pressure and Vacuum Measurement
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36. aaaa High Pressure and Vacuum Measurement
In general, for high vacuum services , either cold cathode or Bayard-Alpert hot cathode gauges are suitable. Neither is particularly accurate or stable, and both require
frequent calibration.
-- For vacuums in the millitorr range the capacitance manometer is a good choice. For intermediate vacuum Applications capacitance manometers are the best in terms of performance, but are also the most expensive.
The lowest priced gauge is the thermocouple type, but its error is the greatest. Digital Pirani gauges can represent a good compromise solution, with accuracy between that of capacitance and thermocouple sensors. For low vacuums (higher pressures) between atmospheric and 10-2 torr, Bourdon tubes, bellows, active strain gages, and capacitance sensors are all suitable
--- 5 – psig
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37. High Pressure and Vacuum Measurement
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38. aaaa High Pressure and Vacuum Measurement
-- A basic manometer can consist of a reservoir filled with a
liquid and a vertical tube .
-When detecting vacuums, the top of the column is sealed evacuated.
-- A manometer without a reservoir is simply a U-shaped tube, with
one leg sealed and evacuated and the other connected to the
unknown process pressure
-- The difference in the two column heights indicates the process vacuum.
-- An inclined manometer can consist of a well and transparent
tube mounted at an angle. A small change in vacuum pressure will
cause a relatively large movement of the liquid.
--Manometers are simple, low cost, and can detect vacuums
down to 1 millitorr.
--- 5 – psig
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39. aaaa High Pressure and Vacuum Measurement
A capacitance sensor operates by measuring the change in electrical capacitance that results from the movement of a sensing diaphragm
relative to some fixed capacitance electrodes
--- 5 – psig
Accuracy is typically 0.25 to 0.5% of reading. Thin diaphragms can measure down to 10-5 torr, while thicker diaphragms can measure in the low vacuum to atmospheric range.
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40. McLeod Gauge: (Originally invented in 1878),
High Pressure and Vacuum Measurement
McLeod Gauge: (Originally invented in 1878),
-- The McLeod gauge measures the pressure of gases by compressing a known volume with a fixed pressure. The new volume is then a measure of the initial absolute pressure.
-- The McLeod gauge has been used until recently for calibrating other gauges.
-- It covers the vacuum range between 1 and 10-6 torr.
--- 5 – psig
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41. aaaa High Pressure and Vacuum Measurement Molecular Momentum:
-- This vacuum gauge is operated with a rotor that spins at a constant speed. Gas molecules in the process sample come in contact with the rotor and are propelled into the restrained cylinder. The force of impact drives the cylinder to a distance proportional to the energy transferred, which is a measure of the number of gas molecules in that space.
-- The full scale of the instrument depends on the gas being measured.
-- The detector has to be calibrated for each application.
--- 5 – psig
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42. Pirani: aaaa High Pressure and Vacuum Measurement Thermal Designs:
The thermal conductivity of a gas changes with its pressure in the vacuum range. If an element heated by a constant power source is placed in a gas, the resulting surface temperature of the element will be a function of the surrounding vacuum. Because the sensor is an electrically heated wire, thermal vacuum sensors are often called hot wire gauges. Typically, hot wire gauges can be used to measure down to 10-3 mm Hg.
Pirani:
In this design, a sensor wire is heated electrically and the pressure of the gas is determined by measuring the current needed to keep the wire at a constant temperature
--- 5 – psig
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43. aaaa High Pressure and Vacuum Measurement Ionization Gauges
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44. aaaa High Pressure and Vacuum Measurement Ionization Gauges
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45. High Pressure and Vacuum Measurement
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46. Summary of Pressure Sensor Characteristics
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