PEC – UET Lahore Dr. Shahid Naveed 7. September 2014 / Dr. –Ing Naveed Ramzan 1 Industrial Instrumentation Dr. –Ing. Naveed Ramzan

PEC – UET Lahore Dr. Shahid Naveed 11. April 2023 / Dr. –Ing Naveed Ramzan 1

Industrial Instrumentation

Dr. –Ing. Naveed Ramzan

Pressure Sensors“In any given plant, the number of pressure gauges used is probably

larger than all other instruments put together”

1. Pressure = Force / Area

2. Pressure can be used inferentially to measure other variables such as Flow and Level

3. Pressure plays a major role in determining the Boiling Point of Liquids

4. Fluids exerts pressure on the containing vessel equally and in all directions

Pressure Measurement

Pressure Measurement

1. Mechanical Methods

2. Electrical Methods

Pressure Measurement Methods

1. Elastic pressure transducers

2. Manometer method

3. Vacuum Pressure Measurement

4. Electric pressure transducers

5. Pressure measurement by balancing forces produced on a known area by a measured force

Pressure Measurement Methods

1. Bourdon tube pressure gauge

2. Diaphragm pressure transducers

3. 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.

Elastic Pressure Transducers

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Bourdon tubes are generally are of three types;

1. C-type2. Helical type3. Spiral type

Bourdon Tube Pressure Gauge

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Bourdon Tube Pressure Gauge

Diaphragm are popular because they required less space and the motion they produce is sufficient for operating electronic transducers

Diaphragm and Bellows Pressure Gauge

They are used to measure gauge pressures over very low ranges.

Two types of diaphragm pressure gauges are:

1. Metallic diaphragms gauge (brass or bronze)2. Slack diaphragms gauge (Rubber)

Diaphragm Pressure Gauge

More sensitive than bourdon type gauge.

Used to measure low pressures

Brass, phosphor bronze, stainless steel, beryllium

copper etc.

Bellows

Mechanical Pressure Transducers to Pneumatic Pressure Transmitters to Electric Pressure Transmitters

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.

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1. Pressure sensing element such as a bellow , a diaphragm or a bourdon tube

2. Primary conversion element e.g. resistance or voltage

3. Secondary conversion element



Strain gauge pressure transducers

Capacitive pressure transducers

Potentiometer pressure transducers

Resonant Wire pressure transducers

Piezeoelectric pressure transducers

Types of Electric Pressure Transducers


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


Strain Gauge Pressure Transducer

Construction and Working


-- 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




Capacitive Pressure Transducer

C=ε0 εr A/dWhere, C = the capacitance of a capacitor in faradA = area of each plate in m2

d = distance between two plates in mεr= 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.

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.

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



-- 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




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.


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


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.


Resonant Wire Pressure Transducer

-- Used for low differential pressure applications

-- Generates inherently digital signal

-- Sensitive to shock and variation

-- Range : From Absolute pressure 10 mm Hg Up to Differential pressure 750 in Water or Gauge pressure 6000 psig

-- Accuracy 0.1 % of Calibrated Spam


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


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 psi

-- Rugged construction, small size and high speed

Where and How have EPTs failed? EPTs require a constant supply of electricity for

them to function. They do not come with built-in power supply.

High performance comes at a cost. Installation of auxiliary display modules and electrical circuitry increases capital cost.

Physical properties, like temperature, which can affect electrical constants may affect the consistency of EPTs.

For this reason, temperature compensation is always required with EPTs.

Some electrical phenomena, like piezolectric energy, have limited applicability. This limits their use in industry.

Electricity exposes personnel to potential hazards.

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Home Work

INDUCTIVE/RELUCTIVE PRESSURE 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:


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.


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



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%.


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





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 requirefrequent 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





-- A basic manometer can consist of a reservoir filled with aliquid 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 transparenttube mounted at an angle. A small change in vacuum pressure willcause a relatively large movement of the liquid.

--Manometers are simple, low cost, and can detect vacuumsdown to 1 millitorr.



A capacitance sensor operates by measuring the change in electrical capacitance that results from the movement of a sensing diaphragmrelative to some fixed capacitance electrodes

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.



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.



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.



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



Ionization Gauges



Ionization Gauges



Summary of Pressure Sensor Characteristics


Discussion & Questions?

Dr. Shahid Naveed 11. April 2023 / Dr. –Ing Naveed Ramzan 48

Discussion Examples


Example 1

Discussion Examples


Example 2

Discussion Examples


Example 3

Discussion Examples


Example 4

It is a device that measures the differential pressure between two inputs.

• Depending on what class the DP-cell is, it will give you feedback with a current signal.

• Normal in Europe is 4-20 mA, where 4 is lowest and 20 is highest.


Differential Pressure Cell (DP Cell)

Documents

PEC – UET Lahore Dr. Shahid Naveed 7. September 2014 / Dr. –Ing Naveed Ramzan 1 Industrial Instrumentation Dr. –Ing. Naveed Ramzan