PRESSURE MEASURING DEVICES

Views:
 
     
 

Presentation Description

There are two types of pressure measurement devices:- A) Pressure transducer and elements- Mechanical B) Pressure transducer and elements- Electrical A) Pressure transducer and elements-Mechanical:- 1) Bourdon tube 2) Helix and spiral tubes 3) Spring and bellows 4) Diaphragm 5) Manometer 6) Single and double inverted bell

Comments

Presentation Transcript

PRESSURE MEASURING DEVICES :

By Abubacker Siddieq abubackersiddieq@gmail.com PRESSURE MEASURING DEVICES

Types of pressure:

Types of pressure

Outline::

Outline: There are two types of pressure measurement devices:- A) Pressure transducer and elements- Mechanical B) Pressure transducer and elements- Electrical A ) Pressure transducer and elements-Mechanical:- 1) Bourdon tube 2) Helix and spiral tubes 3) Spring and bellows 4) Diaphragm 5) Manometer 6) Single and double inverted bell

1) Bourdon tube:-:

1) Bourdon tube :- Principle:- The Bourdon tube works on a simple principle that a bent tube will change its shape. As pressure is applied internally , the tube straightens and returns to its original form when the pressure is released. The tip of the tube moves with the internal pressure change and is easily converted with a pointer onto a scale.

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

Advantages:- Inexpensive Wide operating range Fast response Good sensitivity Direct pressure measurement Disadvantages:- Primarily intended for indication only Hysteresis on cycling Sensitive to temperature variations Limited life when subject to shock and vibration

PowerPoint Presentation:

Application limitation:- These devices should be used in air if calibrated for air, and in liquid if calibrated for liquid . Special care is required for liquid applications in bleeding air from the liquid lines

2) Helix and spiral tubes:-:

2) Helix and spiral tubes :- Helix and spiral tubes are fabricated from tubing into shapes as per their naming. With one end sealed, the pressure exerted on the tube causes the tube to straighten out. The amount of straightening or uncoiling is determined by the pressure applied. The uncoiling part of the tube is mechanically linked to a pointer which indicates the applied pressure on a scale. This has the added advantage over the C-Bourdon tube as there are no movement losses due to links and levers.

PowerPoint Presentation:

Diagram:- SPIRAL TUBE

PowerPoint Presentation:

Diagram:- HELICAL TUBE

PowerPoint Presentation:

Advantages:- Increased accuracy and sensitivity Higher over range protection Disadvantages :- Very expensive Application limitation:- Process pressure changes cause problems with the increase in the coil size

3) Spring and bellows:-:

3) Spring and bellows:- A bellows is an expandable element and is made up of a series of folds which allow expansion. One end of the Bellows is fixed and the other moves in response to the applied pressure. A spring is used to oppose the applied force and a linkage connects the end of the bellows to a pointer for indication. Bellows type sensors are also available which have the sensing pressure on the outside and the atmospheric conditions within. This type of pressure measurement is primarily used for ON/OFF control providing clean contacts for opening and closing electrical circuits.

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

Advantages :- Simple construction Easily maintained Inexpensive Disadvantages :- Sensitive to temperature variations Work hardening of bellows Hysteresis Poor over range protection

4) Diaphragm:-:

4) Diaphragm :- The diaphragm is a flexible disc, which can be either flat or with concentric corrugations and is made from sheet metal with high tolerance dimensions. The diaphragm can be used as a means of isolating the process fluids, or for high pressure applications. It is also useful in providing pressure measurement with electrical transducers. Diaphragms are well developed and proven

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

Advantages:- Provide isolation from process fluid Good for low pressure Inexpensive Wide range Reliable and proven Used to measure gauge, atmospheric and differential pressure

5) Manometer:-:

5) Manometer:- U-type manometer Inclined manometer The reference pressure and the pressure to be measured are applied to the open ends of the tube If there is a difference in pressure, then the heights of the liquid on the two sides of the tube will be different This type of pressure measurement is mainly used for spot checks or for calibration They are used for low range measurements, as higher measurements require mercury. Mercury is toxic and is therefore considered mildly hazardous.

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

Advantages:- Simple operation and construction Inexpensive Disadvantages:- Low pressure range (water) Higher pressure range requires mercury Readings are localized Application limitation:- Manometers are limited to a low range of operation due to size restrictions. They are also difficult to integrate into a continuous control system.

6) Single and double inverted bell:- :

6) Single and double inverted bell:- The Bell instrument measures the pressure difference in a compartment on each side of a bell-shaped chamber. If the pressure to be measured is referenced to the surrounding conditions, then the lower compartment is vented to the atmosphere and gauge pressure is measured. If the lower compartment is evacuated to form a vacuum , then the pressure measured will be in absolute units. However , to measure differential pressure, the higher pressure is connected to the top of the chamber and the lower pressure to the bottom.

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

B ) Pressure transducer and elements- Electrical:- 1) Strain gauge 2) Vibrating wire 3) Piezoelectric 4) Capacitance 5) Linear variable differential transformer 6 ) Optical

1) Strain gauge:-:

1) Strain gauge :- Strain gauge sensing uses a metal wire or semiconductor chip to measure changes in pressure. A change in pressure causes a change in resistance as the metal is deformed . This deformation is not permanent as the pressure (applied force) does not exceed the elastic limit of the metal. If the elastic limit is exceeded than permanent deformation will occur. This is commonly used in a Wheatstone bridge arrangement where the change in pressure is detected as a change in the measured voltage.

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

Advantages:- Wide range , 7.5kPa to 1400 Mpa Inaccuracy of 0.1% Small in size Stable devices with fast response Most have no moving parts Good over-range capability Disadvantages:- Unstable due to bonding material Temperature sensitive Thermo elastic strain causes hysteresis Application limitation:- All strain gauge applications require regulated power supplies for the excitation voltage , although this is commonly internal with the sensing circuits.

2) Vibrating wire:-:

2) Vibrating wire :- This type of sensor consists of an electronic oscillator circuit which causes a wire to vibrate at its natural frequency when under tension. The principle is similar to that of a guitar string. The vibrating wire is located in a diaphragm . As the pressure changes on the diaphragm so does the tension on the wire which affects the frequency that the wire vibrates or resonates at. These frequency changes are a direct consequence of pressure changes and as such are detected and shown as pressure .

PowerPoint Presentation:

Advantages:- Good accuracy and repeatability Stable Low hysteresis high resolution Absolute , gauge or differential measurement Disadvantages:- Temperature sensitive Affected by shock and vibration Non linear Physically large

PowerPoint Presentation:

Application limitation:- Temperature variations require temperature compensation within the sensor, this problem limits the sensitivity of the device. The output generated is non-linear which can cause continuous control problems . This technology is seldom used any more. Being older technology it is typically found with analogue control circuitry

3) Piezoelectric:-:

3) Piezoelectric :- When pressure is applied to crystal s, they are elastically deformed. Piezoelectric pressure sensing involves the measurement of such deformation. When a crystal is deformed , an electric charge is generated for only a few seconds. The electrical signal is proportional to the applied force.

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

Advantages:- Accuracy 0.075% Very high pressure measurement, up to 70MPa small size robust fast response , < 1 nanosecond self-generated signal Disadvantages:- Dynamic sensing only temperature sensitive Application limitation:- Require special cabling and signal conditioning.

4) Capacitance:-:

4) Capacitance :- Capacitive pressure measurement involves sensing the change in capacitance that results from the movement of a diaphragm. The sensor is energized electrically with a high frequency oscillator. As the diaphragm is deflected due to pressure changes, the relative capacitance is measured by a bridge circuit. Two designs are quite common. The first is the two-plate design and is configured to operate in the balanced or unbalanced mode. The other is a single capacitor design

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

Advantages:- Inaccuracy 0.01 to 0.2% Range of 80Pa to 35MPa Linearity Fast response Disadvantages:- Temperature sensitive Stray capacitance problems Vibration Limited overpressure capability Cost

PowerPoint Presentation:

Application limitation:- Many of the disadvantages above have been addressed and their problems reduced in newer designs. Temperature controlled sensors are available for application requiring a high accuracy . With strain gauges being the most popular form of pressure measurement, capacitance sensors are the next most common solution

5) Linear variable differential transformer:-:

5) Linear variable differential transformer :- This type of pressure measurement relies on the movement of a high permeability core within transformer coils. The movement is transferred from the process medium to the core by use of a diaphragm, bellows or bourdon tube. The LVDT operates on the inductance ratio between the coils . The primary coil is located between the two secondary coils and is energized with an alternating current. Equal voltages are induced in the secondary coils if the core is in the center. When the core is moved from the canter position , the result of the voltages in the secondary windings will be different.

PowerPoint Presentation:

Diagram:-

PowerPoint Presentation:

Disadvantages:- Mechanical wear Vibration Application limitation:- The is an older technology, used before strain gauges were developed. Typically found on old weigh frames or may be used for position control applications . Very seldom used anymore

6)Optical:-:

6)Optical :- Optical sensors can be used to measure the movement of a diaphragm due to pressure . An opaque vane is mounted to the diaphragm and moves in front of an infrared light beam. As the light is disturbed, the received light on the measuring diode indicates the position of the diaphragm. A reference diode is used to compensate for the ageing of the light source. Also, by using a reference diode, the temperature effects are nulled as they affect the sensing and reference diodes in the same way.

PowerPoint Presentation:

Advantages:- Temperature corrected Good repeatability Negligible hysteresis Disadvantages:- Expensive

Selection tables: :

Selection tables:

Reference :

Reference Process Control: A Practical Approach Myke King © 2011 John Wiley & Sons Ltd. ISBN: 978-0-470-97587-9 Practical Instrumentation for Automation and Process Control for Engineers and Technicians,

PowerPoint Presentation:

Thank you

authorStream Live Help