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Elastic Pressure Transducers

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Elastic Deformation1:

This pressure sensing system is based on the idea that bending of flexible material is directly relative to the pressure being considered. There are mainly three sensor types that are used in this system of sensing pressure: Bourdon-tubes, diaphragms and bellows.

Flexible Sensors:

Most fluid pressure sensors are of the flexible type, where the fluid is enclosed in a small compartment with at least one flexible wall. The pressure reading is thereby calculated by sensing the movement of this flexible wall, resulting in either a direct readout through suitable connections, or a transducer electrical signal. Flexible pressure sensors are sensitive; they are commonly delicate and liable to throb, however. In addition, they tend to be much more costly than manometers, and are therefore preferentially used for transmitting detected data and sensing pressure differences. A broad diversity of flexible elements could possibly be used for flexible pressure sensors; the majority of devices use some form of a pressure transducers as:

  Bellows

  Bourdon tube

  Diaphragm

1.1 Bellows2:

Bellows elements are cylindrical in form and contain many folds. They deform in the axial direction (compression or expansion) with changes in pressure. The pressure that needs to be detected is applied to one side of the bellows (either inside or outside) while atmospheric pressure is on the contrary side. Absolute pressure can be detected by evacuating either the exterior or interior space of the bellows and then sensing the pressure at the contrary side. Bellows can only be associated to an on/off switch or potentiometer and are used at low pressures, <0.2 MPa with a sensitivity of 0.0012 MPa. The need for a pressure sensing element that was extremely sensitive to low pressures and provided power for activating recording and indicating mechanisms resulted in the development of the metallic bellows pressure sensing element. The metallic bellows is most accurate when sensing pressures from 0.5 to 75 psig. However, when used in conjunction with a heavy range spring, some bellows can be used to measure pressures of over 1000 psig. Figure 1 shows a basic metallic bellows pressure sensing element.

 

The bellows is a one-piece, collapsible, seamless metallic unit that has deep folds formed from very thin-walled tubing. The diameter of the bellows ranges from 0.5 to 12 in. and may have as many as 24 folds. System pressure is applied to the internal volume of the bellows. As the inlet pressure to the instrument varies, the bellows will expand or contract. The moving end of the bellows is associated to a mechanical linkage assembly. As the bellows and linkage assembly moves, either an electrical signal is generated or a direct pressure indication is provided. The flexibility of a metallic bellows is similar in character to that of a helical, coiled compression spring. Up to the flexible limit of the bellows, the relation between increments of load and movement is linear. However, this relationship exists only when the bellows is under compression. It is necessary to construct the bellows such that all of the travel occurs on the compression side of the point of equilibrium. Therefore, in practice, the bellows must always be opposed by a spring, and the movement characteristics will be the resulting force of the spring and bellows.

 

1.2 Bourdon Tube Gauges2:

The principle behind all Bourdon tubes is that an increase in pressure on the inside of the tube in comparison to the outside pressure causes the oval or flat formed cross-section of the tube to try to achieve a circular form. This phenomenon causes the tube to either straighten itself out in the c-type or spiral cases or to unwind itself for the twisted and helical varieties. This change can then be detected with an analog or digital meter associated to the tube. Tube materials can be changed accordingly to suit the required process conditions. Bourdon tubes can operate under a pressure range from 0.1-700 MPa. They are also portable and require little maintenance; however, they can only be used for static measurements and have low accuracy.

Types of Bourdon tubes include C-type, spiral (a more coiled C-type tube), helical and straight tube Bourdon tubes. C-type gauges can be used in pressures approaching 700MPa; they do have a minimum recommended pressure range, though -- 30 kPa (i.e., it is not sensitive enough for pressure differences less than 30 kPa).

The bourdon tube pressure instrument is one of the oldest pressure sensing instruments in use today. The bourdon tube (refer to Figure 3) consists of a thin-walled tube that is flattened diametrically on contrary sides to produce a cross-sectional area elliptical in form, having two long flat sides and two short round sides. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. Pressure applied to the inside of the tube causes distention of the flat sections and tends to restore its original round cross-section. This change in cross-section causes the tube to straighten slightly. Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center. Within limits, the movement of the tip of the tube can then be used to position a pointer or to develop an equivalent electrical signal (which is discussed later in the text) to indicate the value of the applied internal pressure.

 

1.3 Diaphragms1:

Diaphragm elements are made of circular metal discs or flexible elements such as rubber, plastic or leather. The material from which the diaphragm is made depends on whether it takes advantage of the flexible nature of the material, or is opposed by another element (such as a spring). Diaphragms made of metal discs utilize flexible characteristics, while those made of flexible elements are opposed by another flexible element. These diaphragm sensors are very sensitive to rapid pressure changes. The metal type can measure a maximum pressure of approximately 7 MPa; while the flexible type is used for sensing extremely low pressures (.1 kPa - 2.2 MPa) when associated to capacitative transducers or differential pressure sensors. Examples of diaphragms include flat, corrugated and capsule diaphragms. As previously noted, diaphragms are very sensitive (0.01 MPa) . They can measure fractional pressure differences over a very minute range (say, inches of water) (flexible type) or large pressure differences (approaching a maximum range of 207 kPa) (metal type).

Diaphragm elements are very versatile -- they are commonly used in very corrosive environments or with extreme over-pressure situations.

1.     Pressure Sensors Summary2

 

In a bellows-type sensor:

  System pressure is applied to the internal volume of a bellows and mechanical linkage assembly.

  As pressure changes, the bellows and linkage assembly move to cause an electrical signal to be produced or to cause a gauge pointer to move.

 

In a bourdon tube-type sensor:

  System pressure is applied to the inside of a slightly flattened arc formed tube. As pressure increases, the tube tends to restore to its original round cross-section. This change in cross-section causes the tube to straighten.

  Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center. The tip movement can then be used to position a pointer or to develop an electrical signal.

In Diaphragm pressure sensor:

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

   They may be made up of Metallic diaphragms gauge (brass or bronze) Slack diaphragms gauge (Rubber).

 

2.     Functional Uses2

 

Pressure sensors perform the following basic functions:

 

  Indication

  Alarm

  Control

If a pressure sensor becomes inoperative:

 

   A spare sensor element may be used (if installed).

   A local mechanical pressure gauge can be used (if available).

   A precision pressure gauge may be installed in the system.

Environmental concerns:

 

   Atmospheric pressure

   Ambient temperature

   Humidity

 

3.     Merits & Demerits3

Merits of Flexible Pressure transducer

  Flexible Pressure transducer cost is low.

  Flexible Pressure transducer has easy structure

  Flexible Pressure transducers have been time tested in application.

  Flexible Pressure transducer allows high accuracy, spatially in relation to cost.

 Demerits of Flexible Pressure transducer

  Flexible Pressure transducer has a low spring pitch.

  Flexible Pressure transducer is inclined to shock and throb.

  Flexible Pressure transducer is inclined to hysteresis.

 

 

4.     References

 

1.     http://controls.engin.umich.edu/wiki/index.php/PressureSensors  retrieved at 11/10/2010

2.      Kirk, Franklin W. and Rimboi, Nicholas R., Instrumentation, Third Edition, American       Technical Publishers, ISBN 0-8269-3422-6.

-          Wightman, E.J., Instrumentation in Process Control, CRC Press, Cleveland, Ohio.

-          Rhodes, T.J. and Carroll, G.C., Industrial Instruments for Measurement and Control, Second Edition, McGraw-Hill Book Company.

-          Process Measurement Fundamentals, Volume I, General Physics Corporation, ISBN 0-87683-001-7, 1981

3.      http://vpmt.com/transducer.htm retrieved at 11/10/2010

 

 

 

 

 

 

 

 
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