Pressure sensor

A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed. such a signal is electrical.
Pressure sensors are used for control and monitoring in thousands of everyday applications. Pressure sensors can also be used to indirectly measure other variables such as fluid/gas flow, speed, water level, and altitude. Pressure sensors can alternatively be called pressure transducers, pressure transmitters, pressure senders, pressure indicators and piezometers, manometers, among other names.
Pressure sensors can vary drastically in technology, design, performance, application suitability and cost. A conservative estimate would be that there may be over 50 technologies and at least 300 companies making pressure sensors worldwide
Barometer

A barometer is an instrument for measuring atmospheric pressure. Two kinds of barometers are in common use, a mercury barometer and an aneroid barometer. The first makes use of a long narrow glass tube filled with mercury supported in a container of mercury, and the second makes use of an elastic disk whose size changes as a result of air pressure.

Principle

The principle of the mercury barometer was discovered by the Italian physicist Evangelista Torricelli in about 1643. That principle can be illustrated as follows: a long glass tube is sealed at one end and then filled with liquid mercury metal. The filled tube is then turned upside down and inserted into a bowl of mercury, called a cistern. When this happens, a small amount of mercury runs out of the tube into the cistern, leaving a vacuum at the top of the tube. Vacuums, by nature, exert very little or no pressure on their surrounding environment.

As atmospheric pressure pushes down on the surface of the mercury in the cistern, that mercury in turn pushes up with an equal pressure on the mercury in the glass tube. The height of the mercury in the tube, therefore, reflects the total pressure exerted by the surrounding atmosphere. Under normal circumstances, the column of mercury in the glass tube stands at a height of about 30 inches (76 centimeters) when measured at sea level.

Applications:

A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. When used in combination with wind observations, reasonably accurate short term forecasts can be made.Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the nineteenth century. When isobars (lines of equal pressure) are drawn on such a map, we get a contour map showing areas of high and low pressure.
Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activities. Thus, if barometric pressure is falling, it indicates the approach of deteriorating weather or some form of precipitation. Conversely, if barometric pressure is rising, it indicates the coming of fair weather or no precipitation.
Bourdon Guage

Basic Principle of Bourdon tube pressure gauge:

when an elastic transducer ( bourdon tube in this case ) is subjected to a pressure, it defects. This deflection is proportional to the applied pressure when calibrated.

Applications of Bourdon Tube pressure gauge:

They are used to measure medium to very high pressures.

Bourdon Tube Pressure Gauges with brass connections and phosphor bronze tubes are suitable for use on air, water, oils and other fluids compatible with these materials. (When used on steam a syphon should be fitted and filled with water prior to use). Pressure Gauges manufactured in stainless steel are for use with corrosive fluids, in corrosive environments or where food/hygiene requirements demand a higher specification.

Advantages of Bourdon tube pressure gauge:

These Bourdon tube pressure gauges give accurate results.
Bourdon tube cost low.
Bourdon tube are simple in construction.
They can be modified to give electrical outputs.
They are safe even for high pressure measurement.
Accuracy is high especially at high pressures.

Limitations of bourdon tube pressure gauge:

they respond slowly to changs in pressure
they are subjected to hysterisis.
They are sensitive to shocks and vibrations.
Ampilification is a must as the displacement of the free end of the bourdon tube is low.
It cannot be used for precision measurement.
Diaphragm:
Principle

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.

2) The diaphragm has a flexible membrane with two sides. On one side is an enclosed capsule containing air or some other fluid at a predetermined pressure. The other side can be left open to the air or screwed in to whatever system the gauge is meant to measure. The diaphragm also attaches to some sort of meter, which shows how high the pressure is.

Applications:

·         Chemical process industry

·         Petrochemical industry

·         Suitable for corrosive, highly viscous, crystalline or hot pressure media

·         For small flanged process connections.

The following pressure sensors are based on mechanical principles, i.e., deformation based on force.

 Bourdon :  A bourbon 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.

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.