2.8 Pressure Sensor Research Paper

2.2.8 Pressure Sensor

A pressure sensor also known as pressure transducer is used to measure pressure typically of gases or liquids. It converts pressure into an analog electrical signal such as a voltage output or current output which can easily be measured.

Pressure sensors either convert the pressure into mechanical movement or into an electrical output. Complete gauges not only sense the pressure but indicate them on a dial or scale.

Mechanical movement is produced with the following elements.

• Bourdon Tube.

• Spring and Piston.

• Bellows and capsules.

• Diaphragm.

2.2.8.1 LVDT based Pressure sensor

A pressure sensor can be created using the motion of a high permeability core in a magnetic field created by the coils of a transformer.

• They are sensitive to shocks and vibrations.

• Amplification is a must as the displacement of the free end of the bourdon tube is low.

• It cannot be used for precision measurement.

2.2.8.3 Diaphragm pressure gauge A diaphragm pressure gauge is a device that uses a diaphragm with a known pressure to measure pressure in a fluid as shown in Fig (2.25). In the Figure,

A = Annular electrode

D = Disk electrode

S = Substrate

G = Getter (in vacuum space)

Differential capacitance between annulus and disk depends on pressure difference between Test Chamber and Getter.

It uses the elastic deformation of a diaphragm (i.e. membrane) instead of a liquid level to measure the difference between an unknown pressure and a reference pressure. These are similar in principle to the capsule but the diaphragm is usually very thin and perhaps made of rubber. 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. A typical Diaphragm pressure gauge contains a capsule divided by a diaphragm, as shown in figure. One side of the diaphragm is open to the external targeted pressure,

These sensors as shown in Fig (2.26) have the crystal mounted on a dielectric base so that there is no current leakage. Attached to the crystal is a horizontal shaft to which a diaphragm is connected. Whenever the diaphragm senses pressure, it pushes the shaft down which pressurizes the crystal and voltage is produced.

2.2.8.5 Strain Gauge Pressure Transducers

A strain gauge is a long length of conductor arranged in a zigzag pattern on a membrane as shown in Fig (2.27a).

When it is stretched, its resistance increases. Strain gauges are mounted in the same direction as the strain and often in fourth arm to form a full Wheatstone bridge.

If a strip of conductive metal is stretched, it will become skinnier and longer, both changes resulting in an increase of electrical resistance end-to-end. Conversely, if a strip of conductive metal is placed under compressive force (without buckling), it will broaden and shorten. If these stresses are kept within the elastic limit of the metal strip (so that the strip does not permanently deform), the strip can be used as a measuring element for physical force, the amount of applied force inferred from measuring its