Types of Pressure Transducers
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.
The Bourdon tube is a hollow tube with an elliptical cross section. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves. The movement is usually coupled to a needle on a dial to make a complete gauge. It can also be connected to a secondary device such as an air nozzle to control air pressure or to a suitable transducer to convert it into an electric signal. This type can be used for measuring pressure difference.
The pressure acts directly on the piston and compresses the spring. The position of the piston is directly related to the pressure. A window in the outer case allows the pressure to be indicated. This type is usually used in hydraulics where the ability to withstand shock, vibration and sudden pressure changes is needed (shock proof gauge). The piston movement may be connected to a secondary device to convert movement into an electrical signal.
Capsules and Bellows
A bellows is made of several capsules. These are hollow flattened structures made from thin metal plate. When pressurised the bellows expand and produce mechanical movement. If the bellows is encapsulated inside an outer container, then the movement is proportional to the difference between the pressure on the inside and outside. Bellows and single capsules are used in many instruments. They are very useful for measuring small pressures.
These are similar in principle to the capsule but the diaphragm is usually very thin and perhaps made of rubber. The diaphragm expands when very small pressures are applied. The movement is transmitted to a pointer on a dial through a fine mechanical linkage.
Electrical Pressure Transducers
There are various ways of converting the mechanical movement of the preceding types into an electric signal. The following are types that directly produce an electric signal.
· Strain Gauge types.
· Piezo electric types.
· Other electric effects.
Strain Gauge types
The principles of electric strain gauges are covered in Different types of Strain gauges. Strain gauges are small elements that are fixed to a surface that is strained. The change in length of the element produces changes in the electrical resistance. This is processed and converted into a voltage. A typical pressure transducer would contain a metal diaphragm which bends under pressure.
Piezo electric types
The element used here is a piece of crystalline material that produces an electric charge on its surface when it is mechanically stressed. The electric charge may be converted into voltage. This principle is used in the pick up
crystal of a record player, in microphones and even to generate a spark in a gas ignitor. When placed inside a pressure transducer, the pressure is converted into an electric signal.
Other electric effects
Other electric effects commonly used in transducers are CAPACITIVE and INDUCTIVE. In these cases, the pressure produces a change in the capacitance or inductance of an electronic component in the transducer. Both these effects are commonly used in an electronic oscillator and one way they may be used is to change the frequency of the oscillation. The frequency may be converted into a voltage representing the pressure.