Pressure is an important physical quantity to be measured in industrial systems.  

Transducers are one of the popular means of measuring Pressure.   Pressure Transducers work by converting the pressure signal into an analog electric signal usually a 4...20 mA signal. 

Pressure Transducers can be designed using many principles.  The most widely used of these are the capacitive and the Piezo-resistive transducer. 

Capacitive Pressure Transducers
The capacitive transducer consists of a diaphragm which works as one of the plates of a capacitor.  A fixed conductive surface acts as the other plate.  The permittivity of the space in between these plates varies as the diaphragm moves in response to the measured pressure. 

This change in capacitance is measured as the process pressure.  

The capacitive transducer is used to measure very low pressure values.  Very Accurate measurements are possible using the capacitive pressure Transducers. 

Piezo Resistive Pressure Transducers
Piezo Resistive Transducers work on the principle of the piezoresistive effect.  The piezo resistive effect refers to the change in the resistivity of a material in response to force or pressure.  The piezo resistive sensor is used widely in biomedical applications as well as in the automobile industry. 

Piezo Resistive Pressure Transducers
These sensors are low in cost and have high sensitivity.  They can be manufactured for a wide range of pressure measurement.

Piezo Resistive Pressure transducers consist of a diaphragm which is made of silicon.  The diaphragm bends due to the pressure of the system to be measured. 

Mounted on the diaphragm are four piezo-resistors which are usually arranged in the form of a Wheatstone bridge.  When the diaphragm bends due to the pressure, the piezoresistors are subject to either tensile or compressive stress.  This results in a change in resistance values which is measured through the Wheatstone bridge formation and is scaled as a pressure measurement

Inductive Proximity sensors find wide application in the field of industrial instrumentation.  These sensors are extremely popular as they are reliable, robust and have a simple construction.  Inductive Proximity sensors are used to measure speed, detect motion and sense the position of objects.

The inductive proximity sensor consists of an oscillator, a coil and a detector.  The oscillator develops a high frequency signal which is fed to the coil. 

The high frequency signal develops a corresponding high frequency magnetic field at
the tip of the sensor.  When a metallic object comes in front of the sensor, eddy currents are induced in the object.  This acts as a load on the oscillator and the amplitude of the high frequency output drops.  This drop in the voltage is detected by the detector unit which causes the switching on or off of a transistor.  This results in a change of voltage level which is interpreted as a digital signal 0 or 1.

The inductive proximity principle can also be applied to speed sensors.  In speed measurement, the inductive proximity sensor is placed near the rim of a rotating object. The rotating object has a number of teeth along its rim.  When a tooth passes near the inductive proximity sensor, a pulse is produced. 

This sequence of pulses can be converted into an analog signal can be measured as the speed of the device.