Pressure Sensors

Pressure sensors, as the name indicates, measure pressure.  The pressure that it is being measured is typically that of a gas or liquid although pressure applied by a human or animal is also available.  According to Omega.com, pressure transducers “converts pressure into an analog electrical signal”¹.  A strain gage is the most familiar, and common, engineering pressure sensors.  For a strain gage, an electrical signal is generated when “the physical deformation of strain gages which are bonded into the diaphragm of the pressure transducer” ¹.  The strain that is formed from deformation of the diaphragm produces “an electrical resistance change proportional to the pressure” ¹.  Other types of pressure sensors include sensors that measure fluid/gas flow, speed, water level, and altitude².  Some common examples of these are piezometers, manometers, and tire pressure gages.

There are two different categories of pressure gages: force collector types and other types².  Force collector types (e.g. pizoresistive strain gage, capacitive, electromagnetic, piezoelectric, optical, potentiometric) use an item of known area – typically a piston or diaphragm – to measure strain/deflection of the applied force over the applied area².  The other types of pressure gages use different properties (e.g. density) to determine the pressure of the medium.  Some examples of these pressure sensors include resonant, thermal, and ionization².  Wang Chunyi's post describes how resonant and thermal pressure sensors work.  According to his post, resonant pressure sensors have a wire that vibrates at its natural frequency.  As the pressure changes, the resonant frequency of the wire changes in response.  The magnets around the wire create electricity through the wire's vibration which is then transmitted to the readout.  Thermal pressure sensors determine pressure through measuring the heat of a wire.  Under high pressure and constant volume, temperature within the gas, and subsequently the wire, will increase.  This increase in temperature must be calibrated based on the type of gas that surrounds the wire.

There are three types of electrical outputs available for pressure sensors: millivolt, amplified voltage, and 4-20 mA.  Millivolt transducers are typically the most economical sensor and their output is directly proportional to the input power or excitation¹.  However, their output is typically low and so they are not recommended for noisy environments, and distance between the sensor and the readout equipment should be short.  Amplified voltage sensors include “integral signal conditioning which provides a much higher output than the millivolt sensor” ¹.  Because of the higher output level, they are able to be used in noisier environments, like industrial.  The 4-20 mA sensors are the “least affected by electrical noise and resistance in the signal wires” ¹.  This benefit allows the signal to be transmitted long distances (1000+ ft.)¹.

¹ “Pressure Transducers.” Omega.com.

http://www.omega.com/prodinfo/ pressuretransducers.html.

² “Pressure Sensor.” Wikipedia.com. http://en.wikipedia.org/wiki/Pressure_sensor