Week 7 - Pressure Sensors

Pressure is the force per unit area that a fluid exerts on its surroundings. It is the average force of the molecules of the fluid bouncing off of its surroundings. Pressures can be either static (no motion is occurring, like the air in a balloon) or dynamic (affected by the motion of fluid, such as the pressure in a balloon as the air was being let out).

There are three types of pressure measurements: absolute, gauge, and differential. Absolute pressure is measured relative to a vacuum, gauge pressure is measured relative to ambient atmospheric pressure, and differential pressure is measured relative to a reference pressure.

Because of the different kinds of pressures and methods for measuring pressure, there are many different types of pressure sensors. It is typical for pressures to be converted into another form, such as displacement, and then have these values converted into electrical outputs like voltage or current. Three types of pressure sensors that do this are strain gage, variable capacitance, and piezoelectric. Wheatstone bridge sensors are the most common form of pressure sensors. These sensors can be used in both high and low pressure applications, and can perform any of the three types of pressure measurement.

These sensors use strain gauges bonded to the surface of a diaphragm. Changes in pressure cause the diaphragm to deflect, which causes a change in resistance for the strain gage. 

Another type of pressure sensor is the capacitance pressure transducer. These sensors measure the capacitance between a metal diaphragm and a fixed metal plate. As the metal diaphragm deflects under pressure the gap between the two plates narrows, causing the capacitance to change, which can be used to determine the applied pressure.

Piezoelectric pressure sensors use naturally occurring crystals. These crystals generate electrical charge when strained, which can be measured and used to determine pressure. 

Inductive pressure transducers use inductance to convert the flexing of a diaphragm into the linear movement of a ferromagnetic core. This movement changes the induced current generated by an AC powered primary coil on another secondary coil.

Overall, typical pressure sensors produce voltage readings. These voltages can be transformed into corresponding pressures, although signal conditioning may be necessary. 

Pressure sensors like the ones above can be placed in buildings and linked to help with building management. High pressures are typically signs of stress or failures, and these sensors could potentially locate problem areas before they fail. They could be linked with an overseeing computer program which could monitor changes in pressure and report abnormalities. As pressure sensors continue to become cheaper and more effective, they could become standard parts of building systems. 

Comments:

Esther's blog: Good post. It's interesting how many different kinds of pressure sensors there are: I didn't even touch on mechanical or electromechanical pressure sensors in my post. It was also interesting to read about some of the different applications that pressure sensors have - healthcare and touch screens are great examples.

Audrey's blog: Good post, you explained all of the types of temperature sensors really well. It's interesting to see how many different ways there are to measure something, and that almost every method comes back to producing a current or voltage.

Sources:

Keller Pressure UK Ltd. "The Pressure Sensor." How They Work. Sensorland, n.d. Web. 15 Feb. 2014. <http://www.sensorland.com/HowPage004.html>.

"Pressure Transducer." What Is It and How Does It Work. SensorsONE, n.d. Web. 15 Feb. 2014. <http://www.sensorsone.co.uk/pressure-measurement-glossary/pressure-transducer.html>.

National Instruments. "How To Measure Pressure with Pressure Sensors." National Instruments. National Instruments, 15 Nov. 2012. Web. 15 Feb. 2014. <http://www.ni.com/white-paper/3639/en/>.