Pressure Sensors

According to the National Instruments website, “because of the great variety of conditions, ranges, and materials for which pressure must be measured, there are many different types of pressure sensor designs” [1].

My fellow classmate, G. Carpenter, first addresses in her post the most common type of pressure sensor (strain gage, see Figure 1 for cross section) and what it measures and how it is translated to indicate pressure.  She then discusses different categories of pressure gauges (force collector and other) and their electrical outputs (millivolt, amplified voltage, and 4-20 mA).

Figure 1: Strain Gauge Pressure Sensor Cross Section [1]

I will now discuss the other two most universal types of force collector pressure transducers (according to National Instruments): variable capacitance and piezoelectric.

A variable capacitance pressure sensor measures “the change in capacitance between a metal diaphragm and a fixed metal plate” and the capacitance changes when the distance between the two plates changes and the degree of this change is converted into an electrical signal [1] (see Figure 2).  (Capacitance is described as the “the ability of a body to store electrical charge” [2].)  These types of sensors are also described as, “very stable and linear, but sensitive to high temperatures and more complicated to setup then most pressure sensors” [1].

Figure 2: Capacitance Pressure Sensor Diagram [1]

A piezoelectric pressure transducer utilizes the electrical properties of naturally occurring crystals such as quartz [1], and uses these stacks of crystal to convert motion into an electrical output as they become strained [3] (see Figure 3).  They require no external excitation and are "rugged" [1], however, are not effective with dc or steady-state conditions [3].  These sensors are also highly susceptible to shock and vibration [1], and also require special signal amplification as their output signal levels are low [3].

Figure 3: Piezoelectric Pressure Sensor Diagram [1]

Sources:

[1] http://www.ni.com/white-paper/3639/en

[2] http://en.wikipedia.org/wiki/Capacitance

[3] http://www.digikey.com/us/en/techzone/sensors/resources/articles/what-you-need-to-know-about-pressure-sensors.html