Thick film resistor bridges convert the physical quantity of pressure into an electrical quantity proportional to the pressure applied. They measure the pressure in liquids or gases.
In this post, we compare three types of pressure sensors. Then illustrate the construction using thick film resistor bridges as an example.
Typical Pressure Sensor Applications
Pressure sensors are used in a wide range of industrial and military applications. They often operate in harsh environmental conditions where high temperature, high moisture levels and contaminants are present.
Some of the most common applications are fluid and gas level monitoring, hydraulics and alarm systems. They are found in a wide range of automotive and medical applications where reliability and stability are critical.
Pressure Sensor Technology Comparison
There are three basic types of pressure sensors. Thick film resistor pressure sensors, thin-film pressure sensors and piezo (semiconductor)-resistive based. Depending on the application, each technology has specific advantages and disadvantages.
Semiconductor based pressure sensors have the highest sensitivity but are fragile. Accelerated life testing has shown they are less stable than thick film pressure sensors.
They are surrounded by a liquid that acts as the pressure transfer medium. They can, therefore, be complex to construct. Their major disadvantage is they require temperature compensation.
Thin film and thick film resistor bridges require no temperature compensation. Thin-film technology can withstand very high pressures (1000’s of Bar) and has better long-term stability than thick film.
In contrast, thick-film is corrosion resistant and generally more robust. They are generally used in more chemically aggressive environments than thin film. Thick-film is also significantly cheaper and has greater power handling capability than the thin-film equivalent.
Thick Film Resistor Bridges Construction
When pressure acts on a membrane or substrate, the deformation changes the cross-section of the conductor tracks. This, in turn, causes a shift in the electrical resistance. It is this change in electrical resistance that a pressure sensor records.
Thick film pressure sensors use four resistors in a Wheatstone bridge configuration. The resistors are screen-printed onto the back of a ceramic membrane diaphragm and then hermetically joined to a main ceramic body. Connection is made from the membrane through the sensor body to connection points on the surface. As the ceramic membrane deforms, it changes the resistance.
The sensitivity of thick film resistor bridges is related to ceramic thickness. Very thin, lapped ceramic, delivers very sensitive sensors while thicker ceramic allows for higher pressure ranges.
It is important to choose the correct technology for the application, comparing functionality with cost. A range of standard sensors is available. But if they do not fit the demands of the application, a manufacturer of specialist thick film resistors can help.