Technical Article

Introduction to Pressure Transducers

December 08, 2023 by Robert Keim

Learn about pressure transducers: devices used in a variety of industrial, medical, automotive, and consumer applications to sense pressure and convert it into electrical energy.

Sensing conditions and changes in the physical world is one of the fundamental tasks of electrical engineering. In this article, we’ll learn about pressure transducers, which sense pressure and convert it into electrical energy. We’ll review a more detailed definition of “pressure transducer” in the next section, then move on to explanations of the different types of pressure transducers and how they function.

 

What Is a Pressure Transducer?

So, what is a pressure transducer, beyond the one-sentence explanation given above? To answer this question, let's divide it in two. First, what is pressure? And second, what is a transducer?

 

What is Pressure?

In this context, pressure is the amount of force per unit area exerted on an object in the direction perpendicular to the surface of that object. This force is frequently associated with a fluid (i.e., a liquid or a gas). Absolute pressure measurements use theoretical zero pressure as a reference point, and gauge pressure measurements use atmospheric pressure as a reference point.

The distinction between absolute pressure and gauge pressure is an important reminder that what feels like zero pressure to humans isn’t zero pressure at all. We live constantly under atmospheric pressure, which applies about 14.7 pounds of force per square inch (14.7 psi). If living under 14.7 psi from birth to death sounds a bit unfair, living in a 0 psi environment would be much worse—true zero pressure is a perfect vacuum, and would cause rapid death.

 

What is a Transducer?

The terms “sensor” and “transducer” are often used interchangeably. That’s not surprising, since many devices that are sensors are also transducers, and many transducers (including those we’ll discuss today) are also sensors. However, the words themselves indicate a subtle difference.

Sensors sense (as in detect, or measure) physical quantities and make this information available to a user. A mercury thermometer, for example, is a sensor—it allows heat to act upon a substance in such a way that humans can easily measure temperature.

Transducers don’t just measure quantities—they convert energy from one form to another. In most cases, either the “input” energy or the “output” energy takes the form of an electrical signal. A microphone is a transducer, because it changes sonic energy into a voltage signal. Nowadays, signals produced by a transducer are often sampled by an analog-to-digital converter so that the information can be processed and analyzed using digital circuitry.

Decades of innovation have resulted in an impressive array of transducers designed to convert acceleration, temperature, magnetic field strength, light intensity, and other physical quantities into electrical signals. Even if we restrict ourselves to only considering pressure transducers, there are several types. These include:

  • Capacitive.
  • Inductive.
  • Resistive.
  • Piezoelectric.

 

Capacitive and Inductive Pressure Transducers

A capacitor is a passive device, so it can’t generate a signal. However, the capacitor can be integrated into a powered circuit in such a way that the variations in capacitance will lead to signal variations.

The capacitance of a parallel plate capacitor is determined by the following:

  • The area of the plates.
  • The distance separating the plates.
  • The permittivity of the insulating material between the plates.

A capacitive pressure transducer (Figure 1) combines a parallel plate capacitor with a diaphragm that causes the distance between the plates to change when pressure changes.

 

A capacitive pressure transducer intended for industrial applications.

Figure 1. An industrial pressure transducer capable of converting pressure into a 14-bit digital output value. Image used courtesy of TE Connectivity

 

A similar design principle can be applied to inductive devices. In inductive pressure transducers, pressure variations alter the position of a ferromagnetic core, producing signal variations as a result.

 

Resistive Pressure Transducers

We’ll discuss three types of transducers that use resistance to measure pressure:

  1. Strain gauges.
  2. Piezoresistive pressure sensors.
  3. Potentiometric pressure sensors.

Technically, the last two categories are sensors that contain transducers along with additional circuitry. However, it’s more useful to consider the devices as a whole, and “piezoresistive (or potentiometric) pressure sensors containing transducers” is a bit of a mouthful.

 

Strain Gauges

As the name suggests, a strain gauge (Figure 2) measures strain, which is defined as physical deformation in the direction of applied force. A conductive object in a strain gauge undergoes changes in electrical resistance as a result of deformation. Since strain results from force, and pressure is force per unit area, strain-gauge technology can be the basis of a pressure transducer.

 

Textbook diagram of a bonded strain gauge.

Figure 2. Layout of a bonded strain gauge. Image used courtesy of All About Circuits

 

While a strain gauge on its own technically constitutes a transducer, most of the time this technology is combined with a circuit that converts changes in resistance into a voltage signal.

 

Piezoresistive and Potentiometric Pressure Transducers

Piezoresistive pressure sensors are similar to strain gauges in operation, though somewhat dissimilar in construction. Offering high performance and low cost, they are widely used.

Another resistance-based means of measuring pressure involves a potentiometer equipped with a pressure-sensitive element that causes the potentiometer’s wiper to move. As with the strain gauge, changes in resistance can be measured by an external circuit and converted into a voltage signal.

 

Piezoelectric Pressure Transducers

Piezoelectric materials, such as the quartz crystals used in electronic oscillators, produce changes in electric charge in response to mechanical stress. Mechanical stress, in turn, results from applied force. Because of this relationship between force and charge, a pressure transducer can be built around a piezoelectric element and a diaphragm (Figure 3). Piezoelectric pressure transducers tend to be robust and highly responsive.

 

Piezoelectric pressure transducer combining the crystal-plus-diaphragm system with amplifying circuitry.

Figure 3. A piezoelectric pressure transducer that combines the crystal-plus-diaphragm system with amplifying circuitry. Image used courtesy of PCB Piezotronics

 

Wrapping Up

Electrical engineers have a variety of options for incorporating pressure measurements into a design. As usual, some research and careful consideration will be needed to identify the optimal transducer for a particular project. If you’d like to learn more about pressure-sensing technology—particularly in the industrial space—I recommend the following articles from All About Circuits and its sister site Control Automation:

 

Featured image used courtesy of Adobe Stock