Technical Article

# Pressure Sensor Fundamentals and Types

April 18, 2023 by Antonio Armenta

## Pressure sensors are essential tools in industrial applications, required wherever there is a hydraulic or pneumatic system to keep pressure quantities under control.

#### See Our Previous Articles on Industrial Sensors:

The seventeenth century brought many of the greatest discoveries around the physical phenomenon of pressure. In 1647, French mathematician and philosopher Blaise Pascal conducted his famous mountain experiment. Using instruments containing columns of mercury known as barometers, invented by Evangelista Torricelli, he observed that the height of the mercury in a vacuum changed whether he was situated at the top or the bottom of the mountain. He realized that atmospheric pressure depends upon elevation.

##### Figure 1. A representation of Torricelli’s barometer. Image used courtesy of Adobe Stock

Later, Robert Boyle declared his famous law, stating that the pressure and volume of a gas are inversely proportional. Then, in 1738, Daniel Bernoulli advanced Boyle’s work, finding that gas pressure can be increased by heating it while keeping a constant volume.

Pressure sensors are designed starting from all these fundamentals. In addition, it is understood that a pressure quantity is not affected by the shape of its confines. This means that pressure is distributed equally along an entire surface, making it less dependent on the shape of the container.

In the International (metric) System, pressure is measured in pascals. One pascal equals one Newton of force uniformly applied to a surface of one square meter. The atmosphere is another unit of measure, defined as the amount of pressure exerted by a one-meter-high column of water over a surface of one square centimeter. The most common unit of measure in the United States is the pound per square inch (psi).

In the following sections, we will review common types of pressure sensors and applications.

### Diaphragm Pressure Gauge

These are the most basic form of the pressure sensing element. Pressure is calculated by determining the force applied onto a disc with a known surface area. There are rigid and flexible diaphragms, with the latter being the most common. In a flexible diaphragm, the sensing element suffers elastic deformation under pressure. By knowing the sensing material's mechanical properties, the deformation amount can be converted to a force value. Ultimately, the force value is mathematically converted to pressure.

One of the earliest and most common forms of a pressure sensor is the diaphragm pressure gauge. These sensors expose one side of their diaphragm to an external pressure while referencing the other to a known pressure. As a result, the diaphragm will bend according to the pressure differential, which can be measured using needles and displacement sensors.

Diaphragm pressure gauges have been around for decades, and their modern versions are regarded as reliable while offering a broad operating range.

### Capacitance Manometer

These sensors are analogous to the basic diaphragm sensors but incorporate electrodes for interpreting pressure quantities. For example, a capacitance manometer has a diaphragm with an inlet and reference pressure. The electrode is mounted on the side of the reference pressure and sends an electrical signal directly proportional to the pressure differential.

Capacitance manometers constitute excellent calibration devices for other pressure diaphragms purely based on the mechanical deflection. Also, these manometers can operate under a broad range of temperatures and work well under harsh conditions. They typically have low power requirements, which makes them suitable for many industrial control applications.

### Piezoresistive Pressure Sensor

These sensors are also known as strain gauges. They are based on a concept known as resistive pressure measurement, in which a metallic strip is subjected to deflection, causing its electrical resistivity to change. This occurs because the metallic piece becomes shorter under pressure or longer when pressure is relieved. The resulting changes in resistivity are measured to obtain the corresponding pressure quantities.

These sensors are generally considered robust and reliable over time. Their construction is also relatively simple, therefore having lower costs. Conversely, they consume relatively high power compared to other pressure sensor variants.

### Other Types of Pressure Sensors

We have reviewed the most common types of pressure sensors, but there are certainly several others. One of them is the aneroid barometer. This sensor can measure environmental pressure and work solely on mechanical deformations. Another type of sensor is the bourdon tube, which is based on the aneroid barometer. These tubes have a helicoidal shape that also calculates pressure based on the deformation of the sensor.

Pressure sensors also have many more industrial applications. They are essential tools in systems that require high pressure and vacuum. In the manufacturing sector, they are required everywhere where there is a hydraulic or a pneumatic system to keep pressure quantities under control.

### Sensing in Automation

In the next article in our series, we will take a closer look at level sensors and the types used in industrial automation.