The Science Behind Pressure Sensors: How They Work?

A pressure sensor is a device that is used to measure the pressure of a fluid or gas. It works by converting the physical pressure of the fluid or gas into an electrical signal that can be read by a computer or other electronic device. The working principle of a pressure sensor is based on the concept of piezoresistance which is the change in the resistance of a material when it is subjected to pressure. In this article, we will explore the working principle of a pressure sensor in detail.

Introduction

Pressure is all around us. Most people don’t think about it much but it’s something the manufacturing and industrial worlds deal with on a daily basis. A large portion of the machines out in the industry utilize at least one form of fluid power.

Fluid power is a general term that can refer to gas or liquid. Gas would be defined as an air-like fluid substance that expands freely to fill any space available, irrespective of its quantity; a liquid is a substance that flows freely but is of constant volume, having a consistency like that of water or oil. Pneumatic systems utilize gases while hydraulic systems utilize liquids.

Many fluid power systems, either hydraulic or pneumatic, have to be monitored for pressure. There are many ways to detect pressure and the most common is a Bourdon tube gauge. Every gauge has the potential to be a pressure sensor.

Working Principle of Pressure Sensor

A pressure sensor has a measuring cell that converts (or transduces) the mechanical strain of the pressure applied and converts the force into an electrical signal. Pressure in a pipe, hose or duct applies force on the measuring cell of a sensor causing a deflection which is measured by an electrical circuit. This measurement is then converted into a current or voltage output.

Pressure sensors can be used in many different types of applications from pressure monitoring to level and flow detection.

There are different types of pressure measurement.

Absolute sensors will have a pressure zero reference to absolute pressure. Absolute pressure is detected relative to 0 Pa, i.e., the static pressure of a vacuum. The sensor is designed with one port for the fluid to enter and exert pressure on the sensing element. The pressure applied produces a positive change in output, of magnitude proportional to the pressure applied.

Gauge sensors will have a pressure zero reference to the atmosphere. Generally, if you want to measure or control a pressure that is influenced by changes in atmospheric pressure. This style sensor is used in any application where you want to overcome the atmospheric conditions to produce a task or pull a vacuum to accomplish another type of task. The applications for gauge pressure sensors are quite vast. Some examples are pump discharge pressure, fire hose discharge pressure, tank level, steam pressure in a commercial boiler and many more.

Differential sensors use two measurement cells; one for high pressure and one for low pressure. The differential pressure is Cell 1 – Cell 2. Remember that differential pressure is the difference in pressure between two points of measurement. You can measure very low to high pressures in all kinds of different media including liquids, gases, water, refrigerants and air. Differential applications can be quite varied, some examples are supply and return pressure in a chiller, airflow stations, leak detection systems, pressurized tank levels, hospital isolation or protection rooms and many more.

Understanding the difference between gauge and absolute pressure is critical for most applications. Remember that gauge pressure is measured relative to current atmospheric pressure (subject to change with changes in the barometric pressure) and that absolute pressure is measured relative to a perfect vacuum. Thus, your application will determine which of these approaches is required.

Sensing principles

The sensing principle employed by a pressure sensor, can influence accuracy, reliability, measurement range and compatibility with the target environment. 5 different ways the mechanical displacement taking place inside a sensor is turned into an electrical output:

• Resistive
• Capacitive
• Piezoelectric
• Optical
• MEMS

Resistive

Resistive pressure sensors utilize the change in electrical resistance of a strain gauge bonded to the diaphragm that’s exposed to the pressure medium.

The strain gauges often comprise a metal resistive element on a flexible backing bonded to the diaphragm or deposited directly using thin-film processes. The metal diaphragm gives high over-pressure and burst-pressure capabilities. Otherwise, strain gauges can be deposited on a ceramic diaphragm using a thick-film deposition process. Over-pressure and burst-pressure tolerance are typically much lower than for metal diaphragm devices. Piezoresistive sensors take advantage of the change in resistivity of semiconductor materials when subjected to strain due to diaphragm deflection. The magnitude of the change can be 100 times greater than the resistance change produced in a metal strain gauge. Hence piezoresistive sensors can measure smaller pressure changes than metal or ceramic sensors.

Capacitive

Capacitive sensors which display a capacitance change as one plate deflects under applied pressure, can be highly sensitive, can measure pressures below 10mbar and withstand large overloads. Constraints on materials and joining and sealing requirements, however, can restrict applications.

Piezoelectric

Piezoelectric pressure sensors utilize the property of piezoelectric materials like quartz, to generate a charge on the surface when pressure is applied. The charge magnitude is proportional to the force applied, and the polarity expresses its direction. The charge accumulates and dissipates quickly as pressure changes, allowing measurement of fast-changing dynamic pressures.

Optical

Optical sensors, which utilize interferometry to measure pressure-induced changes in optical fiber, are undisturbed by electromagnetic interference, allowing use in noisy environments or near sources such as radiography equipment. They can be created using tiny components or MEMS technology, can be medically safe for implantation or topical use and can measure the pressure at multiple points along with the fiber.

MEMS technology

MEMS (Micro-Electro-Mechanical System) sensors contain a piezo or capacitive pressure-sensing mechanism fabricated on silicon at micron-level resolution. Co-packaged signal-conditioning electronics convert the small-magnitude MEMS electrical output to an analog or digital signal. They are tiny surface-mount devices typically only about 2-3mm per side.

Sensor references

All pressure sensors have a reference point and there are different types of reference points available. The reference point helps keep the zero point of the transmitter at a constant value. Depending on the application, it may be necessary to use different types of pressure references for the device.

Pressure sensors get their reference via a hole on the side, top or bottom of the sensor which is usually sealed by a hydrophobic material. Alternatively, pressure sensors can get their reference via a vent tube. This is the only way a submersible sensor can get its reference, unless sealed or absolute. This is achieved by having a tube that runs through the electrical cable to allow a vent between the atmosphere and the backside of the measuring cell.

References: The design engineer’s guide by Avnet, Turck pressure sensor catalog