Pressure sensors keep us safe, but most people do not even know that they exist. They monitor the hydraulics in our cars and alert us if the brakes are faulty. This article explains the types of pressure sensors available as well as their applications.

What is a pressure sensor?

A pressure sensor is a device or instrument which measures the pressure in gases or liquids. The pressure sensor usually acts as a transducer i.e. it converts variations in one physical quantity into an electrical signal.

Pressure sensors are used in the monitoring and control in very many everyday applications. They can be used to indirectly measure other variables such as the speed or flow of a fluid or gas, water level, as well as measuring the altitude.

Pressure sensing principles

These are the principles used in the various types of pressure systems.

Sensing principles

The sensing principles used by the various types of pressure sensors are shown below:

1.      Resistive

These types of pressure sensors utilize the change in the electrical resistance of a strain gauge bonded to the diaphragm that directly interacts with the pressure medium. The strain gauges present are often made up of a metal resistive element placed on a flexible backing bonded to the diaphragm. Alternatively, it can be deposited directly through the use of the thin-film process. The metal diaphragm gives high over-pressure and burst-pressure capability. Otherwise, the strain gauges can use the thick-film depositing process to deposit the strain gauges on a ceramic diaphragm. Burst pressure and over pressure are usually much lower than for metal diaphragm devices.

2.      Capacitive

These are pressure sensor types that display a change in capacitance as one plate deflects under applied pressure. They are highly sensitive, and can measure pressures that are well below 10 millibars. In addition they can withstand large overloads. Constraints on joining and sealing requirements, and materials may restrict the applications of these pressure sensor types.

3.      Optical

These are pressure sensor types that use interferometry to measure the pressure induced changes in an optical fibre. They have the following desirable features:

They are undisturbed by electromagnetic interference, thus, allowing use in noisy environments.

  • They can be used near sources such as radiography equipment.
  • They can be medically safe for implantation or topical use.
  • They can measure pressure at multiple points along the fibre.

4.      Piezoelectric

These are types of pressure sensors that make use of the piezoelectric property of materials such as quartz to generate a charge on the surface when a pressure is applied to the surface.

5.      Micro Electro-Mechanical System (MEMS)

These are pressure sensor types that contain a capacitive or piezo pressure sensing mechanism fabricated on silicon. This is made at micron level resolution. This shows how small they are. Co packaged signal conditioning electronics are present in order to change the small magnitude MEMS electrical output to an analogue or digital signal. These are tiny surface mount devices that are about 2 to 3 mm per side.

 

Types of pressure sensors

The various types of pressure sensors can be categorized into three groups.

1.      Absolute pressure sensors

These types of pressure sensors use measure the pressure present relative to the pressure in a perfect vacuum i.e. relative to zero. One side of the sensor is exposed to the medium to be measured, and the other side is sealed to effect a vacuum.

 

2.      Gauge pressure sensors

These types of pressure sensors measure the pressure relative to the atmospheric pressure. One side is connected to the system such as a pump like a suction pump, and the other side is vented to the atmosphere. Great care needs to be taken to ensure that the vent hole does not become obstructed.

 

3.      Differential pressure sensors

These types of pressure sensors measure the difference between the pressures experienced at two exposed ports. They are used in applications such as measuring the liquid or gas flow in pipes or ducts. They can also be used for detecting a seized valve or a blockage in a pipe.

 

4.      Sealed pressure sensor

These types of pressure sensors are quite similar to the gauge pressure sensor. The key distinction between the two is that the sealed pressure sensor measures the pressure present relative to some fixed pressure rather than the ambient atmospheric pressure. This ambient atmospheric pressure is seen to vary according to the location and the weather.

 

5.      Vacuum pressure sensor

This pressure sensor type is a sensor that measures the pressures that are below the atmospheric pressure, showing the differences between that low pressure and atmospheric pressure. The may also be used to describe a sensor that measures absolute pressure relative to a vacuum.

Applications of pressure sensors

1.      Automobile brake systems

In this application, pressure sensors may be used to detect fault conditions that exist in the hydraulic brakes. They immediately alert the user of the vehicle that a condition exists that could impact their ability to function properly.

2.      Collision detection

The various types of pressure sensors can be used to detect collisions and trigger the activation of safety devices such as airbags.

3.      Ventilators

In medical ventilators, various types of pressure sensors are used to monitor the oxygen pressure present. In addition, they also help control the mix of air and oxygen that is supplied to the patient.

4.      IV fluid delivery

Automated drug delivery systems that infuse medication into a patient in the form of IV fluids use pressure sensors to deliver the proper dosage at the proper time of the day.

5.      Fluid level monitoring

The fluid level in a tank can be monitored by using pressure sensors located at the bottom of the tank. As the fluid level decreases, the head pressure which is caused by the weight of liquid above the sensor also decreases. This measurement gives a direct indication of the amount of liquid in the tank, and is independent of the shape of the tank as it only depends on the height of the fluid.

Conclusion

This article gave the different types of pressure sensors. Their sensing principles and applications were given a thorough examination.

We hope you learnt something new.