What is a Pressure Sensor?

• Definition of Pressure as a Measured Quantity: Pressure is defined as the applied force by a liquid or gas on a surface and it is usually measured in units of force per unit of surface area. Common units are Pascal (Pa), Bar (bar), N/mm2 or psi (pounds per square inch).
• Definition of a Sensor: A sensor is a device that measures a physical quantity and translates it to a signal. The quantity can be for instance temperature, length, force or – of course – pressure. The signal is in most cases electrical but can also be optical.
• Definition of a Pressure Sensor: Therefore, a pressure sensor is an instrument consisting of a pressure-sensitive element to determine the actual pressure applied to the sensor (using different working principles) and some components to convert this information into an output signal.

Types of Pressure Measurements

There are a variety of properties that can be used to classify pressure sensors including the pressure range they measure, the temperature ranges of operation or the type of pressure they measure.

Different pressure types can be absolute, gauge, sealed gauge and differential pressure.

• Absolute pressure sensors measure the pressure relative to a reference chamber (nearly vacuum).
• Gauge pressure sensors – or relative pressure sensors – are used to measure the pressure relative to the currently present atmospheric pressure.
• Sealed gauge pressure sensors are like a gauge pressure sensor but they measure pressure relative to a fixed pressure rather than to the current atmospheric pressure.
• Differential pressure sensors determine the difference between two pressures and can be used to measure pressure drops, fluid levels and flow rates.

The obvious advantage of an absolute pressure sensor is to always measure against the same reference pressure (vacuum) and therefore be unaffected by atmospheric pressure changes and less affected by temperature changes.

Working Principles of Pressure Sensors

There are a variety of different technologies used within pressure sensors to provide accurate results. The following section will highlight some of these.

• Strain gauge-based pressure sensors also use a pressure-sensitive element where metal strain gauges are glued on or thin-film gauges are applied on by sputtering. This measuring element can either be a diaphragm or for metal foil gauges measuring bodies in can-type can also be used. The big advantages of this monolithic can-type design are improved rigidity and the capability to measure the highest pressures of up to 15,000 bar. The electrical connection is normally done via a Wheatstone bridge which allows for a good amplification of the signal and precise and constant measuring results.

• Capacitive pressure sensors use a pressure cavity and diaphragm to produce a variable capacitor. The diaphragm is deformed when pressure is applied and capacitance decreases accordingly. This change in capacity can be measured electrically and is then set with the applied pressure. These sensors are limited to low pressures of roughly 40 bars.

• Piezo-resistive pressure sensors consist of a diaphragm – mostly made of silicon – with integrated strain gauges to detect strain as a result of applied pressure. These strain gauges are typically configured in a Wheatstone bridge circuit to reduce sensitivity and increase the output. Due to the material being used the pressure limitation is at around 1,000 bars.

• Unlike the technologies – which all use the deflection of a measuring body – resonant pressure sensors use the changes in resonance frequency in a sensing mechanism to measure stress caused by applied pressure. Depending on the design of these sensors, the resonating element can be exposed to the media, where the resonance frequency then depends on the density of the media. Sometimes these sensors are also sensitive to shocks and vibration.

Other pressure sensors that also do not use a measuring body can be thermal or ionization sensors, which use changes in thermal conductivity due to density changes in the flow of charged particles to measure the applied pressure.

Pressure Sensor uses – Pressure Switches

Pressure switches are available with either one port (known as static or gauge pressure switches) or two ports (differential pressure switch. They are well known for their quick response time and dependable long life. They are small and generally a low-cost solution. 

Below are some uses for pressure switches, many of these are common in everyday life.

• Alarm systems on HVAC boiler room controls
• Monitoring the oxygen levels in pressure tanks in the medical industry
• Water levels in household appliances such as washing machines
• Liquid dispensing in coffee machines
• Medical Breathing apparatus
• Detecting blocked filters in fans and extraction units

Industrial Applications of Pressure Sensors

Energy conservation is increasingly important to reduce power consumption and its associated costs for any enterprise as well as minimizing the environmental impact including a businesses’ environmental footprint. For improved energy conservation accurate pressure sensor measurements are required in portable and stationary weather stations, testing devices for diesel truck emissions, wind energy systems, wind engineering about new building design aerodynamics, ocean research, high-altitude weather research balloons, water pollution devices, smokestack mercury sampling and atmospheric studies.  

The technology industry requires accurate pressure sensors in its products and in the regulation of environments that house massive data centers around the world. Pressure sensor applications consist of altimeters, barometers, sensing printer ink levels, airflow rate in equipment, IT center/ computer cooling systems, semiconductor process equipment and laser measurement as well as cleanroom monitoring devices.

Manufacturing industry applications for MEMS pressure sensors continue to increase based on the need for more tightly controlled processes with their associated quality control requirements. Increased monitoring of pressure and/or vacuum can detect the need for equipment maintenance before untimely failures that cause excessive downtime and increase manufacturing costs. In addition to general instrumentation requiring static measurements, pressure sensors play an increasing role in applications such as bottle and equipment leak detection, Variable Air Volume (VAV) systems, air blades, compressed air pressure monitoring, industrial flow monitoring, filter pressure monitoring, duct airflow, gas detection, pneumatic controls, mine safety instrumentation, industrial degassers and suction check-in pick and place applications such as the printed circuit boards and semiconductor process equipment.