Pressure Sensor

Overview of the Pressure Sensor

Put plainly, a pressure sensor is any instrument capable of sensing pressure and converting it into a signal of measurement (usually an analogue electrical one), the magnitude of which changes in proportion to the degree of pressure the device is subject to. Because of the conversion of one energy type into another, pressure sensors are also commonly referred to as pressure transducers.

These devices are generally used to measure changes to gas or fluid pressure in various applications. Pressure is measured in terms of force per unit area and is generally defined as the amount of force required to prevent the expansion of a fluid.

Today, literally thousands of common applications depend on pressure sensors for their smooth functioning. Pressure sensors now embrace an extraordinarily wide range of technologies (at least 50) and come in myriad designs.

 

History

Although he was at a loss as to how it happened, the great Galileo Galilei of Pisa discovered in 1594 that he could make water rise to a height of 10 metres within a syringe like a suction pump. He knew, however, that this was a measure of something, and the emerging scientific community began experimenting to find out what. Fifty years later in 1644, his fellow Italian, Evangelista Torricelli, filled a 1 metre length of glass tube with mercury (one end was hermetically sealed), and upended it into a bowl of mercury, whereupon the mercury column within fell to 760 mm. Speculating on this finding, the French philosopher and physicist Blaise Pascal theorised that the 760 mm space above the mercury in the tube was related to the weight of air inside it. To test this, he conducted experiments with the mercury column at different altitudes – the weight of air at the top of a mountain was likely to be less than that at the bottom, and Pascal indeed confirmed that the “empty” space at the top of the cylinder was smaller at high altitudes than at ground level. He called the force pressing on the surface of the mercury “pressure.”

Using J-shaped glass tubes sealed at one end in 1661, the chemist Robert Boyle discovered that, provided the quantity of gas and the temperature remained constant, it was possible to calculate the change of pressure within the tube if the volume of gas was changed by expansion or contraction.

But it wasn’t until 1843 that the first pressure sensor came into being – the spring balance-operated ‘aneroid barometer’ invented by the Frenchman Lucien Vidie. Electrical pressure sensors, however, did not appear until 1930, when distortions to a diaphragm as a result of pressure were converted into alterations in electrical capacitance. In the 1930s and 50s, bonded strain gauges and foil gauges were developed to measure fluctuations in pressure, but the “sensor age” really began to arrive in 1967, when the Honeywell Research Centre in the USA patented the first edge-constrained silicon diaphragm. From 2000 onwards, piezoresistive pressure sensors began to take off and have effectively become universal today.

 

Technical aspects

The standard SI unit of pressure is the Pascal, where 1 Pascal = one newton per square metre, which is approximately equivalent to the amount of force one £5 note exerts when resting flat on a table. Because this is such a tiny quantity, typical pressures in industrial and manufacturing applications are measured in kilopascals, where one kPa = 1,000 Pascals. Other metrics for pressure include pounds per square inch (psi), where 1 psi (1 lb per square inch) = 6891 Pa; Bars, where 1 Bar = 105N/m2 = 100 kPa; Tors, where 1 Tor = 1 millimetre of mercury (mmHg) = 133.3 Pa; and atmospheres (atm) were 1 atm = 760mmHg.

Today, Force Collector Pressure Sensors use the deflection induced by pressure acting on a spring or diaphragm (the force collector) to give a measurement of the magnitude of the force deployed. Materials such as Polysilicon Thin Film, Monocrystalline Silicon and Bonded Metal Foil, which alter their electrical resistance as a result of pressure (piezoresistive sensors), are now widely used to measure differential pressure (the difference in pressure on either side of the force collector), absolute pressure (the difference between ambient pressure and a perfect vacuum), gauge pressure (the difference between ambient and atmospheric pressure) and vacuum pressure (the difference between atmospheric pressure and lower-than-atmospheric pressures).

Force Collector Sensors incorporating diaphragms made of silicon, metal or ceramic measure changes in the electrical capacitance of these materials in response to applied pressure. They are typically deployed in applications measuring pressures at the lower end of the scale. The measurement of more dynamic pressure changes typically requires piezoelectric technology in the sensor.

Besides Force Collector Pressure Sensors, other pressure transducers rely on properties such as the density of a gas or fluid, the resonant frequency set up by pressure changes, the changes to a material’s thermal conductivity in response to pressure fluctuations and  changes in ionisation (as in Hot and Cold Cathode Gauges) resulting from pressure alterations.

 

Product application

Pressure sensors are increasingly used in the manufacture of computers and smartphones, where they facilitate the functioning of touch-sensitive screens. They are widely used in the automotive industry, monitoring the pressure of oil and coolant in car engines, for example, and regulating the engine’s power in response to changes to foot pressure on the accelerator pedal. They are also used to ensure the reliable operation of anti-lock braking systems and the activation of car air bags.

Pressure sensors are widely used in aviation to regulate the difference between atmospheric pressure and intra-aircraft air pressure, not only for passenger and cabin staff to breathe properly but also to ensure the proper functioning of the aircraft’s inner components and electrical circuitry.

The marine industry relies on pressure sensors to gauge the depth at which submarines are operating and numerous biomedical instruments (such as ventilators and digital blood pressure monitors) depend on accurate pressure sensors to ensure patient well-being.

How the Pressure Sensor differs from other sensors

Pressure sensors of all varieties specifically respond to changes in ambient pressure rather than to, say, heat energy, electrical potential, humidity, light or motion.

Current product advantages and limitations

While they have helped revolutionise the industries mentioned in the Product Application section above, there are some stubborn limitations to the effectiveness of pressure sensors in some applications. Touchscreens on smartphones and computers become considerably less sensitive, for example, when multiple touches are exerted simultaneously.

As with a number of transducers, extensive use  (as in automobiles and computer touchscreens) can rapidly degrade the efficiency of a pressure sensor - although industrial research is focusing on this limitation as a matter of priority and a new generation of more rugged pressure sensors may well be available in the foreseeable future.