There are a number of pressure sensors on the market today, meaning that narrowing down which works best for your application can be difficult. That being said, sensors can be categorized according to their function.
For example, they can be differentiated by the type of pressure measurement they make, the sensing principle employed, the output signal, and the media they are measuring. One of the other differentiating factors relies on whether or not they are Micro Electro-Mechanical System (MEMS) sensors, or whether they are medically approved.
There are three main measurement modes: absolute, gauge, and differential. Ensuring that you are using the appropriate pressure sensor for your intended application is necessary in the control of most industrial, medical, automotive, and consumer processes.
Absolute Pressure Sensor
An absolute pressure sensor is best suited for your application if you are measuring air pressure. For example, if you need to record barometric measurements for weather or in altimeters, such sensors are best. The absolute measurement is acquired by measuring the target pressure relative to the known pressure of an absolute vacuum.
The absolute pressure sensor is used by exposing one side of the sensor to the medium being measured, and the other side is sealed off to effect a vacuum. This comparison is measured using Kelvin wherein the lowest possible temperature is zero degrees Kelvin.
Gauge Pressure Sensor
A gauge sensor measures the pressure relative to atmospheric pressure. This can be accomplished by using a multimeter’s DC measurement range, where the display shows the voltage at the positive probe with reference to the negative probe.
One side of the sensor is connected to the vacuum, which may be a pump, while the other side is vented to the atmosphere. It is important to note that the ventilation cavity won’t become obstructed.
Differential Pressure Sensor
A differential pressure sensor measures the difference between pressure experienced at two exposed openings. Unlike gauge sensors, differential sensors are unaffected by atmospheric pressure. Some of the most typical uses include measuring gas or liquid flow as well as detecting a blockage or seized valve. Differential pressure sensors come equipped with ports on either side where pipes can be attached. The pipes are then connected to the system where the measurement is being made. Typically, the two pressures that are being measured are applied to opposite sides of a single diaphragm. The deflection of the diaphragm, either positive or negative with respect to the resting state, determines the difference in pressure.
Additionally, some industrial differential sensors use two separate absolute sensors, utilizing internal electronics to calculate and provide the difference in pressure to the control system. Alternatively, the differential measured can also be acquired using two absolute pressure sensors, then calculating the difference on an industrial control system. This may be necessary in cases where two different types of sensors are required, for example: the medium being measured is a liquid or gas, or the environment needs more than one type sensor.
The accuracy, reliability, measurement range, and compatibility of the aforementioned pressure sensors are all influenced by the sensing principle employed. The most common sensing principles include: resistive, capacitive, piezoelectric, optical, and MEMS.
Resistive pressure sensors use the change in electrical resistance of a strain gauge bonded to the diaphragm that is exposed to the pressure system. The strain gauge, which measures the strain of an object, is composed of a metal resistive element. The metal diaphragm gives high over-pressure and burst-pressure capability.
Capacitive pressure sensors display a capacitance change, or the ability of a system to store an electric charge, as one plate deflects under applied pressure. This is a highly sensitive process that can measure pressures below 10mbar.
Piezoresistive sensors take advantage of the change in resistivity of semiconductor materials, like quartz, to generate a charge on the surface when pressure is applied. The charge magnitude is relative to the force applied, and the polarity indicates its direction. Since the charge accumulates and dissipates quickly as pressure shifts, this allows for the measurement of fast-changing dynamic processes.
Optical sensors, which use interferometry, are used to measure pressure-induced changes in an optical fiber. This is ideal in environments where there is noise pollution because the sensors are uninterrupted by electromagnetic interference. Additionally, optical sensors can be made from MEMS technology which in turn makes it medically safe for implantation or topical use, and can measure multiple points of pressure on a single fiber.
Micro Electro-Mechanical System sensors (MEMS) contain a piezoelectric or capacitive pressure-sensing component bound to silicon at micron-level resolution. Co-packaged signal-conditioning electronics convert the small-magnitude MEMS electrical output to an analogue or digital signal.
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