Pressure sensor
A pressure sensor belongs to the group of pressure gauges , which as the first element of a measurement chain , the physical quantity -pressure (= force per surface ) in an electrical output as a measure of the pressure forming . The SI unit for pressure is Pascal with the unit symbol Pa. According to DIN 1301 , the bar , the unit symbol bar, is also permitted. Pressure sensors are available for measuring stationary pressure, a pressure difference or pressure fluctuations up to sound pressure .
Pressure sensor types
There are different pressure sensors on the market depending on the measuring method, such as:
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Absolute pressure sensors: This type of sensor relates its pressure measurement to a vacuum with a pressure of almost 0 bar.
- Barometric pressure sensors: This type is an absolute pressure sensor that is specially designed for the pressure range 0–1.3 bar and measures the barometric air pressure.
- Differential pressure sensors: This type of sensor measures the pressure difference between two measuring chambers, in which different pressures can prevail. Changes in pressure that affect both measuring chambers are not recorded.
- Bidirectional differential pressure sensors: like differential pressure sensors, this type measures the pressure difference that exists in two chambers. The special thing about it is that the measuring membrane can bend in both directions, hence the term bidirectional.
- Relative pressure sensors : This type is similar to the differential pressure sensor . The relative pressure in relation to the atmosphere (atmospheric air pressure) is measured as a reference.
Manufacturers, especially in English-speaking countries, refer to absolute pressure sensors as "absolute" or "a". Pressure sensors designed to measure pressures relative to atmospheric pressure are referred to as "gauge" or "g". A further distinction is made here between enclosed (and thus structurally sealed) atmosphere of, for example, 1013 mbar (sealed gauge) and a measurement relative to the currently actually existing atmospheric pressure. In the last-mentioned pressure sensors, the atmospheric chamber is usually coupled to the atmospheric pressure through a small bore.
Pressures can be measured in gaseous and liquid media as well as on solids as a force-transmitting component.
Sensor materials
Silicon , quartz or metals are used as sensor materials . With the help of semiconductor technologies it is now also possible to apply piezoelectric thin films directly to measuring bodies. This is mostly zinc oxide (ZnO) or aluminum nitride (AlN).
Monolithic pressure sensors consist of only one material, e.g. B. Quartz; In addition to an elastic base material, strain gauges are sometimes used.
Physical measurement effects
Various physical measuring effects are used to record the measured variable . The pressure sensors are therefore divided into the following types:
Piezoresistive pressure sensor
For piezoresistive pressure sensors, either metallic strain gauges or pressure-sensitive semiconductor chips are used. The materials used for pressure measurement show a strong temperature dependency, but since this influence is the same on all resistances, it can be made ineffective by an electrical circuit that creates a difference . The following sensor technologies are common:
Ceramic thick film sensors are made from aluminum oxide. The base body is formed by a massive ring, the face of which is the membrane. On the back of the membrane - the side facing away from the pressure - the strain gauges are applied and burned in using a screen printing process. The aluminum oxide ceramic is almost ideally elastic up to the breaking point. Ceramic thick film sensors generally have an excellent price-performance ratio and are resistant to many chemicals.
Metal thin film sensors consist of a circular ring membrane made of stainless steel. The conductor material for the strain gage is applied to the membrane by means of gas phase deposition and the structures are then etched photolithographically . The resulting strain gauges are thinner than 1 µm. The sensors are relatively small and suitable for high pressure applications and are extremely vibration resistant.
Silicon pressure sensors have a membrane made of silicon and on it diffused deformation-dependent resistors (DMS).
Piezoresistive silicon sensors have a similar functional principle as metallic strain gauges. However, the measurable effect is not based on a change in resistance, but on a change in the crystal lattice structure and thus the electron mobility. The actual sensor element is a silicon chip a few millimeters in size, which is attached and bonded to a TO header . The header sits in a round metal capsule that is closed with a metal membrane. The interior is filled with oil, which transfers the pressure from the membrane to the chip. The measuring cells are very variable in terms of pressure range and size. They are also particularly overload-proof and can be used for many media thanks to the appropriate membrane materials.
Piezoelectric pressure sensor
In a piezoelectric sensor , an electrical voltage is generated in a crystal by means of pressure through charge separation. This is called the piezoelectric effect . Pressure shifts ions inside the crystal, creating an electrical charge proportional to the force on the surface . The charge is converted into a proportional electrical voltage by a charge amplifier . The direct measurement of the voltage is not possible, as the small charge generated must be very well insulated and no electrical capacitance change must be allowed. Any pressure can be set as the zero point of the charge amplifier by deriving (short-circuiting) the charge; this makes pressure changes directly measurable.
Piezoelectric sensors basically only measure forces. If the sensor is to be used in pressure measurement technology, the pressure must first be proportionally converted into a force via a membrane.
Advantages of piezoelectric sensors:
- insensitive to high temperatures
- no external power supply required
- high sensitivity
- mechanically very rigid, which means that there are only minor natural vibrations or post-oscillation effects
- suitable for pressure fluctuations at high frequencies up to> 100 kHz.
Disadvantages of piezoelectric sensors:
- Cannot be used without charge amplifier
- Cannot be used for static measurements such as water level or air pressure, because even with the highest possible insulation an actually constant charge drains off over the course of hours.
Pressure sensors in the vacuum range
Reference is also made here to:
- Thermal conduction vacuum meter according to Pirani, measuring range approx. 100… 0.1 Pa
- Ionization vacuum gauge with cold cathode according to Penning, measuring range approx. 10 −1 ... 10 −5 Pa
- Ionization vacuum meter with hot cathode according to Bayard-Alpert, measuring range approx. 10 −1 ... 10 −10 Pa
Frequency analog pressure sensor
This is usually a piezoresistive pressure sensor that measures the pressure by means of strain gauges and is expanded by a ring oscillator in terms of circuitry . By changing the pressure, the resistance of the strain gauges changes and the frequency of the ring oscillator is detuned. The frequency output is directly proportional to the applied pressure.
Pressure sensor with Hall element
It works according to the Hall effect , whereby the magnetic field around the Hall element is changed when pressure is applied.
Capacitive pressure sensor
Capacitive pressure sensors contain two capacitors diffused into a silicon chip . When pressure is applied, the distances between a membrane and two capacitor plates opposite on both sides and thus the capacities of the capacitors are changed in opposite directions. Usually the capacitors are part of an internal amplifier, the output signal of which depends on the difference in capacitance.
Inductive pressure sensor
Inductive pressure sensors work with an inductive displacement transducer which is connected to a membrane. Changes in pressure create a force on the membrane and move it. As a result, the position of an iron armature changes in opposite directions in two coils: the inductance increases in one, and decreases in the other. The difference can be determined very precisely electrically. This arrangement can also be replaced by a differential transformer .
More sensors
For the detection of small pressure fluctuations , such as those that occur with sound pressure, further measuring effects and their applications in sensors of the microphone are listed under the associated keyword.
Commercially available pressure sensors
The sensors available on the market emit a standardized electrical signal that can be easily processed by the downstream device (e.g. DDC ). Analog unit signals may 0 ... 10 V , 0.5 ... 4.5 V rat or 4 ... 20 m A to be; there are also standardized digital interfaces such as I²C or CAN bus . These pressure sensors are passive components that require a power supply to process the signal. The characteristics of the sensors can be linear or correspond to an exponential function; Disturbance variables (e.g. temperature) must be compensated and the characteristics linearized appropriately. Therefore, today one differentiates between:
- non-compensated pressure sensors (e.g. normal bridge circuit, piezoresistive pressure sensor , microsystem technology )
- analog, but calibrated sensors (e.g. calibrated bridge circuit, piezoresistive pressure sensor, microsystem technology)
- digital, calibrated and linearized as well as temperature-compensated pressure sensors (e.g. calibrated bridge circuit combined with analog-digital converter and parameter memory, piezoresistive pressure sensor, microsystem technology)
Application restrictions
Pressure sensors are sensitive to overload. If the measuring range is exceeded, the sensor can easily be irreparably damaged, depending on the technology used. For correct measured values, the correct installation position of the sensor must also be taken into account. For sensors that measure liquids, make sure that the sensor line is vented. It is often necessary to separate the (aggressive) medium from the actual pressure sensor in order to protect the chemically sensitive pressure sensors from corrosion or contamination.
Web links
- Markus Bogdahn, Eric Egert, Sebastian Kassner, Timo Kober and Matthias Staab, pressure measurement (PDF file; 3.65 MB)
Individual evidence
- ↑ JUMO: JUMO Campus - Electronic pressure measurement technology. Retrieved September 3, 2018 .