Pressure sensor

A pressure sensor belongs to the group of pressure gauges, which transform as the first element of a measuring chain, the physical size pressure (= force per unit area ) into an electrical output as a measure of the pressure. The SI unit for pressure is Pascal with the unit symbol Pa. According to DIN 1301 and the bar, bar unit symbol is approved. Pressure sensors are available to measure the steady pressure, a pressure difference or pressure fluctuations up to the sound pressure.

Pressure sensor types

There are a variety of different pressure sensors available on the market, such as:

• Passive pressure sensors
• Relative pressure sensors
• Absolute pressure sensors
• Differential pressure sensors.

From the makers, especially in the English - speaking world, absolute pressure sensors are referred to as " absolute " or "a". Pressure sensors which are intended for measurements of pressures relative to the atmospheric pressure, are designated " gauge" or "g". Here we distinguish further between trapped (and thus constructively sealed ) atmosphere, for example, 1013 mbar ( sealed gauge ), and a measurement relative to the current actually existing atmospheric pressure. In the latter, the atmospheric pressure sensor chamber is mostly coupled through a small hole with the atmospheric pressure.

Pressures can be measured in gaseous and liquid media and on solid bodies as a force-transmitting component.

Sensor materials

As sensor materials, silicon, quartz, or metals. With the help of semiconductor technologies, it is now also possible to administer piezoelectric thin films on measuring bodies directly. This is usually to zinc oxide (ZnO) or aluminum nitride ( AlN).

Monolithic pressure sensors consist of one material, such as quartz; partially strain gauges are used in addition to an elastic base material in addition.

Physical Measurement effects

To capture the measure various physical measurement effects are used. Therefore, one divides the pressure sensors in the following types:

Piezoresistive pressure sensor

It comprises a diaphragm with applied electric resistors, and is mainly produced as a silicon pressure sensor. About a pressure-dependent deformation of the membrane and then diffused deformation- dependent resistors results in the formation of an electrical voltage. These pressure sensors are inexpensive to produce and have a comparatively high sensitivity. Although, the materials used for pressure measurement, a strong temperature dependence, but since this effect on all resistors is the same, it can be rendered ineffective by a difference -forming electrical circuit.

A piezoelectric pressure sensor

In a piezoelectric sensor, an electric voltage is generated in a crystal by means of pressure due to charge separation. This is called the piezoelectric effect. By shifting pressure in the interior of the crystal ions, thereby forming electrical charge on the surface is proportional to the force. The charge is converted by a charge amplifier to a proportional electrical tension. The direct measurement of the voltage is not possible because the small charge generated must be very well insulated and can find no electrical capacitance change. Any pressure can be obtained by derivation (short circuit) are the charge set as the zero point of the charge amplifier; be pressure changes measured directly.

Piezoelectric sensors generally measure only forces. If the sensor is used in the pressure measuring technique, it must be converted via a diaphragm of the pressure proportional to a force only.

Advantages of piezoelectric sensors:

• Insensitive to high temperatures,
• No external power supply required
• High sensitivity
• Mechanically very rigid, resulting in only slightly self-oscillations or Nachschwingeffekten
• Suitable for pressure fluctuations at a high frequency to> 100 kHz.

Disadvantages of piezoelectric sensors:

• Can not be used without charge amplifier
• Not suitable for static measurements such as water level or air pressure, because even with a full insulation actually a constant charge over flows of hours.

Pressure sensors in the vacuum region

Reference is made to

• Heat conduction vacuum gauge according to Pirani, measuring range 100 ... 0.1 Pa
• Ionization gauge cold cathode Penning, range about 10-1 ... 10-5 Pa
• Hot cathode ionization vacuum gauges with Bayard -Alpert, range about 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 extended by a ring oscillator circuitry. By varying the pressure, the resistance of the strain gauge changes and consequently the frequency of the ring oscillator is detuned. The reported frequency is directly proportional to the applied pressure.

Pressure sensor with Hall element

It operates on the Hall effect, whereby when pressure is applied, the magnetic field is changed by the Hall element.

Capacitive pressure sensor

Capacitive pressure sensors contain two diffused into a silicon chip capacitors. When pressure is applied, the distances to two opposite sides of a membrane condenser plates, and thereby the capacities of the capacitors are varied in opposite directions. Most of the capacitors are part of an internal amplifier, the output signal depends on the difference of the capacitances.

Inductive Pressure Sensor

Inductive pressure sensors work with an inductive position transducer, which is connected to a diaphragm. Changes in pressure produce a force on the diaphragm and move it. Thus, the position of an iron armature changes in opposite directions in the two coils: In the inductance increases, the other decreases it. The difference can be electrically determined very accurately. This arrangement may also be replaced by a differential transformer.

Other sensors

For the detection of small pressure fluctuations, as occur in the sound pressure, further measurement effects and their applications are listed in sensors of the microphone under the associated keyword.

Commercially available pressure sensors

The on the market sensors output a standardized electrical signal that is by the following device (eg DDC) easy to process. Analogue standard signals can be 0 ... 10 V, 0.5 ... be 4.5 Vrat or 4 ... 20 mA; Furthermore, there are standard digital interfaces, such as I ² C or CAN bus. These pressure sensors are active components, which require a power supply for the processing of the signal. The characteristics of the sensors may be linear or correspond to an exponential function; Disturbances ( for example, temperature) to be compensated, and to linearize the suitable characteristics. Therefore, a distinction today between

• Not compensated pressure sensors (eg, normal bridge circuit piezoresistive pressure sensor, microsystem technology )
• Analog, but calibrated sensors (eg, calibrated bridge circuit piezoresistive pressure sensor, microsystem technology )
• Digital, calibrated and linearized and temperature compensated pressure sensors (eg, calibrated bridge circuit combined with analog -to-digital converter and memory parameters, piezoresistive pressure sensor, microsystem technology )

Restrictions on the application

Pressure sensors are sensitive to overloading. If the measuring range is exceeded, the sensor can be easily damaged irreparably depending on the technology used. For accurate measurements also the correct installation position of the sensor is taken into account. For sensors that measure liquids, it should be ensured that the sensor line is vented. Often, separation of the ( aggressive ) medium from the pressure sensor itself is required in order to protect chemically sensitive pressure sensors from corrosion or contamination.