How Digital Pressure Switches & Sensors Work
In a lot of systems, the mechanical pressure switches are replaced by electronic versions. These digital pressure switches offer a variety of additional features like a digital display and the possibility to adjust the switch points electronically. Digital pressure switches are mainly based on electronic pressure transmitters. This provides the switch with the complete functionality of a transmitter. Simple control tasks can be performed using the integrated pressure switch. The switching points can be set individually using the digital display or an I/O link.
Electronic pressure measurement principles
Electronic pressure sensors are required to detect the pressure (changes) and convert them with high accuracy to an electrical signal. The electrical signal is used to represent the magnitude of the pressure (changes). The main four principles are discussed below.
Resistive pressure measurement
Resistance pressure measurement uses the deflection-dependent resistance of electrical conductors. This principle makes it possible to correlate the pressure-dependent deflection with the electrical resistance. For this purpose, the following relation applies:
R: Electrical resistance
A: Cross-sectional area
If the length of a conductor increases due to a tensile force, the cross-sectional area decreases. The electrical resistance is inversely proportional to the cross-sectional area. This means that the electrical resistance rises as the cross section decreases.
Pressure sensors that use this principle usually have a diaphragm on which four metallic strain gauges are placed. They are distributed over the elongation and compression areas. The resistance thus changes according to the deflection (compression or elongation) of the diaphragm. For accurate measurements, Wheatstone measuring bridges are used.
Figure 1: Resistive pressure measurement principle
Piezo-resistive pressure measurement
The piezo-resistive pressure measurement principle is similar to the previously mentioned resistive measurement principle. The piezo-resistive principle however makes use of semiconducting materials. In addition to a change in the cross-sectional area, their resistivity will change due to deformation, this is called the piezo-resistive effect. This effect is insignificant for conducting metals but has a significant effect in semiconductors such as silicon. These semiconducting materials are integrated within the diaphragm. This allows both the diaphragm and the strain gauges to be of the same semiconducting material. Usually, four strain gauges are embedded and connected to a Wheatstone bridge.
The semiconducting materials are not suitable for a wide range of media. For this reason, the censoring material must be protected. This is done by transferring the pressure indirectly to the semiconductor using a metal membrane and a transmission medium such as oil.
An advantage of piezo-resistive pressure sensors is that they can be used for very low pressure ranges. A disadvantage is the strong dependancy on temperature and manufacturing related variation. This requires that each sensor has to be temperature compensated.
Capacitive pressure measurement
This principle is based on the capacitance measurement of a capacitor. The capacitance of a dual-plate capacitor is calculated using the following relation:
C: Capacitance of the dual-plate capacitor
d: Plate separation
A: Cross-sectional area
This principle is realized using two plates. Deflection due to pressure changes results in a change of separation between the plates. Due to the fact that the area and the permittivity are constant, the capacitance only depends on the plate separation. This results in a high sensitivity of the sensor. This makes capacitive pressure sensors suitable or very low pressure ranges (one-digit mbar range). A high overload safety for the sensor is ensured since the moving diaphragm can be deflected up to the fixed plate.
Figure 2: Capacitive pressure measurement principle
Piezo-electric pressure measurement
Note, this is a different principle than the piezo-resistance pressure measurement. This principle is based on the Piezo-electric effect found in certain non-conductive crystals like monocrystalline quartz. Due to an applied pressure or tensile force, the opposed surfaces of the crystal are respectively charged positive and negative. This is caused by the induced displacement in lattice elements which are electrically charged. The displacement causes an electric dipole moment responsible for the surface charges.
The voltage difference between the surface is measurable and can be converted to a pressure change. Piezo-electric pressure measurement is only suitable for dynamic pressure measurement. These sensors are restricted to specific applications.
Figure 3: Piezo-electric pressure measurement principle
The previously mentioned measurement principles can be found in the three main sensor types. For resistive pressure measurement ceramic thick-film and metal thin-film sensors are the most commonly used. The third sensor type is the piezo-resistive pressure sensor.
Metal thin-film sensor
Both the diaphragm and main housing of a metal thin-film sensor are constructed out of stainless steel. The strain gauges, insulation layers, conducting paths and compensating resistors are applied to the side of the diaphragm that is not in contact with the medium.
Thin-film sensors are constructed under cleanroom conditions and sometimes vacuum conditions. Thin-film sensors are very stable because of the materials used. Furthermore, they are very shock and vibration resistant. This also makes them very suitable for dynamic loads. Because of the weldable material, thin-film sensors can be welded onto the system connection without any further use of sealing materials. Due to the ductility of steel, the sensor has a low overpressure range but a high burst pressure.
Ceramic thick-film sensor
The thick-film sensor’s main body and diaphragm are constructed out of ceramic. The most common ceramic used is Aluminum oxide (Al2O3) because of its processability and stability. The four strain gauges are applied to the diaphragm side which is not in contact with the medium. They are applied as a thick-film paste, hence the name. The paste is burned into the diaphragm at a high temperature after which it receives a protective coating. Ceramic thick-film sensors are fabricated in cleanroom conditions. Ceramic is very corrosion-resistant, the additional seal that is required however is not resistant against all media. It is a brittle material, which leads to a lower burst pressure with respect to thin-film sensors.
Piezo-resistive sensors have a far more complex structure than the previously mentioned sensors. The sensor itself is fabricated out of a silicon chip. This silicon chip contains the diaphragm in which piezo-resistive resistors are placed. The surface area of these chips is a few square millimeters, which is a lot smaller than the thin or thick-film diaphragms.
The piezo-resistive chip must be encapsulated due to its susceptibility to ambient influences. This is done by installing the chip into a stainless steel case. This case is sealed using a thin stainless steel diaphragm. The free volume within the case is filled with transmission fluid. This transmission fluid transfers the pressure from the outer steel diaphragm to the inner sensor diaphragm. Special displacements bodies are used to minimize the transfer fluid’s thermal expansion influence on the measurement.
A header is used to mount and electrically connect the sensor chip. The header can be welded hermetically to the housing. Bond wires are used to connect the sensor to the pins. A ventilation tube is located in the center of the header and leads to the rear side of the sensor diaphragm. Absolute pressure can be measured when the chamber behind the sensor is evacuated and the ventilation tube is closed. In the case of an opened ventilation tube, the relative pressure is measured. The ventilation tube is connected to the environment through the outer casing or a cable that is ventilated. The ventilation tube needs to be protected from contamination and especially moisture.
Figure 4: Piezo-resistive sensor
Chart with applications and sensor technologies
The chart below shows different measurement applications with the sensor technologies that are best suited. Digital pressure transmitters are cost-effective if a large amount of monitoring points are needed and controlled.