Find out more about the function, structure, types and areas of application of pressure sensors
How does a pressure sensor work?
A pressure sensor is essential for accurate pressure measurement. Baumer pressure sensors are based on resistive and piezoresistive pressure measurement. Changes in electrical resistance are measured and converted into an electrical signal. The basis for this is a metal, ceramic or piezoresistive material whose electrical resistance changes depending on the pressure acting on the material.
Pressure measurement is one of the most important and widely used technologies for monitoring and controlling machines and systems in process technology. Baumer offers a comprehensive portfolio of pressure sensors for a wide range of applications.
Monitoring or controlling of physical processes with respect to absolute pressure, for example steam pressure
Relative:
Measurement of pressure with respect to atmospheric pressure
Sensor with relative compensation
“Open” sensor, therefore influenced by its surroundings
Monitoring or controlling of physical processes with respect to ambient conditions
Ex. underpressure: holding force through vacuum process for workpiece handling
Ex. overpressure: hydrostatic level measurement in ventilated tanks
Compound:
Measurement of gauge or differential pressure from negative to positive values
Differential:
Measurement of the difference between two pressures
Structure and types of pressure sensors
Baumer pressure sensors basically work with two different pressure measurement methods. The method to be used depends on the intended use of the sensor.
Resistive pressure measurement: In resistive pressure measurement, the deformation of a thin metal body or a ceramic membrane leads to a change in electrical resistance. When the metal strip is stretched by pressure, it becomes longer and its electrical resistance increases. When the metal strip is compressed, its cross-sectional area increases and its electrical resistance decreases. The change in resistance is then recorded as an electrical signal and converted into pressure.
Piezoresistive pressure measurement: This principle is also based on the change in electrical resistance when a material is deformed. However, this measurement principle uses a piezoresistive material. This material has the property that the mechanical stress that occurs during deformation (stretching or compression) also causes a change in electrical conductivity. The change in resistance is greater than with resistive pressure measurement.
In addition to the types of pressure measurement and pressure sensors listed here, piezoelectric, capacitive and inductive pressure sensors are also mentioned, as well as vacuum pressure sensors, frequency analogue pressure sensors and pressure sensors with Hall element.
In semiconductor materials, the change in the specific resistance and thus of the signal results from the variable mobility of the electrons in the crystalline structure. This mobility is affected by the mechanical load. The sensitive silicon chip and the process medium are separated by a stainless steel membrane (encapsulation). Depending on the application paraffin or silicone oil is used as transmission fluid for internal pressure transmission.
In semiconductor materials, the change in the specific resistance and thus of the signal results from the variable mobility of the electrons in the crystalline structure. This mobility is affected by the mechanical load. The sensitive silicon chip and the process medium are separated by a stainless steel membrane (encapsulation). Depending on the application, paraffin or silicone oil is used as transmission fluid for internal pressure transmission.
Transmitters with piezo-resistive silicon technology stand out due to their high measuring accuracy and long-term stability. Thanks to their fully welded housing, they are durable and can also be used in potentially explosive areas (ATEX).
They are even suitable for small measuring ranges, particularly for hydrostatic level measurements from a height of 0.5 m.
The base body is made of a ceramic monolith, onto whose membrane the resistors are imprinted on the back. On this side, the ambient air pressure acts as a reference pressure. Therefore only relative pressure measurement is principally possible. Ceramic measuring cells stand out due to their good long-term stability and corrosion resistance. Because ceramic cannot be welded to the process connection, sealing is required for media separation. In ceramic thick-film technology four resistors are interconnected to create a Wheatstone bridge. During pressurization, the resistors are exposed to the highest strain in the middle of the diaphragm and the greatest compression in the edge areas. In ceramic cells, the measuring membrane is simultaneously the separating membrane from the medium. No internal transmission fluid is required.
The measuring film is located between a thin ceramic membrane disk and a ceramic base body. The required room for the bending of the membrane is created by the specifically generated gap. This created volume can be ventilated or evacuated with ambient pressure, thus enabling relative or absolute pressure measurement. Ceramic measuring cells stand out due to their good long-term stability and corrosion resistance. Because ceramic cannot be welded to the process connection, sealing is required for media separation. In ceramic thick-film technology four resistors are interconnected to create a Wheatstone bridge. During pressurization, the resistors are exposed to the highest strain in the middle of the diaphragm and the greatest compression in the edge areas. In thin-film cells, the measuring membrane is simultaneously the separating membrane from the medium. No internal transmission fluid is required.
The base body is made of stainless steel. The resistance structure is produced by photolithography. Thin-film measuring cells stand out due to their excellent resistance to pressure peaks and bursting pressure. Even extremely high pressures can be measured – even when exposed to high shock and vibration loads. In metal thin-film technology four resistors are interconnected to create a Wheatstone bridge. During pressurization, the resistors are exposed to the highest strain in the center of the diaphragm while the strongest compression is present in the edge region. In thin-film cells, the measuring membrane is at the same time the separating membrane from the medium. No internal transmission fluid is required. As a rule, thin-film technology is only offered for relative pressure measurement, because creating a vacuum on the back of the membrane requires extensive effort constructionally.
Suitability for gas applications
Pressure in sterilization processes
Hot steam is used to sterilize devices and equipment. Small elements, such as sensors (PBMH autoclavable), can be sterilized in a suitable chamber (autoclave). In larger installations, hot steam is fed through the system, which is described as “Sterilization in place” (SIP). Accordingly, a sensor must be designed to be robust, although the signal is generally not transmitted during the sterilization process. It must survive the prevailing temperature and pressure for the relevant time span (e.g., 134 °C at more than 3 bar for 30 min). In physical terms, pressure and temperature are coupled directly with each other, which is shown in the saturated steam curve.
Pressure of saturated steam with respect to temperature
Baumer PBMx and PFMx pressure sensors are ideal for controlling the sterilization process. They provide accurate values even in the event of fast changes in temperature, and thus control the process reliably by monitoring pressure, which leads to the corresponding temperature.
Definition of the pressure ranges
Explanation of terminology and relationships
Precision: This describes the possible deviation of a single measurement from the average of many measurements and can be interpreted as a dispersion circle. High precision: small dispersion circle, low precision: large dispersion circle.
Accuracy: This describes the distance (offset) of the average value of many measurements from the true value. High accuracy: small offset, low accuracy: large offset.
Standard error of measurement: This information is obtained through the best fit straight line, BFSL, and describes precision (dispersion circle).
Maximum error of measurement: This contains the standard error of measurement and the offset of a sensor.
Temperature dependence
The application may deviate from the reference temperature (e.g. 20 °C), so that the standard or the maximum error of measurement must be regarded in a differentiated manner.
The “Temperature Coefficient zero point” (TC zero point) describes the temperature dependence of the zero point and thus the influence on accuracy.
The “Temperature Coefficient span” (TC span)describes the temperature dependence of the measuring range and thus the influence on precision, that is, the standard error of measurement.
TC zero point and TC span together describe the temperature dependence on the maximum error of measurement.
Temperature dependence of the maximum error of measurement
In many cases, a temperature-stable sensor with lower initial accuracy is to be preferred to a more unstable sensor with higher initial accuracy if the operating temperature deviates from the reference temperature (e.g. 20 °C).
Error indication
Baumer specifies the “maximum error indication”, i. e. statistically, 99.7% of the sensors comply with the specification. Some competitors enter the “typical error indication”, in which 32% of the products do not comply with the specification.
