Fluid Sensors

Jan 3, 2023

Level sensors, pressure sensors, flow sensors, temperature sensors

The optimal monitoring of the process parameters is the essential basis for efficient and resource-saving processes. For this purpose, SICK offers a broad product portfolio of solutions for process control, storage as well as monitoring of liquids, bulk goods, or gases. Our sensors measure parameters like pressure, temperature, filling level and flow.

Pressure measurement for liquids and gas

In many areas of plants and mechanical engineering, machine tool construction, process engineering, manufacturing industry and in the productions and refinement of food and beverages the measuring of the pressure is crucial. 

The measuring method determines the sensor: A distinction is made between absolute pressure sensors, differential pressure sensors and relative pressure sensors. Absolute pressure sensors use a vacuum with a pressure of almost 0 bar as a reference and relative pressure sensors use atmospheric air pressure as a reference, while differential pressure sensors measure the pressure difference between two measuring chambers. Depending on the measuring range, a piezoresistive silicon sensing element or a metal thin-film sensing element is used for the measurement.
The metal thin-film measuring cells are used for measuring ranges of 0 ... 10 bar and higher. These cells consist of a stainless steel diaphragm on which strain gauges are applied. Strain gauges are measuring elements which change their electrical resistance even at slight deformations. The measured change is then converted into a standard output signal such as 4 ... 20 mA or 0 ... 10 V.
The piezoresistive measuring cell is used for sensors with measuring ranges from 0 ... 0.4 bar to 0 ... 6 bar. The operating principle of the piezoresistive silicon measuring cell is similar to the strain gauges. However, the measurement is not based on the change in resistance. When deforming, the silicon chip changes the crystal lattice structure and thus the mobility of the electrons. The silicon chip together with the header is located in a capsule, which is closed with a diaphragm. Inside the capsule is oil, which transfers the pressure from the diaphragm to the chip.
SICK offers a portfolio of electronic pressure transmitters and switches that can be optimally adapted to individual customer requirements due to the intelligent and versatile configuration options. Typical for SICK, the devices combine the use of high-quality material, robustness and precise measurement technology with simple operation and installation.
Find out more about the PBS plus, our multifunctional IO-Link sensor for pressure management, control and monitoring.

Temperature measurement for liquids and gases

Whether it is a question of monitoring operating conditions in plant and machine construction or of the control and regulation of sensitive processes: the reliable and accurate recording of temperature is fundamental in many areas of industry.

The measuring element of our temperature sensors is a platinum measuring resistor, which changes its resistance depending on the temperature. In case of so-called cold conductors, the resistance rises with rising temperature and the other way around. Platinum is used because the change in resistance is almost linear to the change in temperature. Platinum measuring resistors are named according to the material and the nominal resistance at a temperature of 0°C. SICK's temperature sensors are equipped with either a Pt100 or a Pt1000. The measured resistance is then converted into a common electrical output signal such as 4 ... 20 mA, 0 ... 10 V, with the measuring circuit being a 2-wire, 3-wire or 4-wire.
SICK's product range includes screw-in and insertion thermometers as well as temperature switches, which offer high solutions for contact measurement of temperature in liquids and gases.

Solutions for filling level and limit level measurement

The innovative offer includes e.g., guided radar wave sensors, ultrasonic devices, capacitive sensors, devices based on the vibration principle, and various optical technologies. For continuous filling level monitoring, limit level measurement, or both combined, SICK offers numerous solutions with different operating principles for liquids and bulk solids.

  • The continous level sensors for liquids can detect the level of almost all liquids with dielectric constant ≥ 1.8 thanks to the guided radar shaft measuring principle. The free-space radar works in all types of liquids and bulk solids, even under extreme process conditions. Level sensors of the LFR and LBR series based on radar technology are modulated continuous wave radars (=FMCW - Frequency Modulated Continuous Wave). The radar signal is emitted from the antenna and reflected from the surface of the liquid or bulk material. The time measured between transmission and reception is directly proportional to the distance to the surface. Because such short times are difficult to measure accurately, FMCW uses frequency modulation and mixes the transmitted and received signal. The resulting frequency is the difference between the frequencies of the two signals and indirectly corresponds to the time difference, which indicates the distance.

    Another radar technology can be found in our sensors of the LFP series, the so-called Time Domain Reflectometry (TDR). In these sensors the radar pulses are guided along a probe, which is designed as a metal rod or steel cable. When the transmitted radar pulse hits the surface of the liquid, part of this pulse is reflected and travels along the probe back to the receiver module. The time difference between the transmitted and received pulse is proportional to the distance. The advantage of this principle is that possible installations in the tank do not lead to interfering signals, which is already the case when measuring with unguided radar.
  • The capacitive level sensors can detect aqueous as well as oily liquids and measure the process temperature. The sensors of the CFP series work with the MCiM (Multi-capacitive intra-probe measuring) technology developed by SICK. This technology is based on a capacitive operating principle and enables the measurement of liquid levels in plastic tanks without the need for additional internals. The CFP measures the capacity, which is influenced by the dielectric constant of the liquid in which the probe is located. Due to the design of the electrode and counter electrode in the probe, the measurement takes place within the probe. The integrated electrodes allow the calibration of the surrounding medium. The subdivision of the probe allows an exact level measurement independent of the medium. Capacitive impedance spectroscopy provides good measurement performance at a reasonable price for oil and water-based media without buildup.

  • Ultrasonic sensors measure the level of liquids and bulk solids without contact. The sensors of the UP56 series work similar to our sensors based on radar technology. They work with ultrasonic waves and the time-of-flight principle. An ultrasonic pulse is transmitted from the sensor, reflected by the surface of the liquid or bulk material and sent back to the sensor. The time required between sending and receiving the ultrasonic pulse determines the distance between the sensor and the medium.

    Ultrasonic sensor UP56
    Ultrasonic sensor UP56
  • Hydrostatic sensors like the LFH series detect the level of liquids in demanding applications. These sensors are a pressure sensor with piezoresistive measuring cell and inserted into the tank from above, hanging on a cable. The hydrostatic level measurement is a relative pressure measurement, with a vent tube inside the connecting cable providing pressure compensation to the atmosphere. The deeper the sensor is in the liquid, the higher the pressure. For example, the pressure in water increases by approximately 0.1 bar per meter of depth. The level can then be calculated with the aid of the density of the liquid.

    LFH series
    LFH series
  • On the one hand, level sensors work according to the vibronic measuring principle and can therefore detect the level of almost all liquids, whether the liquids are conductive or non-conductive or bulk solids. At SICK, level sensors can be found in the LFV and LBV series. These sensors are equipped with a piezoelectric drive, which makes a tuning fork oscillate in its natural frequency. The natural frequency decreases the more medium is coupled to the spring-mass system of the tuning fork. When this frequency falls below a certain value, the sensor detects the limit level previously determined by the installation position. The GRF18S is a level sensor that uses optical technology to detect the level. The transmitting LED of the sensor sends a light beam, which is returned to the receiving LED by means of a prism. If the prism comes into contact with water or a water-based liquid, the light beam is no longer directed to the receiving LED, but is scattered into the medium and the sensor switches. The optical level sensor can detect both, water and water-based liquids.

Flow measurement technology

Flow sensor technology from SICK is based on innovative transit time measurement methods. This method is based on ultrasonic and laser technology. These non-contact technologies are particularly characterized by their flexible areas of application and their versatility. Whether the quantity measurement is made with analog values or pulse scans as well as the wide variety of media and harsh conditions, SICK sensors work reliably and safely.

  • The flow sensor FTMg (as well as the FTS series) measures the flow by means of the calorimetric measuring principle. The sensor probe is heated and the medium flowing past cools the probe. The temperature drop is proportional to the flow velocity. This means: The higher the temperature drop the faster the medium flow. The FTMg flow sensor can be used to measure compressed air and non-corrosive gases such as argon and helium, carbon dioxide and nitrogen and provides also values for temperature, pressure and energy consumption. 

  • The FFU and DOSIC series sensors use ultrasonic technology. If the ultrasonic signal is transmitted against the flow direction, a longer transmission time (TOFBA) is required. When the ultrasonic signal is transmitted in the direction of flow, the transmission time (TOFAB) is shorter. Thus, the signal is delayed or accelerated by the movement of the liquid. The time difference between TOFBA and TOFAB is proportional to the flow rate.

  • The detection of volumetric flows on industrial conveyor belts can be very demanding depending on the environmental conditions. Dust, moisture and vibrations take their toll on mechanical solutions, such as belt scales. The Bulkscan® flow sensor uses time-of-flight technology to measure the volumetric flow of bulk materials on conveyor belts contactless. Independent of bulk material properties and weather influences, the Bulkscan® generates a reliable volume flow signal from laser transit time and belt speed thanks to multi-echo technology.