Frequency sweep technology:
An electrode integrated in the tip of the sensor together with the environment forms a capacitor. Depending on its dielectric constant (DC value), the medium determines the capacity value. This, together with a coil in the sensor electronics, creates a resonance circuit. Depending on the measured resonance frequency and the programmable trigger thresholds, the switching signal is triggered.
Hydrostatic level measurement:
About half of all process automation level measurements in tanks is performed with pressure sensors. Small pressure measurement ranges and simultaneously high accuracy are required. The hydrostatic pressure per meter of measuring height is only approximately 100 mbar for water or water-based media. The density of the medium must be known and accordingly offset or calibrated for accurate level detection. If the tank is unpressurized, a single relative pressure sensor on or near the tank bottom is sufficient. In pressurized tanks, the information must be calculated via the so-called head pressure in the tank. A second pressure sensor is required for this. Accurate measurement becomes increasingly challenging as the measuring height decreases and the head pressure increases because the hydrostatic pressure information formed by the difference between both the two values continuously decreases according to the selected pressure range of the sensor.
Ultrasonic level measurement:
Ultrasonic sensors are based on the measured propagation time of the ultrasonic signal. They transmit high-frequency sound waves which are reflected on the surface of the medium to be measured. The media can be liquid, granular or in a powder form. Ultrasonic sensors clearly identify transparent and other objects where optical sensors might fail.
With ultrasonic sensors for continuous level measurement, the measured distance value from the sensor to the media surface is output as a voltage value. The output current or the output voltage is proportional to the level or distance of the media surface.
Capacitive level measurement:
A capacitive sensor basically operates like an open condenser. An electrical field is formed between the measuring electrode and the GND electrode. If a material with a dielectric constant εr greater than air enters the electrical field, the capacity of the field increases depending on the εr of this material. The electronics measure this capacity increase, the generated signal is conditioned during subsequent signal processing and causes output switching at a corresponding magnitude.
Photoelectric level measurement:
Level and leakage sensors with media contact: The operating principle of optical level sensors is based on the alteration of the critical angle for total internal reflection, depending on whether the sensor tip is surrounded by liquid or air. If the sensor tip is surrounded by a liquid, the light beam is deflected into the liquid and the sensor output changes its switching status. The liquid medium may be electrically conductive, turbid or clear. The same operating principle is used in the leakage sensors.
Level sensors wihtout media contact: The level sensors for tube/ascending pipe installation operate according to a similar principle. For example, the FFDK 16 also utilizes the light-deflecting property of liquids. In a state without liquid, the transmitted light directly meets the receiver. If liquid enters the detection area, a part of the transmitted light is deflected so that less light meets the receiver. This variation in light intensity can be analyzed by the sensor accordingly. In the fiber optic version FSL 500C6Y00, this works exactly the other way around. In a state without liquid, no light meets the receiver. A part of the transmitted light is deflected to the receiver only after liquid enters the detection area of the array. This variation in light intensity can be analyzed at the receiver. The advantage of array design with a monitoring area of approx. 5 mm is that interference caused by foam and small air bubbles can be suppressed with a powerful fiber optic sensor.
Potentiometric level measurement:
A potentiometer is an angle sensor with ratiometric operating principle. The mathematical relationship provides the independence of the digital output at the AD converter from the drive current UP and the resistance value RP.
The implementation of this principle uses for the virtual slide way an electrically conductive pipe which is supplied by a potential-free voltage source UP. A linear voltage drop is produced by the current flow along this pipe. The medium in which a corresponding dipole field develops for the equipotential surface of the conductive tank wall or an auxiliary electrode forms the slide. The two equivalent substitute resistors RM1 and RM2 represent this as a 1:1 voltage divider. The unsymmetrical voltage between the tank or auxiliary electrode and one pole of the voltage source for the measuring signal UM, which is linear to the level and radiometric to the half the amplitude of the voltage source UP based on the pipe length.