The range of liquid level sensors of First Sensor are based on pressure and optoelectronic measurement technologies and offer point level detection as well as continuous level measurement. Important factors which influence the appropriate sensor selection include the size, geometry, material of the tank/container, existing tank installations such as e.g. agitators as well as the type of process media.
A fluid column generates a hydrostatic pressure at the bottom of a tank or reservoir which is a function of the height and density of the liquid. A pressure sensor can measure the hydrostatic pressure and the liquid level can be determined after calibration to the specific density of the process media. In vented tank or open container applications gage pressure sensors which measure relative to atmospheric pressure are used to avoid false results due to barometric pressure fluctuations. In pressurised tanks the difference between the pressure at the bottom of the tank and the gas pressure above the liquid surface has to be measured. This can be done by two individual gage pressure transmitters or one differential pressure sensor.
First Sensor offers two different versions of hydrostatic liquid level transmitters: submersible level probes for immersion into the liquid or sensors with threaded ports which will be externally mounted through the tank wall.
Figure 1: hydrostatic level measurement
Optoelectronic level switches contain a light source (LED) and a light receiver (phototransistor). In air or gas light emitted from the LED is reflected back to the phototransistor in the tip of the sensor by total internal reflection. With liquid surrounding the sensor tip light is refracted into the liquid and a weaker signal reaches the phototransistor, which actuates a switching output.
First Sensor optical point level sensors include basic devices with analog phototransistor outputs, models with microprocessor compatible TTL signals, or high power switches with transistor outputs. With their housings of plastic (either polysulphone or TROGAMID®) or stainless steel the sensors provide excellent media compatibility.
Figure 2: optoelectronic level measurement
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