The WIKA portfolio of measuring instruments for level monitoring encompasses both continuously measuring instruments and level switches. Depending on the application, different measurement principles are used for level monitoring:
The WIKA portfolio of measuring instruments for level monitoring encompasses both continuously measuring instruments and level switches. Depending on the application, different measurement principles are used for level monitoring.
Level sensors achieve continuous level measurement by detecting the filling height of a liquid or material in a vessel. The measured value is then transmitted in various absolute and relative values and measurement units. Some level sensors provide the value as a percentage, whereas others output the value as voltage or current. The level value can either be displayed directly on site or integrated into a control system. In the process industry, the most common applications for continuous level measurement are tanks and mobile containers.
Different measurement procedures are used, on which the level sensors are based. The level measurement techniques used vary depending on the application. The following measurement principles, among others, can be used for continuous level measurement:
For level monitoring, the WIKA product portfolio includes bypass level indicators, submersible pressure sensors, float switches, glass level gauges, optoelectronic switches and continuous measurement with floats. If the liquid is chemically aggressive, there is also suitable level measurement technology for this. The most important factor in the choice of measurement technology is whether continuous measurement or limit level detection is required.
A permanent magnet built into the float is lifted by the rising liquid level in the water tank and, through its magnetic field, actuates the resistance measuring chain built into the guide tube. The entire assembly corresponds to a 3-wire potentiometer circuit. The measured resistance signal is proportional to the level.
With magnetostrictive level monitoring, the position of the float can be determined very precisely. Instead of mechanical connections, magnetostrictive transmitters use the speed of torsional waves along a wire to locate the float and report its position.
For indirect level monitoring, pressure measuring instruments are used. Depending on the application, either a submersible pressure sensor is submersed into the tank or another pressure measuring instrument is attached to the bottom of the exterior of the tank and exposed to pressure of the tank contents through an opening in the bottom of the tank. With closed tanks, for the level monitoring, the gas pressure existing above the liquid in the tank must also be measured and be subtracted from the hydrostatic pressure. This can be carried out in two ways: either by using two independent pressure measuring instruments and then generating the pressure difference in the downstream control unit, or by using special differential pressure measuring instruments with two process connections designed for this application. Pressure measuring instruments for level monitoring are characterised above all by their media resistance and their relatively small measuring ranges. A further requirement for submersible pressure sensors is that the medium must not enter neither the cable nor the probe itself, even at submersion depths of several hundreds of metres. In hazardous applications, such as boreholes or refineries, the measuring instrument must feature the corresponding ignition protection type. For applications in wells, shafts and boreholes, the slimmest possible design is required and there are also high demands on the robustness of the (usually very long) cable.
If not every filling height is important, as it is with continuous level measurement, a level switch can also be used for limit level detection. Here, only when a certain filling height in a vessel is reached - the limit level - is it recorded. Level limit switches feature a mechanical switch contact, in electronic design mainly a PNP or NPN transistor output, via which the measured value is output.
A wide range of measuring technologies are also used for level switches. The following measuring principles can be used for limit level detection:
The most important criterion for selecting the right level measuring instrument is therefore the area of application. If the level measurement only consists of detecting the limit levels in a tank, e.g. dry running (empty tank) or overfilling (full tank), a level limit switch is sufficient. However, if the contents of a tank are to be monitored fully, i.e. the current level between 0 and 100 % level is to be measured, a level sensor is used to give continuous level measurement. The WIKA submersible pressure sensors, also called level probes, are suitable for this purpose. With combined electronics, these can transmit both analogue signals and switching signals.