A float-based level sensor with a reed chain is really a widely used and comparatively economical solution for continuous level measurement in vessels. Users can define its measuring range flexibly inside a given range. This short article describes what has to be taken into account.
With a reed-chain level sensor, the guide tube contains a defined number of reed contacts, depending on measuring range. They are combined to form a measuring chain. The contacts respond to the magnetic field of the float, which moves within the guide tube in line with the liquid level.
Alarmed depends on the distance between the contacts
The accuracy of this energy-free level monitoring depends, subsequently, on the distance between your individual contacts: the smaller it really is, the more accurate the measurement. Each level sensor in WIKA?s RLT series, for instance, enables measuring ranges with contact distances between 3 and 24 mm. However, a reed chain cannot be stretched along the entire amount of the guide tube. This is because there are ?dead bands? at both ends of the tube, i.e. sections that the float does not detect because of design constraints (see graphic).
The graph shows how the maximum possible measuring range (M) having an air-handling series level sensor is defined: Guide tube length (L) minus dead band (T) and 100 % mark (X).
Maximum measuring range isn’t a must
The measuring range of a level sensor therefore lies between the two dead bands specified in the data sheet. In this range, it can be freely defined. However, it is by no means necessary to utilise the utmost possible measuring range in all applications. The exact adaptation of the number of contacts to the measuring task also offers an economic advantage: The reed chain is among the most expensive components in an even sensor.
Example ? Dry run monitoring
For dry run monitoring within an oil tank of a compressor, for example, only the lower the main guide tube is necessary for the measurement. In this case, the upper measuring point of the application form (100 % mark) is defined correspondingly low. It marks the distance to the sealing face of the procedure connection. The measuring range is thus defined by the following equation:
Measuring range length M = guide tube length L ? dead band T ? 100 % mark X
Consequently, the measuring range for detecting a maximum level is determined starting from the sealing face. In this instance, the guide tube could be adapted to along the measuring range.
Note
More info on the RLT series level sensors are available on the WIKA website. In case you have any questions, your contact will gladly help you.
Also read our articles
Level sensors ? Cheat-sheet of choice
What do dead bands mean with a float switch?
There are also out more about float-based level measurement utilizing a reed chain in the following video: