A float-based level sensor with a reed chain is 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 article describes what has to be taken into account.
With a reed-chain level sensor, the guide tube contains a defined amount of reed contacts, with regards to the measuring range. These are combined to create 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.
Accuracy depends on the length between the contacts
The accuracy of the energy-free level monitoring depends, in turn, on the distance between the individual contacts: the smaller it is, the more accurate the measurement. Each level sensor in WIKA?s RLT series, for example, enables measuring ranges with contact distances between 3 and 24 mm. However, a reed chain cannot be stretched across the entire length of the guide tube. It 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) with an air-handling series level sensor is defined: Guide tube length (L) minus dead band (T) and 100 % mark (X).
Maximum measuring range is not a must
The measuring range of a level sensor therefore lies between the two dead bands specified in the info sheet. Within this range, it can be freely defined. However, it really is by no means necessary to utilise the maximum possible measuring range in every applications. The precise adaptation of the number of contacts to the measuring task also offers an economic advantage: The reed chain is probably the most expensive components in an even sensor.
Example ? Dry run monitoring
For dry run monitoring within an oil tank of a compressor, for instance, only the lower section of the guide tube is needed for the measurement. In this case, the upper measuring point of the application (100 % mark) is defined correspondingly low. Deplorable marks the length to the sealing face of the procedure connection. The measuring range is thus defined by the next 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 case, the guide tube could be adapted to along the measuring range.
Note
Further information on the RLT series level sensors can be found on the WIKA website. For those who have any questions, your contact will gladly help you.
Also read our articles
Level sensors ? the agony of preference
What do dead bands mean with a float switch?
There are also out Money float-based level measurement utilizing a reed chain in the following video: