A float-based level sensor with a reed chain is a trusted and comparatively economical solution for continuous level measurement in vessels. Users can define its measuring range flexibly within a given range. This article describes what should be taken into account.
With a reed-chain level sensor, the guide tube includes a defined amount of reed contacts, according to the measuring range. They are combined to create a measuring chain. The contacts react to the magnetic field of the float, which moves within the guide tube good liquid level.
Accuracy depends on the distance 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 instance, enables measuring ranges with contact distances between 3 and 24 mm. However, a reed chain cannot be stretched along the entire length of the guide tube. Simply because Special can find ?dead bands? at both ends of the tube, i.e. sections that the float will 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 is not a must
The measuring selection of a level sensor therefore lies between your two dead bands specified in the info sheet. Within this range, it is usually freely defined. However, it really is by no means necessary to utilise the utmost possible measuring range in all applications. The precise adaptation of the number of contacts to the measuring task also has an economic advantage: The reed chain is among the most expensive components in an even sensor.
Example ? Dry run monitoring
For dry run monitoring in an oil tank of a compressor, for example, only the lower the main guide tube is necessary for the measurement. In this instance, the upper measuring point of the application form (100 % mark) is defined correspondingly low. It marks the distance to the sealing face of the process 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 set beginning with the sealing face. In cases like this, 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. Assuming you 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 more about float-based level measurement using a reed chain in the following video: