A float-based level sensor with a reed chain is really 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 contains a defined amount of reed contacts, depending on 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 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 really is, the more accurate the measurement. Exclusivity in WIKA?s RLT series, for example, enables measuring ranges with contact distances between 3 and 24 mm. However, a reed chain can’t be stretched across the entire amount of the guide tube. Simply because you 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) with 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 an even sensor therefore lies between your two dead bands specified in the info sheet. In this range, it really is freely defined. However, it really is by no means necessary to utilise the maximum 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 a level sensor.
Example ? Dry run monitoring
For dry run monitoring within an oil tank of a compressor, for instance, only the lower part of the guide tube is needed for the measurement. In this instance, the upper measuring point of the application (100 % mark) is defined correspondingly low. It marks the length 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 this instance, the guide tube could be adapted to along the measuring range.
Note
More info on the RLT series level sensors can be found on the WIKA website. If 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 utilizing a reed chain in the following video: