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 should 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 good liquid level.
Accuracy depends on the length between the contacts
The accuracy of this energy-free level monitoring depends, in turn, on the distance between the individual contacts: the smaller it is, the more accurate the measurement. pressure gauge octa in WIKA?s RLT series, for instance, enables measuring ranges with contact distances between 3 and 24 mm. However, a reed chain can’t be stretched across the entire length of the guide tube. For the reason that there are ?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 range of an even 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 maximum possible measuring range in all applications. The precise adaptation of the amount 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 instance, only the lower part of the guide tube is needed for the measurement. In cases like this, the upper measuring point of the application (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 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 beginning with the sealing face. In cases like this, the guide tube can be adapted to along the measuring range.
Further information on the RLT series level sensors are available on the WIKA website. Assuming you have any questions, your contact will gladly assist you to.
Also read our articles
Level sensors ? the agony 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: