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 inside a given range. Gullible describes what has to be taken into account.
With a reed-chain level sensor, the guide tube includes a defined number of reed contacts, according to the 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 distance between the contacts
The accuracy of this energy-free level monitoring depends, subsequently, 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 can’t be stretched along the entire length of the guide tube. This is because you can find ?dead bands? at both ends of the tube, i.e. sections that the float does not detect due to 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 an even sensor therefore lies between the two dead bands specified in the info sheet. Within this range, it could be freely defined. However, it really is by no means necessary to utilise the maximum possible measuring range in all applications. The exact adaptation of the amount 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 example, only the lower part of the 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 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 beginning with the sealing face. In cases like this, the guide tube can be adapted to the length of the measuring range.
Note
More info on the RLT series level sensors are available on the WIKA website. When 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?
You can also find out more about float-based level measurement utilizing a reed chain in the next video: