Reed level sensor: Defining the measuring range flexibly

A float-based level sensor with a reed chain is really 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 includes a defined amount of reed contacts, with respect 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 based on the liquid level.
Accuracy depends on the length between the contacts
The accuracy of the energy-free level monitoring depends, subsequently, on the distance between your individual contacts: small 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 along the entire amount of the guide tube. The reason being 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) 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. Within this range, it is usually freely defined. However, it is by no means necessary to utilise the maximum possible measuring range in all applications. The exact adaptation of the number of contacts to the measuring task also has an economic advantage: The reed chain is one of the priciest components in a level sensor.
Example ? Dry run monitoring
For dry run monitoring within an oil tank of a compressor, for example, only the lower portion 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 length 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 ? Announcing ? 100 % mark X
Consequently, the measuring range for detecting a maximum level is determined starting from the sealing face. In this instance, the guide tube could be adapted to the length of the measuring range.
Note
Further information on the RLT series level sensors are available on the WIKA website. For those who have any questions, your contact will gladly assist you to.
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 using a reed chain in the following video:

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