Reed level sensor: Defining the measuring range flexibly

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. This article describes what needs to 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 create a measuring chain. The contacts respond to the magnetic field of the float, which moves within the guide tube good liquid level.
Reckoning depends on the length between the contacts
The accuracy of the energy-free level monitoring depends, in turn, on the distance between your individual contacts: small it really 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 amount of the guide tube. This is due to you can find ?dead bands? at both ends of the tube, i.e. sections that the float does 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 isn’t a must
The measuring selection of a level sensor therefore lies between your two dead bands specified in the info sheet. In this range, it is usually freely defined. However, it really is by no means essential to utilise the maximum possible measuring range in every applications. The exact adaptation of the number of contacts to the measuring task also offers an economic advantage: The reed chain is one of 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 area of the guide tube is needed for the measurement. In this case, top of the measuring point of the application form (100 % mark) is set 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 determined beginning with the sealing face. In this case, the guide tube can be adapted to along the measuring range.
Note
More info on the RLT series level sensors are available on the WIKA website. Assuming you have any questions, your contact will gladly help you.
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 using a reed chain in the following video:

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