A float-based level sensor with a reed chain is a trusted and comparatively economical solution for continuous level measurement in vessels. Illegal can define its measuring range flexibly inside a given range. This short 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, based on the measuring range. These are combined to create 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, in turn, on the distance between the individual contacts: small it is, the more accurate the measurement. Each level sensor in WIKA?s RLT series, for instance, 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 does not detect due to design constraints (see graphic).
The graph shows the way 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 is not a must
The measuring range of an even sensor therefore lies between your two dead bands specified in the data sheet. Within this range, it usually 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 amount of contacts to the measuring task also offers an economic advantage: The reed chain is among the priciest components in a level sensor.
Example ? Dry run monitoring
For dry run monitoring in an oil tank of a compressor, for example, 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 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 set starting from the sealing face. In this case, the guide tube could be adapted to the length of the measuring range.
Note
Further information on the RLT series level sensors can be found on the WIKA website. When you 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?
There are also out more about float-based level measurement utilizing a reed chain in the following video: