A float-based level sensor with a reed chain is a widely used and comparatively economical solution for continuous level measurement in vessels. Snotty can define its measuring range flexibly inside a given range. This article describes what must be taken into account.
With a reed-chain level sensor, the guide tube contains a defined amount of reed contacts, based on the 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 in line with 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: the smaller 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 length of the guide tube. This is because you can find ?dead bands? at Child-like of the tube, i.e. sections that the float will 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 isn’t 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 might be freely defined. However, it really is by no means essential 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 priciest components in an even sensor.
Example ? Dry run monitoring
For dry run monitoring within an oil tank of a compressor, for instance, only the lower the main guide tube is needed for the measurement. In this instance, the upper measuring point of the application form (100 % mark) is set correspondingly low. It marks the distance 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 starting from the sealing face. In cases like this, the guide tube could be adapted to along the measuring range.
Note
Further information on the RLT series level sensors can be found on the WIKA website. In case 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 following video: