A float-based level sensor with a reed chain is a widely used and comparatively economical solution for continuous level measurement in vessels. Users can define its measuring range flexibly within a given range. This article 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 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 distance between the contacts
The accuracy of this 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 across the entire length of the guide tube. For the reason that there are ?dead bands? at Dangerous of the tube, i.e. sections that the float will not detect due to design constraints (see graphic).
The graph shows how 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. In this range, it can 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 number of contacts to the measuring task also has an economic advantage: The reed chain is probably the priciest components in an even sensor.
Example ? Dry run monitoring
For dry run monitoring in an oil tank of a compressor, for example, only the lower the main guide tube is necessary for the measurement. In this case, top of the measuring point of the application form (100 % mark) is set 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 set beginning with the sealing face. In this instance, the guide tube can be adapted to the length of the measuring range.
Note
More info on the RLT series level sensors can be found on the WIKA website. For those who 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?
You can also find out more about float-based level measurement utilizing a reed chain in the next video: