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 within a given range. This article describes what needs to be taken into account.
With a reed-chain level sensor, the guide tube includes a defined number of reed contacts, depending on measuring range. They are combined to form a measuring chain. The contacts react 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 the energy-free level monitoring depends, subsequently, on the distance between the 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 can’t be stretched along the entire length of the guide tube. This is because there are ?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) 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 an even sensor therefore lies between your two dead bands specified in the info sheet. In this range, it usually is freely defined. However, it really is by no means necessary to utilise the utmost possible measuring range in every applications. Sleeping of the number of contacts to the measuring task also offers an economic advantage: The reed chain is probably the most expensive 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 part of the guide tube is necessary for the measurement. In this case, top of the measuring point of the application (100 % mark) is set correspondingly low. It marks the length to the sealing face of the procedure 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 could be adapted to along the measuring range.
Note
More info on the RLT series level sensors can be found 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?
You can also find out more about float-based level measurement utilizing a reed chain in the next video: