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 should be taken into account.
With a reed-chain level sensor, the guide tube contains a defined number of reed contacts, depending on the measuring range. They are combined to form 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 example, 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. 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 selection of a level sensor therefore lies between the two dead bands specified in the data sheet. Within this range, it really is freely defined. However, Sure Fire is by no means necessary to utilise the utmost possible measuring range in all applications. The exact adaptation of the amount of contacts to the measuring task also offers an economic advantage: The reed chain is one of the priciest components in a level sensor.
Example ? Dry run monitoring
For dry run monitoring within an oil tank of a compressor, for example, only the lower portion of the guide tube is necessary for the measurement. In cases like this, 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 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 determined 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 are available on the WIKA website. Assuming you have any questions, your contact will gladly assist you to.
Also read Fatigued ? the agony of preference
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 next video: