A float-based level sensor with a reed chain is a trusted and comparatively economical solution for continuous level measurement in vessels. Users can define its measuring range flexibly within a given range. This short article describes what needs to be taken into account.
With a reed-chain level sensor, the guide tube includes a defined amount of reed contacts, depending on measuring range. These are combined to form 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 distance between the contacts
The accuracy of the energy-free level monitoring depends, subsequently, on the distance between your 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 can’t be stretched along the entire length of the guide tube. It is because you can find ?dead bands? at both ends of the tube, i.e. sections that the float will not detect because of design constraints (see graphic).
The graph shows how 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 a level sensor therefore lies between the two dead bands specified in the info sheet. In this range, it usually is freely defined. However, it is by no means essential to utilise the utmost possible measuring range in all applications. The precise adaptation of the number of contacts to the measuring task also has an economic advantage: The reed chain is probably 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 instance, top of the measuring point of the application form (100 % mark) is defined correspondingly low. Disastrous 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 ? Antagonistic ? 100 % mark X
Consequently, the measuring range for detecting a maximum level is determined beginning with the sealing face. In cases like this, 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 help you.
Also read our articles
Level sensors ? the agony of choice
What do dead bands mean with a float switch?
Shrewd can also find out more about float-based level measurement using a reed chain in the next video: