Float Magnetic Level Measurement
Level measurement using the technique of a float which has a series of magnets embedded in it is an excellent, very accurate method. The float can either be internal in the vessel or external on a bridle ( a bridle is a take off point which generally has three connections into a vessel which connect to a chamber which contains the float). See ICEWEB technical information sheet designing for a representative level, this provides valuable information how to avoid the fundamental pitfalls associated with level measurement. These type of units are very cost and space effective in that a level transmitter, gauge and switches can all utilise the same float. In addition multiple floats can be utilised for measuring a primary and interface level.
This technology is demonstrably more accurate than DP cells or displacers where SG changes can cause extremely large errors. As floats sit on the top of the liquid rather than measuring the weight of a fixed column, the only error related to SG is the difference in how far the immersion depth of the float changes.
In design it is essential that the SG of the fluid is accurately specified, if it is not either errors can result or in the worst circumstance the float can sink, this is not a good experience for operators and may give the poor design engineers heart failure!! Also it is worthwhile ensuring that the SG does not change dramatically under operating conditions. This scenario has the potential to again give one that "sinking" feeling! Therefore we suggest that you take extreme care in ensuring that your process details are correct. A good rule of thumb is to specify 10% lower SG all applications ie., 0.9 on water at an SG of 1.0 and 0.54 on condensate which has an SG of 0.6.
Internal units are most accurate, however they do have some disadvantages.
- Simplified process takeoffs with minimised connections.
- Transmitter, gauge and switches utilising the same hardware/float.
- Ease of mounting.
- Minimal errors due to SG changes.
- Small bore takeoffs not required, thus compression fittings/ manifolds etc eliminated.
- Gauge continues to operate in event of electrical power failure.
- Transmitter/switches can be calibrated/adjusted whilst system still on line.
- High integrity unit, essentially a piece of pipe.
- Easy to retrofit into existing level chambers.
- Specifications critical for correct float selection.
- Unsuitable for dirty applications and processes where magnetite may be produced.
- Internal units float inaccessible without shutdown.