Basics of industrial level measurements 3

Level measurement techniques in industry maybe has the most diverse and multifaceted methods. From Silos where solids level are measured to single or dual level measurement techniques. Some we have already discussed in our previous articles and here one of the most important one is the displacer type level measurement. Very common since the beginning of industrial revolution, initially it was used with pneumatic transmitters and still is in use in the same fashion in many installations.

Displacer is normally placed in an adjoining chamber or level troll, along with sensing devices which convert the motion into an equivalent level signal (pneumatic or electrical) for remote indication alarm or control. There is isolation from the main process so it can be maintained, as well as to check its calibration when required.

The level in the chamber corresponds to the process level and the displacer will be immersed in the liquid, as the level rises resulting in the weight loss and causing the LVDT core or torque tube to reposition itself, and the more there is liquid in the chamber, the more weight loss will occur.

Torque tubes were employed in earlier measuring devices, and are still in use, which converted this motion due to up thrust into a rotary motion, driving a potentiometer, or other transducing device. The pneumatic version used to reposition the flapper against the nozzle and varied the output generated due to this movement into equivalent pneumatic signal. Whatever the case the basic sensing element, the displacer is the main level measuring device.

Later, more simplified version used an enclosure tube above the chamber which encapsulate a ferrous core rod which moves linearly with the displacer movement. Linear voltage differential transformer (LVDT) is used to directly transfer the linear motion into electrical signal. This technique was not possible for pneumatic level transmitter or indicator. It is more simple and is widely used for electronic signal transmission of a level instrument.

Since a displacer is working on the Archimedes principle of liquid buoyancy, the specific gravity of various liquids cause different weight losses and it can be used to detect interface level, if two liquids are present in a vessel, tank or column. The more dense the liquid, the more it will be exerting the up thrust on the displacer. For that, the liquid with lower specific gravity is filled in the measuring chamber of the troll where the displacer is hanging, and zero of the instrument is set. Then the chamber is drained and filled with the liquid having higher specific gravity and the maximum span is set. When subjected to process, it will read. Hence it is possible to measure a single or dual (interface) liquid level using a displacer.

Displacer can be made of hollow cylinders of various material suitable for each process, type of liquid and process conditions like pressure and temperature. Sometimes TEFLON, PVC and other materials are also used in the construction of a displacer. It should always be considered when selection of a displacer is being done to take into account liquid properties, process conditions etc. Most of the time, the new level instrument installed in the industry comes as a whole package with integrated transmitter, level chamber and the displacer of course.

There are other methods which we will be discussing in our upcoming articles, specially the liquid head measurement or use of a differential pressure transmitter in the measurement of level or interface level as well.

The articles here are just recapturing the basics of industrial automation and control in general. But the concepts discussed here are just to refresh memory of some of our professionals while helping the novice to gain the basic knowledge about the working of these instruments and how it is implemented in the real life scenario in various industry.

Thank you very much if you have reached this far to read the article. Newsletter INSTRUMENT REVISITED is about the providing general information about the industrial instrumentation aspects. Feel free to share it if you like it. Also suggestions, comments and feedback will help to improve the articles.

(Please note that though the accuracy of the information provided here is based not only on reliable sources, but also based on personal experiences, but it is for providing a general picture and the author is not liable for any loss or damages arising from the use of this information in any instance)