Basics of industrial level measurement 9

We have seen so far many level measurement principles in a series of eight previous articles and though many different variants exist for the basic level measurements, since we are only exploring the basic concepts behind these measurement principles and where these are normally employed in industry, so keep things from complicating we will be concluding our series on level measurement basics in this article. Hopefully most of the fresh professionals now have idea about what is most common industrial level measurement principles. For more seniors, it is a refresher, as the knowledge base is already there.

Today we will be discussing a level measurement method which is known as the capacitance or capacitive probe. This is more common with storage tanks having metallic walls. The probe rod works as one of the pole and the tank as another. The amount of liquid, solids, powder or slurries act as the dielectric between the two. So we can say that a capacitive sensor is a big capacitor whose change in the dielectric will determine the amount of level inside the tank, vessel, container or bin. A radio frequency signal is introduced as was the case in the inductive probes discussed earlier. Since the change in capacitance will change the frequency, the same is translated into an equivalent change in the level using electronic circuits. The fluid being measured however needs to be non conductive in nature as is the case in normal capacitor's dielectric material.

Having no moving parts, it is easily maintained and only thing which affects the reading is the diversity in the dielectric. If the liquid or solid being measured is homogenized it will give correct readings, but for some emulsions or liquids/solids having inconsistency the level readings may prove to be erroneous. Apart from this drawback, it is a quite reliable method in general. These are also good where there is a strong formation buildup in the tank and works equally good for viscous liquids.

Another method of liquid level measurement is the radioactive sensor. As the name describes, radiation from a radioactive source is used to detect the level in a vessel or tank.

The source is mostly Gamma radiations from Cesium 137 or Cobalt 60 which passes through the tank walls and also passes through the liquid or solid being measured, which weakens the radiation signal. The signal strength is measured at the opposite side by detector which then turns this variation in signal strength into a level measuring signal.

The source part must comply with the international standards for radiation safety and should have a lockable shutter of mechanism to isolate the source if some maintenance work needs to be performed on the tank or instrument itself

Since it is using a radioactive source, it is not feasible in general use, but is still used in some applications where the fluid being measured is highly corrosive, abrasive or reactive, or having a high temperature or pressure. This measuring method is also good if the tank or vessel geometry is not uniform or there is an agitator mechanism in use which can cause unnecessary vortices. It is a non intrusion type of level measurement and can be used with proper care and handling of the instrument when other conventional level measuring techniques are failing.

Here we come to and end on basics of industrial level measurement, but one thing which needs due consideration that no matter which method of level measurement is employed, it should meet the required measurement accuracy for the process and should be compatible with the fluid properties being measured to get the desired results.

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(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)