ELECTRICALLY OPERATED :
Common electrical type level instruments are
1. Capacitance type
2. Ultrasonic type
3. Radar type
4. Conductivity type
Here i will cover capacitance type method, other method i will cover in next article.
One of the most desirable characteristics of level measuring apparatus is that there are no moving parts in the tank or vessel. This requirement is met by a system that is based on changes in the capacitance of the probe as the level changes. This type of probe is suitable for the measurement of liquid level, interface level (an interface liquid level has two or more liquids of different specific gravity), and granular solid level. The advantages of capacitance probe are that,
* It has no moving parts.
* Construction is simple and rugged.
* The probe can be anti-corrosive and Easy to clean and able to operate in excessive temperature.
A capacitor consists of two conducting plates, separated by an insulator called dielectric. In our case dielectric is medium which we want to measure. The electrical characteristic that expresses the dielectric quality of a material is called dielectric constant (k). When the liquid rises, liquid acts as the dielectric of the capacitor. When the level drops, an exposes the probe, more air or vapor above the liquid acts as the dielectric. This alters the capacitance of the probe, thus affecting the output of the electronic bridge circuit, in which it is used.
Capacitance type of method used for both conductive and non-conductive liquid where in conductive type of liquid it generate capacitance in between wall of tank acts as one plate and probe acts as a other plate. while in non conductive type ground probe are used so it does not affect measuring capacitance.
In conductive type of liquid conductivity >100 micro s/m require calibration and in case of non conductive type liquid conductivity < 1 micro s/m require calibration.