Capacitance Level Measurement

Yes, capacitance level measurement are used to continuously measure a level. The factor that your overlooking is that there are two different dielectric constants at work here, air (free space) and the fluid. Now if the fluid does not have a dipole moment and thus has a dielectric constant identical to free space then a capacitance level measuring technique will not work at all. Hydrazine levels can be very easily measured capacitively. Gasoline can be measured capacitively but not as easily. Mercury and liquid Nitrogen are impossible to measure capacitively. Water level appears to be very easily measured with a capacitance level. However, the effects of dissolved salts can induce significant errors in the measurement since the constant varies with these impurities.

When air ll be the dielectric the total capacitance / voltage in the circuit what change can happend up or down ? same for liquid ? please clarify me

There must always be two different media in the same capacitance level measurement probe. Air will have nearly identical electrostatic properties as an ideal vacuum, so what will be the other media to be measured?

Non-metallic tanks, like fiberglas tanks, require a separate reference probe (which might be integrated into the capacitance probe), which steel walled tanks supply by nature of the material itself.

A word of caution with capacitance level measurement. If the properties of the fluid are not constant, then the setting for the capacitance equipment will not give the correct reading. For example, an absurd illustration: If the equipment were set-up on seawater at 6degC then it will not give the correct indication on fresh water at 6degC, 12degC or isopropanol at 15degC, and one might reasonbly expect the wrong volume to be dispensed and/or vessel overfills were one to try it.

So use capacitance as a last resort and only when the fluid properties are constant throughout the whole of the use of the vessel.

what ll be the capacitance change when no liquid appears ( air ) and liquid present (water) increase or decrease please

As indicated in #4↑, it depends upon the temperature and the level of dissolved substances in the water, both of which may vary.

For water, use ANYTHING other than capacitance!

See also #8↓.

Cap =DKxL/D squared,

When level changes , the only change from the above equation is DK.Thats basically how it works.

Yes, as long as the conductivity/capacitance of the measured fluid does not change.

If the fluid conductivity/capacitance changes, the level indication will not be anywhere near accurate.

These systems work off the measured current flow between the probe conductors and/or the tank wall which is directly affected by the dielectric constant of the fluid.

The dielectric constant is the ability of the material/fluid to alter, change, affect the orbit of the conduction band electrons which directly affects curent flow.

One of the recent projects I went on was using this type of level detection in sewage vaults which is totally unacceptable as there is no way the fluids (and other objects) are in any way constant in nature.

I replaced the capacitance level measurement systems with Piezo pressure sensing devices from Siemens and the system has not had any level control issues since.

Hope this helps.