Capacitance type Level Transmitter principle
There are different kinds of level measuring techniques .one of the simplest method is capacitance type method.
A capacitor is formed when a level sensing electrode is installed in a vessel. The metal rod of the electrode acts as one plate of the capacitor and the tank wall acts as the other plate. As level rises, the air or gas normally surrounding the electrode is displaced by material having a different dielectric constant. A change in the value of the capacitor takes place because the dielectric between the plates has changed. RF (radio frequency) capacitance instruments detect this change and convert it into a relay actuation or a proportional output signal. The capacitancerelationship is illustrated with the following equation:
Calibration of capacitance type level transmitter
1.Remove the level transmitter from the system(tank).
2.check whether transmitter shows zero reading by connecting with multimeter otherwise release the pressure.
if the transmitter is smart
3.connect 475 hart communicator and multimeter to the level transmitter
4.put multimeter to ma.
5.Fill the corresponding liquid in correct density and note down the readings . Fill liquid at 25%, 50%, 75% and 100% in both ascending and descending orders and note down the readings.
6.check for errors if there is zero and span adjust should be done.
7.for zero calibration :drain the liquid and check the multimeter if it is not 0 then go to sensor trim option in the HART then go to zero trim and the HART communicator will automatically trim the sensor in to zero
8.For span calibration: fill 100% and wait for some time then go to sensor trim and select span trim in HART communicator the 475 will automatically trim the sensor into 20ma.
9. After doing zero and span trimming again check the reading at 0%,25%,50%,75% and 100%.
In case of non smart capacitance type transmitter
10.Connect a multimeter and rotate the zero pot and stop when multimeter shows 4ma.
11.Fill the chamber to maximum liquid level and rotate the span screw to 20ma.
12.Repeat these steps and check all readings.
Advantages:
Very little force is required to operate them and hence they are very useful in small systems.
They are extremely sensitive.
They have a good frequency response and can measure both the static as well as dynamic changes.
A resolution of 2.5 x 10^-3 mm may be obtained with these transducers.
Disadvantages:
The metallic part of the capacitor must be insulated from each other.
Their performance is severely affected by dirt and other contaminants because they change the dielectric constant.
They are sensitive to temperature variations and there are possibilities of erratic or distorted signals due to long lead length
