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The principle of capacitive level measurement is based on the capacitance change of a capacitor. The probe and the tank wall form a capacitor whose capacitance is dependent on the amount of product in the tank: An empty tank has a lower, a filled tank a higher capacitance
The working principle is based on the based on the change of capacitance. The insulated electrode of the electrode acts as the one plate of the capacitor and the tank wall (or the reference electrode in a non metallic vessel) acts as the other plate. The capacitance depends on the fluid level. From the equation below
it is clear that as the dielectric constant varies as the level increases, capacitance increases and this change in capacitance is directly proportional to level of the liquid
Calibration of transmitter
Isolate the level transmitter from the field .Bleed off the trapped pressure and check whether transmitter shows zero reading.
Connect HART to the level transmitter. Clean the probe if contact type transmitter is used. 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. See if there is any error in readings. If there is considerable error do zero and span calibration.
For zero calibration fill liquid up to 10mm or fully drain the liquid and wait for stabilization and then go to sensor trim in HART and then go to zero trim and do zero trimming. In case of non smart capacitance type LT connect a multimeter in series and rotate the zero screw (pot) until the multimeter shows 4 mA or press the zero button until multimeter shows 4mA
For span calibration, fill 100% of range in the chamber and wait for stability, then go to sensor trim in HART and then go to span trim and after steady reading is achieved do span trim. In case of non smart devices, fill the chamber till the maximum level where we require 20mA and then rotate the span screw (pot) to set 20mA in multimeter or press span button until the 20mA reading is achieved.
After doing zero and span trimming again check the reading at 0%, 25%, 50 %,75 %and 100%. If everything is OK we can take the transmitter in line
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