Capacitive level measurement
Capacitive level instruments operate on the basic principle of variation of the electrical
capacity or capacitance of a capacitor formed by the sensor,vessel wall and dielectric material.
• A capacitor is made up of two conductive plates which are seperated from each other by a
dielectric.
• The storage capability of a capacitor defined by the capacitance,C,is directly dependant on
the plates area(A),their distance apart(d)and the dielectric constant of the material between
the plates.
Principle:
• The principle of capacitive level measurement is based on change of capacitance.
• The capacitance depends on the fluid level. An empty tank has a lower capacitance while a
filled tank has a higher capacitance.
• A simple capacitor consists of two electrode plate separated by a small thickness of an
insulator such as solid, liquid, gas, or vacuum. This insulator is also called as dielectric.
• Value of C depends on dielectric used, area of the plate and also distance between the plates.
Capacitive level instrument uses the principle of capacitance equation
C - capacitance
E - Permittivity of dielectric material between plates.
A - overlapping area of plates
d - Distance between plates
• The dielectric constant of the process material is the most important aspects of this level
measurent application.
• it should be noted that temperature ,moisture content,humidity and density can change the
dielectric constant of the process material.
• The higher the dielectric constant of the process material,the easiest and most reliable would
be the level measurement.
• materials with low dielectric constant are not suitable for capacitancr level measurement
application.
Types of capacitive level transducers
Depends upon type of liquid material and measurements,they are categorized as
1. Measurement of non-conductive material
2. Measurement of conductive material
3. Non-contact measurement
1.Non-conducting material:
• For measuring level of non conducting liquids, bare probe arrangement is used as liquid
resistance is sufficiently high to make it dielectric.
• In the case of measuring level of non conducting liquids,the parallel plates of the capacitors
are made up of conductive materials.
• There is no need for insulation over the plates since there is insulation in between the plates
due to non conductive liquid material.
• Since the electrode and tank are fixed in place, the distance (d) is constant, capacitance is
directly proportional to the level of the material acting as dielectric.
2.Conducting Material:
• In conducting liquids, the probe plates are insulated using thin coating of glass or plastic to
avoid short circuiting. The conductive material acts as the ground plate of the capacitor.
3.Proximity measurements (Non-contact type measurements):
• In Proximity level measurement is the area of the capacitance plates is fixed, but distance
between plates varies.
• Proximity level measurement does not produce a linear output and are used when the level
varies by several inches.
