Level measurement with capacitance probe, advantages, and limitations.

The capacitance instrument measures the capacitance between the two pieces of a capacitor.

The capacitance of the capacitor increases when a dielectric is placed between the plates. The circuit supplies a high-frequency signal to the probe.

C = K E0 A D

Where:

C = capacitance

K = dielectric constant of the material

E0 = permittivity of vacuum

A = plate area (conductor)  

D = distance between conductors (plates)  

When one plate of a capacitor moves toward the other, the capacitance between them changes. If one of the plates is the diaphragm of a pressure sensor, the capacitance is related to the pressure that is applied.

This change in capacitance can be used to vary the frequency of an oscillator or to be detected by a circuit bridge type. If the dielectric material remains constant, the mechanism provides a very repeatable meter

Level measurement of Non-conductive, conductive Liquids

Non-conductive liquid:

The process fluid is a dielectric barrier.

The tank wall is the second plate.

The magnitude of the dielectric variation is .the measurement value.

The variation in plate size (liquid) is the quantity measured.

The change in level is equivalent to the dielectric difference that occurs.

Conductive Liquid:

The process fluid is considered the second plate.

The insulator on the probe is dielectric.

The variation in plate size (liquid) is the quantity measured.

The level of magnitude is equivalent to the change in plate area.

Advantages:

Its main advantages are

  • Low hysteresis,
  • Good linearity,
  • Stability and repeatability.
Disadvantages:
  • Complicated electronics are required because the capacity that varies is very small and is of the order of the parasitic capacities of the structure.
Limitations of Capacitance
  • Dielectric variations cause errors.
  • Coating of probes because the product can cause errors.
  • Tank walls made of non-metal or tanks that do not have vertical walls, the addition of probes as a reference is needed.
  • Calibration is not easy, especially if one of them requires “Bench calibration”.
  • Changes in vapor space may affect output.

Author: Abraham Moses

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