Capacitance Level Probe Calculator
Estimate the capacitance a coaxial (or stilling well) capacitance level probe sees from empty to full, and the level that a measured capacitance represents, for non conductive liquids.
- k = 2πε₀ ÷ ln(D/d) = 46.21 pF/m
- C = k × [εr × h + (L − h)] = 143.2 pF
Coaxial Capacitance Formula
Span = 2πε₀ × (εr − 1) × L ÷ ln(D ÷ d)
ε₀ = 8.854 pF/m, εr the liquid dielectric constant, h the wetted length, L the active probe length, D the inside diameter of the outer tube (or an equivalent vessel diameter) and d the probe diameter. This applies to bare probes in non conductive liquids; insulated probes in conductive liquids behave differently, and installation capacitance adds to the empty value.
Worked example
A 12 mm rod in a 40 mm tube, 2 m long, reads 92.4 pF empty and 194.1 pF full in oil (εr 2.1). At 1 m level it reads about 143.2 pF.
Frequently Asked Questions
How does a capacitance level probe work?
The probe and vessel or outer tube form a capacitor. Liquid with higher dielectric constant than air replaces vapour as the level rises, increasing capacitance.
Why use a coaxial tube?
It fixes the geometry, raises the capacitance per metre and makes the measurement independent of the vessel shape.
What about conductive liquids?
They need an insulated probe; the insulation then sets the capacitance and the liquid acts as the second plate.
Learn more on the blog
Types of level transmitters
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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