Humidity

Capacitance humidity dependance

The capacitance of a plastic film capacitor will undergo a reversible change of value in relation to any change in the ambient humidity. Depending on the type of capacitor design, both the dielectric and the effective air gap between the films will react to changes in the ambient humidity, which will thus affect the measured capacitance. The humidity coefficient bc is defined as the relative capacitance change determined for a 1% change in humidity (at constant temperature).

b = 2 x(C₂-C₁)/C₂+C₁/(F₂-F₁)
C₁ Capacitance at relative humidity F₁
C₂ Capacitance at relative humidity F₂

Dielectric		PP	PET	PEN
C humidity coefficient b	10-6%RH	40 ... 100	500 ... 700	700 ... 900
(50 ... 95%)

The values of bc given in table are valid for a relative humidity range of 50% to 95%. At relative humidity below 30%, the humidity coefficient is relatively low. Wide variations are to be expected at relative humidity above 85%.

Source: Epcos-TDK, modified by Garmanage: Roland Gallay