SaM - Capacitive Humidity Sensor

• $\text{RH}$ : humidity level.
• The dielectric (or electric) permittivity $(\epsilon )$ is a function of humidity $(\text{RH})$.
• The relative humidity of water $@25°\text{C}$ is approximately ${\epsilon }_{r_{{\text{H}}_2\text{O}}}\approx 80$.
• So the capacitance will be equal to: $C=\frac{A}{d}\epsilon (\text{RH})$And if we use some common real world values we obtain a variation of capacitance:$\frac{\Delta C}{C}=1.7\frac{\text{pF}}{\text{RH\%}}$Where: $\text{RH\%}$ is “humidity percentage”.
• And a typical value we find of $C$ is:$375\text{pF}+1.7\frac{\text{pF}}{\text{RH\%}}\cdot \text{RH\%}$So we dont actually have the problem of a small capacitance, this is a measurable capacitance, it’s big enough.