Capacitance gauge dielectric?

• A. Oil.
• B. Water.
• C. Fuel and air.
• D. Vacuum.

Answer: (C)

A capacitance gauge works by measuring how a capacitor’s capacitance changes with the dielectric material between its sensing plates. The capacitance depends on the dielectric constant of that material. In an aircraft fuel system, the space between the electrodes is filled with fuel when the tank is topped up and with air when the tank is empty. Since the dielectric constant of fuel is different from that of air, the gauge changes its reading as the level changes.

That’s why the correct dielectric options are fuel and air—these are the two media that actually occupy the sensor area during operation. Vacuum isn’t present in a fuel tank, and while oil or water could influence readings if present as contaminants, they aren’t the intended dielectrics the gauge relies on in normal operation.