In capacitive gauges, what changes with the fuel-air ratio?

• A. The physical distance between plates.
• B. The electrical supply voltage.
• C. The gauge pointer.
• D. The amount of charge stored.

Answer: (D)

Capacitance between the gauge’s plates depends on the dielectric between them. The fuel–air ratio changes that dielectric, because fuel has a higher dielectric constant than air. As the mixture shifts toward more fuel, the effective dielectric constant rises, increasing capacitance.

In many capacitive gauges, the supply voltage is kept constant, so the charge on the plates follows Q = C × V. When C increases due to a richer fuel mixture, more charge is stored. The gauge then uses that change in charge (or the corresponding signal) to indicate the reading.

The other aspects—plate spacing being fixed, the voltage being stable, and the pointer responding to the electrical signal—don’t themselves vary with the fuel–air ratio; what actually changes is the amount of charge stored on the plates as the capacitance changes.