In a capacitive fuel gauge, how does the fuel-air ratio affect the gauge reading?

• A. Voltage increases with density.
• B. Ambient temperature changes stored charge.
• C. The amount of charge stored changes with the ratio.
• D. Pressure changes stored charge.

Answer: (C)

Capacitive fuel gauges read changes in capacitance caused by the dielectric between the plates, which is the fuel–air mixture in the sensing region. As the fuel–air ratio changes, the fraction of fuel present between the plates changes, altering the dielectric constant and thus the capacitance. In many gauge circuits the voltage is kept constant and the stored charge on the plates, Q = C V, varies with the capacitance. That change in stored charge is what the gauge uses to indicate fuel quantity. So, the gauge reading follows how the amount of charge stored changes with the fuel–air ratio. Temperature or pressure can have secondary effects, but the primary mechanism is the dependence of stored charge on the dielectric change caused by the ratio.