The two cadensicons used on A320 are float type, density is determined by measuring Archimedes force (buoyancy). A float is maintained by springs and the force on the springs is measured.
The force "is equal to the weight of the fluid that the float displaces".
This weight depends on different elements: volume, gravity field and density.
Volume is constant, gravity field is assumed constant, leaving only a dependency on density.
A cadensicon, or capacitive density condensator, is a densitometer with a measure of fuel permittivity (dielectric constant), to calibrate the capacitive probes.
On the A320, there is a cadensicon for each tank (2 or 3 if there is a center tank), all float type. View of the A320 cadensicon:
A320 cadensicon. Left: Boeing site, right: Airbus A320 documentation
From Airbus technical training manual for A320 fuel system:
Each cadensicon houses 2 sensors covered by fuel. A fixed capacitor covered by fuel is the primary source for dielectric constant of the fuel in each tank and a densitometer float measures fuel density. Both parameters are used for calculations of the fuel quantity.
As density of the fuel grade in use changes with temperature, a fuel thermistor makes an allowance for temperature variations.
A partially immersed probe can be seen as the sum of two variables capacitors with different dielectrics (air and fuel) and which total capacity is a function of fuel level. Each tank has a probe fitted with a capacitance index compensator, which measures fuel permittivity at the bottom of the tank.
A320 fuel level probes, from Airbus A320 documentation
The location of the probes is chosen to ensure at least one probe cuts the fuel level in any aircraft attitude.
The FQI has two channels, one working to compute the mass, the other to monitor the first one. Mass is computed from the levels sensed by capacitive probes and:
- Permitivity from cadensicon
- Permitivity from capacitance index compensators
- Density from cadensicon
- Fuel temperature
- Pitch and roll angles.
The conversion of levels to volume takes into account the attitude determined using the probes and a table in the computer memory containing the geometry of the tanks.