Fuel Gauging - Component Confusion

[fueldrinker]

I have read from various conflicting sources that the capacitance gauging systems used in many aircraft fuel tanks may have compensators and/or densitometers and /or cadensicon sensors fitted.

Can some scientist give me a dumbed down explanation of what each of these components do?

[Wodrick]

Trying to keep it simple. They all do the same thing. In older, analog systems, 1-11, B727, 737-2, and even some of the pseudo digital systems which are basically analogue with a digital display each tank will contain one compensating probe amongst all the other probes. The compensating probe has an extra capacitor.
A densitometer (American terminology) and a Cadensicon (French terminology) are similar components.
All three attempt, with varying success, to correct for variation in Specific Gravity of the fuel.
The accuracy of a fully serviceable, modern, digital system would astound those used to a simple analogue capacitance bridge system. I know it did me the first time I saw one !

[Spanner Turner]

give me a dumbed down explanation of what each of these components do

Tank Unit
The Tank Sensor changes capacitance as the fuel level changes. Each
tank contains a series of tank sensor units. Each tank unit extends
from the top of the tank to near the bottom of the tank. Together the
combination of tank units provides a profile of fuel level in the tank.
Compensator
The Compensator changes capacitance as the dielectric constant of
the fuel changes. The dielectric constant is directly related to fuel
density.
Physically the Compensator is a short tank unit with a larger
diameter. This allows it to be mounted in the lowest part of the tank
The FQIS Processor uses the density value from both the
Densitometer and the Compensator. These values are compared for
accuracy. Fuel weight is computed from fuel volume, determined
from the tank units, and the density measured by the Densitometer
and Compensator.
Densitometer
The Densitometer provides a measure of density of the fuel in a
tank. This unit is tuned to a specific mechanical vibration based on a
nominal fuel density. As density varies from this nominal value, the
tuning of the Densitometer changes proportionally.
The Densitometer is mounted in the lowest part of the tank, where it
is always covered with fuel.

[NutLoose]

can't beat an MLI or dripstick

[fueldrinker]

Thanks guy's for your detailed explanations and comments. My understanding of the subject is much clearer now.

Does anyone know of any aircraft (civilian or military) that uses ultrasound for fuel gauging?

[jettison valve]

fueldrinker (awesome nickname, by the way...),
It is my understanding that the B777 uses a Smiths Ultrasound System. Apparently, neither the perfect solution, they need to fight with air bubbles, froth and sloshing. Maybe somebody else can comment...?

NutLoose,
MLIs and dripsticks are far less accurate, around 10% on the A330/A340 if I remember correctly. A FQIS in good shape brings that down to max. 1% at high quantities.
A380 will come without MLIs, which themselves can be cause for trouble.

Cheers,
J.V.

[G-STAL]

Capacitive systems are often set up by firstly measuring the volume of fuel in the tanks with a dip stick. Hydrometers are used to measure density and the mass is calculated. The system is therefore only as accurate as the initial measurement using the dip-sticks. My experience is that this is where most mistakes are made. Often the SG is made up, as a hydrometer is not used.

The ultrasonic system is much less reliant on the human element in this respect. The bubbles are a problem for both systems, but software tunes out the effect in ultrasonics.

1% accuracy is impossible. fuel itself varies too much for that. 3% is more realistic for a modern system.

[brine]

One thing that makes capacitance systems so good is that the fuel and vapour in the tank are measured.

The aircraft attitude makes no difference (or shouldn't!) to the reading. In aerobatics, the guages should read the same whatever the attitude.

Bri