Tuesday, 30 December 2014

Fuel Gauging Amplifiers

The fuel gauging on the Victor works on a capacitance fuel reading system, meaning there are no fuel level floats in any of the fuel tanks. The fuel is measured by a capacitance positive and a capacitance negative column. These are tubes with no moving parts extending from top to bottom in the tank and work on a 'Wheatstone Bridge Principle' by reading level changes by a change in capacitance in the fuel itself.  The reading is transmitted to the fuel gauges by co-axial cabling via an amplifier that drives the fuel gauges at 115volts AC/400Hz. The system is very sensitive and requires absolutely reliable parts in order to be accurate.

The amplifier units are full of valves, capacitors and resistors and with old age become unreliable. Effectively these items are still being expected to work 20 years after the expected end of useage!

Original Spec capacitors
Replacement capacitors

All of the spare amplifiers that we recovered from scrap aircraft have now being used up and the ones in the aeroplane were starting to weaken with low and stagnant indications. Therefore it called for drastic action and 'The Brains Trust' got to work (namely Ken 'Radio' Sanderson) who came up with the up to date replacement specs for the capacitors. Some are no longer available down to the absolute identical spec but, we've found every type to replace them. There are 9 in each amplifier and much 'soldering & burnt fingers' has resulted in a batch of 6 out of 7 repaired ones now working, There are more to do now the formula has been set!  The next job is to fine-tune them with the fuel gauge test set before they are fitted back into XL231.

Original spec capacitors
Replacement capacitors