tank sensor fault

[captainforbes]

I have a VDO capacitative water tank sensor (2 wire rods, no float), and the sensing system has failed. Nigel Calder's book does not mention this type of sensor, so I don't know how to proceed. I have removed the sensor from the tank and it has no obvious gross fault The circuit runs from the battery bus thru a press switch to activate the circuit, thru an analogue meter to show level, and to the tank. The battery has 13.6 v, and the voltage at the sensor contacts is 8.6v with the sensor disconnected. I have a second tank with a similar circuit runing thru the same display meter, it clearly has a separate press switch, and has the same sensor. It is working fine, but it not readily accessibile to check terminalvoltage at the sensor.
*is the voltage drop 13.6 to 8.6 due to the meter in the circuit, or might it be a symptom of a poor connection ect?
*all ideas on fault tracing welcome!

Mike

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[William_H]

Hello Captain Forbes you have a wonderfull oportunity having a system that works. If there is an electronics box for each tank then swap them if not obviously there is no problem in the box and it can only be in the wiring. ( perhaps you havn't found the electronics box it would seem unlikely that the 12 volts would go through a meter coil possibly electronics in meter case)Try lots of swapping. Waffling on as I do ...Capacitance fuel gauges are used in all big aircraft it has the advantage that fuel can be measured in pounds or Kgs rather than by volume. Fuel density varies with temperature ie expands and contracts and on a jet we are talking big temp changes from minus 50 to plus 45c. The coeficient of capacitance changes also so a readout in pounds is a readout of actual energy rather than volume. (energy of course dictates how far you can fly) I had a Ford Australian built 1982 which had a capitance fuel gauge which was exelent with the electronics built into the top of the sensor which was concentric cylinders. Time goes by now I have a 1997 model same car with variable resistor float type gauge I can't imagine why. Enough waffle maybe it will promote comment good luck with your fault finding

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[Alex_Blackwood]

Hi, First question! Have you confirmed that your press switch is OK? These systems usually have an output in milliamps. ie your meter is probably an ammeter reading milliamps. If you have a digital multimeter you could try switching to the ammpere ranges, try the highest range, normally 10 amps first.
Connect the meter across the press switch input and output terminals, don't operate the switch. Reduce the meter range until you get a reading. I suspect the output from the sensor will be 4 - 20 milliamps (Tank empty at 4, full at 20). Try this on the good system first and then compare with the faulty system. If you get a reading on either system you could operate the switch with the meter connected, the reading should disapear.
This will at least prove if you are getting anything out of the system and that the switch is working.
In my experience it is usualy the sensor that goes U/S on these systems. Although the electronic box mentioned by the previous poster can go faulty and sometimes one going will take the other out as well. It is common practice to replace them as as unit.
Sorry cannot be of greater help, as usual, difficult without seeing the system. Try the above and then as previously suggested it may be a case of swopping bits to find the culprit.

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[captainforbes]

Thanks for the info........I'll try that with my meter. Can't easily get to the other tank to compare readings and swop bits around.

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[franky045]

You sound like a man who knows a thing or two. I want to fit a fuel gauge in my Bowman 44 (1973) which previously didn't have one. There are three tanks, two beneath the floor either side (30 gallons each) and one in the keel (60 gallons). The one in the keel is more than a metre deep and by the time you get to the top of the side tanks we are talking a total depth of nearly 1.5 metres. Of course they are all funny shapes with sloping floors. My thought is to fit a pressure sensor in the keel tank (from which the only supply pipe comes) level with the bottom of the supply pipe and gauge the contents from top to bottom. The top tanks both feed the keel tank and have their own outlet valves.

In am not sure what a pressure sensor output is, probably milliamps, but the output could be measured as the tanks were filled from empty, 10 gallons at a time, and a conversion chart made, or with a bit of clever computer electronics I am sure a gauge could be made to read the contents on a linear scale.

I am a retired accountant (and did classics at school) so although I regard myself as an engineer manque I have no training in anything that matters on this earth.

Can you give me any guidance on this. I know that some small homebuilt aircraft are using pressure sensors for fuel gauges, which is what gave me the idea.

Frank Mycroft



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[halcyon]

You can do it without electricity, just use air.
Fit a pipe to near the bottom of the lower tank, allow for sludge in tank, fit low pressure gauge to top. You need a little pump to pump air through the pipe, this gives a constant reading, then open valve to gauge, and the air pressure read off gauge gives a level. This is due to wieght of fuel, the deeper the level the greater the pressure. empty tank is zero, full tank is what ever pressure you read. Can be done for the two side tanks as well.

Brian

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