Scott,
No, we're talking about the same thing. The device uses a simple oscillator with the variable element being the capacitance in the RC network. Therefore, the frequency varies inversely with level. The total capacitance is fairly small, but not too small. As long as the DE of the fuel is consistent, the electronic components don't drift or change with temperature or supply voltage, there are no stray capacitance issues, and no water is present the sensor will work very well. The header tank is probably a good application since it is in a relatively constant temperature dry environment. In the IV/ES wing tank application what would happen if the wing/fuselage joint were to leak just above the sensor and wires? Water could find its
way into the electronics, which aren't sealed, and that would create a large error or even total failure.
The comments are based on science and the design of the sensor, not experience. As Bob suggested, if one were using experience alone the total field population isn't enough. Perhaps if there were 100 million out there, then maybe. In summary, our comments(if I can speak for Bob), are that the design and construction of the sensors don't justify total faith in their accuracy or reliability. They're adequate, but I would like to see better.
Gary
Bob and Gary,
I have no idea how the capacitance probes you talk about work. Here is how the VM Fuel system probes work:
There are three wires emanating from the probe. One is ground. One is a regulated 5 VDC to the probe. One carries a square wave frequency output from the probe that varies with the interaction of the fluid level on the probe. For Example, the 17 gallon wing tanks go from about 7500 counts at empty to 4500 counts at
full. The header setup got to be quite different - VM said the min probe length was about 14" - I wanted my header indicator to be very accurate and the probe vertical - that meant it was only 8" long. The fuel computer could not handle frequencies above 20,000. So, after studying some simple circuits from some books at Radio Shack, I built a CMOS frequency divider with Schmidt triggers to clean up the square wave - it is just a bulge in the connecting cable. Piece of cake - the header frequencies run between 12000 and 18000 and are very accurate (calibrated by 1/2 gallons). The probes do not seem to be affected by radio frequencies - of course, I am not looking at fuel level gauges during radio transmissions.
Oh well, you
use your experience and I'll use mine.
Scott Krueger
Lancair 320
PS The wing tank calibrations are so close that if I had to switch to a more modern display that only shows two tanks instead of the three I can see now, I could switch the wing output to one tank display and leave the header on the other.