g3x touch fuel calibration

[Alex Edwards]

Originally Posted by grayforge

My impression of Matt's responses is that regardless of the voltage drop that the G3X sees from having steam gauges wired in parallel to the G3X, you can calibrate the G3X to show the proper fuel levels.

Example: With just gauges, let's say they see 2v to 10v and read accurately.
You add the G3X in parallel, now, both steam gauges and G3X see 1V to 7v. Then you calibrate the G3X to show the proper level at various voltage steps. No problem. However, the steam gauges still read incorrectly.

The only issue with this setup is that if one day the analogue gauge fails and goes to an open circuit then the voltage will go back to the single gauge voltage. This would mean that the G3x will over estimate the fuel left in the tank.

[grayforge]

True. Some experimentation would tell by how dramatic the change would be so one could watch out for that type of failure mode.

[g zero]

Why not pick one source of display and go with that ? Multiple fuel gauges are not really necessary .
Tom

[drmax]

I am going to procede with the calibration and see how everything jives. I'll report back. The reason I want to keep it, is the way I wanted it. It was was said, prior to having the panels cut, (by a couple others that have accomplished, but with a different efis system, that it works.) They may have wired in a variable resistor with the gages. At the end of the day, if I'm not satisfied, by either the anolog gages are severely affected, or "if" the g3x for some reason, (which it should not be the case) is affected, then I'll yank the gages and have a new panel cut. Thx, DM

[penguin]

Be careful here, the analogue gauges take 12v, what voltage does the G3x run at? It could well be 5v. If so you are now effectively applying 12v to a 5v circuit which will make it read in strange ways to start with and then burn up.

The analogue gauges are current monitors - a fixed voltage is applied across a variable resistance to vary the current. How does the G3x measure the resistance? Probably in the same way, but need to be sure before proceeding. Paralleling up 2 measurement systems will make both read inaccurately. Therefore some kind of interface box will probably be required to measure the resistance of the gauges and transmit it on 2 independent circuits. I'm told by people who know more about electronics than me that this is not difficult to achieve, but I don't know of any 'off-the-shelf' box.

Good luck!

Pete

[grayforge]

I'm sure the G3X reads the variable 14v voltage directly from the resistive fuel senders. This is how they have it shown in the install manual. Rev P, page 24-2.

[drmax]

Went smoothly, the calibration. The g3x GA24 is 10 volts per Garmin. (for the guy who posted last) Anyway, I see that my analog gages don't register fuel till around 6 gals, and stop registering fuel at 14 gallons. I see that with my analog fuel gages c/b out, while initially no fuel in tank, the efis showed a voltage of 1.7 V on either tank fuel sensor. With analog fuel gage c/b in, it raised this value to 3.1 V. Being that my floats did not show a reading on the analog gages until around 6 is (i'm assuming at this point) the error due to the having the analog gages wired in parallel to GA24, as it was confirmed that after adding 1 gallon increments to either tank, from zero, showed a correct curve (and respective voltage drop) on the efis calibration. At the top end, however (14 gallon area) the curve stopped and voltage value did not change. (floats have no more travel) Going to go out and fly this and manage fuel as always and confirm my suspicions. I know my analogs, when reading just on the empty peg, still have 6 gals in each tank, and will see what my efis reads. I also made a fuel dipping stick at the 10 gal mark, for each tank. If it turns out, that my efis gives inaccurate readings, or inconsistent readings, I'll tank the analogs and gut a new panel. The red cube fuel flow will also be of considerable help. I'll report back with additional findings. Take care, DM

Part 121 operators use only one set of fuel gauges why do you feel the need for two?

[Bill.Peyton]

I agree. No need for two sets of gauges especially with the fuel flow computer built into the G3X. My remaining fuel is always within a few tenths of a gallon. My resistive gauges are accurate within +- 1 gallon using the G3X cal. procedure.

[grayforge]

Some people have 2 batteries and 2 alternators. Every project is individual. If he can get it to work well, more power to him. :-)