Correction of fuel gauges ?

[wanabigaplane]

Thanks all.

 

Thanks for the circuits Sain. Interesting. However, the tanks with resistive senders are already installed.

 

Ozzie - the existing gauges give a different reading when the engine is running. I will overcome this by using a voltage regulator as you suggest. Also, yes, we could put known quantities of fuel in the tank, and mark the face of the gauge for each amount, but the aim of the exercise is to make the gauge linear and read full scale from full to empty.

 

Maj - you say you calibrate fuel gauges, so what type are they and how do you do it ?

 

And Facthunter, this type of gauge may be notoriously erroneous, but they are consistent. I often hear a car owner say something like “when the meter shows empty, you have a quarter of a tank left†In New Guinea, the Nationals would say “E†stood for “enoughâ€.

 

The consensus indicates that there is nothing available to do this - I had to check because I don’t want to start out on a design, only to find there was something already commercially available.

 

Thanks all. Jack. :):)

 

 

[sain]

Do the sensors have the same resistance for a given amount of fuel every time, or does it tend to wander all over the place?

 

If its reasonably stable then the engine monitor circuit here:

 

KP0000

 

might be of use....

 

It uses an op-amp to do a resistance to voltage conversion, which then drives a digital meter. If you want to do it cheaper you could probably just use a panel meter from Dick Smith or Jaycar.

 

If you'd like a hand with a circuit design feel free to PM me..

 

 

[Guest Maj Millard]

Ozzie, those fuel level sensors look pretty interesting, how do they handle slosh, is there a bit of a time delay, or is it just averaged out in the circut ?.

 

Wanabigaplane....The fuel cal I speak of is the standard fuel guage cal required on GA aircraft periodically, and required by CASA regs. It is generally for onboard elect, sensor -type guages and is to eliminate any error present in those guages, due because the guages are never exactly the same. Also required after a fuel cell/tank change or sender or guage change. Basically is carried out as follows:

 

1. Drain all fuel from fuel tanks.

 

2. From a low fuel point at the engine, drain any remaining fuel from lines (usually line into carb) Note quantity drained.

 

3. This fuel becomes unusable fuel, and is noted IE: 'Unusable fuel 4 lts' or whatever.

 

5. The following needs to be done from an accurate fuel bowser, or from containers that are a known quantity, so that you know exactly what qnty is going into the aircraft.

 

6. On a sheet of paper list guage markings. Usually E 1/4 1/2 3/4 F., but could be different, on you'r particular aircraft.

 

7. Place known qnty of unusable fuel into aircraft IE: known qnty removed in step 2. After this is done guage should still read empty.

 

8. With aircraft power on, fill each tank to the first guage indication IE: 1/4. Note quantity put into each tank, accuracy is key here IE: Right 1/4 22 lts. Left 1/4 20 lts.

 

9. Continue to do this for the whole tank, at each guage indication, alternating between wings to maintain wings- level for accuracy. You will end up with something like the following.

 

Right tank Left tank

 

E 0 E 0

 

1/4 22 lts 1/4 20 lts Note: Unusable fuel 4 Lts.

 

1/2 43 lts 1/2 41 lts

 

3/4 60 lts 3/4 58 lts

 

Full 82 lts Full 79 lts

 

10. You may be able to put more fuel into a tank, above the full reading on the guage, this is additional, and is usually marked as part of 'tank capacity' by the filler cap.

 

11. Place the results you have got on the panel close to the fuel guages, Congratulations...you have done a fuel cal ! Now at each guage indication you will know within a couple of lts how much fuel you have in that tank.

 

Please note the above is for guidance only, quantities are arbatrary and do not represent any actual aircraft fuel tanks, if in doubt about the procedure consult CASA regs, a LAME, or RAA level 2 or Tech Advisor.

 

Whenever conducting fueling operations always connect the required grounding lead/s to you'r aircraft to eliminate the danger of static spark.

 

 

[Guest ozzie]

Major those sensors will trip as soon as the liquid comes off them. the unit they plug into has a 5sec timer to stop any "slosh" trips. it would be easy to get a IC timer and make a imer circuit to stop this. but yo could set it below your normal reserve limit so it will trip at a level that is obviously a "time to land NOW" level.

 

 

[Guest ozzie]

just came across this in a EAA hombuilders story. seems relevant

 

EAAFuel Gauge Calibration

 

I learned a very important lesson a couple years ago while on a return flight from western Michigan to my home base at Twelve Oaks Airport in Martinsville, Indiana (II87). Shortly after passing Indianapolis Metropolitan Airport, the engine quit. A quick scan of the fuel gauges indicated about six gallons remaining in the left tank (the tank from which the engine was drawing fuel) and about 10 gallons in the right tank. I immediately switched tanks, turned on the second fuel pump, and said a quick but urgent prayer. I then remembered that Metro Airport had just passed under my right wing, so I made a quick right turn and found myself perfectly set up on downwind for Runway 15.

 

I continued to look for the source of the problem on downwind, believing it to be fuel related because of the way the engine died - a smooth rpm reduction over a period of about three seconds. But I had very carefully calibrated the fuel gauges while building the airplane and had marked the gauges at several points with the actual fuel level, and the gauge still showed six gallons. As I turned onto a short final, with the prop windmilling, the engine restarted on its own. I landed and taxied to the ramp, with everything performing normally. A visual check of the left wing tank found it to be completely dry.

 

After thinking about this for a few minutes, it dawned on me that the procedure used for calibrating the fuel gauges was faulty. The gauges were calibrated while the plane was still in the basement, with the plane on about 12 volts of battery power. In flight, with the alternator charging, the electrical bus is carrying about 14 volts. That two-volt difference caused the gauge to read six gallons higher on the low end of the scale. Needless to say, the fuel gauges were recalibrated with the engine running.

 

 

[Guest Maj Millard]

Another perfect example of why I don't trust elect fuel guages, I doubt if a calibrated dipstick would have done that to him. Cheers Oz

 

 

[Guest ozzie]

Other than indicating how much fuel was in the tank at engine start a dip stick would have not been much help once the flight commenced. i'm pretty sure that the author of that story was a responsible pilot and would have dipped the tanks as part of his preflight. once the flight commenced the pilot would have had to rely on known comsumption rates at given power settings with a cross reference to the gauges. As it was a new unknown product with fuel consumptions more an educated guess rather than actual experience. I'm sure what happened to him has happened to just about everyone who has flown a Warrior. Luckily a Warrior has an electric pump to hasten the flow from the other tank for a quick restart.

 

If you only have fuel gauges, once in flight you have to rely on them, knowing how much fuel burn per hour is of absolutley no help when a leak in the system, missing cap or an increase in fuel burn caused by a mechanical problem crops up the gauge is the only method that will give you any indication. If you don't have a fuel flow rate indicator accurate gauges are a must. What was mentioned in that homebuilder experience reinforced what i posted earlier about having gauges that run on lesser voltage than is supplied by the system. As experience builds with hours on a new aircraft one can gain an accurate fuel usage if the pilot dips before and after a flight before refuelling. but with a new aircraft with unknown preformance you better be good at guesstiments for a while.

 

Ozzie

 

 

[wanabigaplane]

Prototype

 

Prototype built. Now I just have to program it! :):)