Fuel level sender scales and offsets

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N333M
Posts: 15
Joined: Tue Feb 20, 2018 1:58 am
Location: Hangar at Shelby County Airport (2H0)

Fuel level sender scales and offsets

Post by N333M »

Struggling with these calibrations. Craft is an RV9A.
I began by siphoning the fuel from the right tank, then removing the quick drain and draining the remaining fuel in the tank.
set the EIS to 100 and 10 scale and offset, per manual.
noted the reading on the EIS was 4.2
added fuel a gallon at a time.
EIS started to change on the 4th gallon...wrote it down.
continued to add fuel a gallon at a time.
EIS stopped changing after the 7th gallon..at 5.7..Hmmmmm
continued to fill up the tank.
Went thru the worksheet..came up with 533 scale factor and 440 offset..

Upon reviewing it all, I realized this is not going to be satisfactory for a 16 gallon tank!.. reading only from 3 to 7 or 8 gallons...

The craft had the standard Vans 2 1/4 round fuel gauges in it. they seemed to work ok in the 75 hours I flew it before I started the GRT upgrade..
IF I understand this GRT EIS fuel level calibration correctly, They recommend using the reading for full tanks as the level where the float sender stops changing..and the empty reading as the level it stops when going empty..in my case, it seems that I will have an empty reading at 3 gallons, and a full reading at 7. hmm..as I said, thats not going to work for a 16 gallon tank.OR at least, not very beneficial.
I went thru the math several times on the GRT worksheet.
Im scratching my head. If the sender output stopped increasing after the 7th gallon added...I guess it could be faulty..hard to believe it would only have a 3 to 7 gallon range of motion. (if its a float, which I think it is)
aircraft was level sitting in the hanger.

Any comments or Advice?
N333M
Don G.
Central Illinois
RV9A 160 hp O-320 carb.
GRT_Jeff
Posts: 662
Joined: Tue Dec 11, 2012 12:11 am

Re: Fuel level sender scales and offsets

Post by GRT_Jeff »

It should read the same range as the Vans gauges. Normally the EIS would be wired to provide 4.8V to the float sender through a 470 ohm resistor, and the resistance of the float sender to ground should vary through the entire range of the float. The float sender is like a variable resistor so you can measure this resistance directly at the sender while disconnected from the EIS.
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N333M
Posts: 15
Joined: Tue Feb 20, 2018 1:58 am
Location: Hangar at Shelby County Airport (2H0)

Re: Fuel level sender scales and offsets

Post by N333M »

yes..I am begining to think after checking my wiring that the sender is screwed up.
Should I be reading exactly 4.8 volts on the blue wire when checking it? also after the resistor...
I have not checked that yet.
N333M
Don G.
Central Illinois
RV9A 160 hp O-320 carb.
Bobturner
Posts: 429
Joined: Mon Mar 11, 2013 6:34 pm

Re: Fuel level sender scales and offsets

Post by Bobturner »

I'll repeat my suggestions from over on VAF.
1. Go into the wing root, remove the wire from the sender.
2. Power off, wire removed, measure the resistance from the sender terminal (the one you just removed the wire from) and a good ground. It should read somewhere between 30 and 250 ohms.
A. If it does not, check that the sender has a good ground connection
B. If you want to check the range of motion, remove all the gas again, and refill. Measure the resistance every few gallons. It should change until you get up to the last 3 or 4 gallons. (on my -10' 30 gal tanks, the float tops out at 25 gal). [I would check steps 3 and 4 first, easier to do.]
3. Wire still off the sender. Don't let the wire touch ground. Power up the EIS. Put a good voltmeter on that wire, the other probe on a good ground. You should see very close to 4.8 volts. If not, there's an issue with the power coming from the EIS, or the 470 ohm resistor is open, or....
4. If (3) is okay, repeat step 3 but this time hold another 470 ohm resistor between the voltmeter probes while also in contact with the wire on one probe and ground on the other, power on (use alligator clips if you have them). You should see something close to 2.4 volts. If you see something close to zero, the "470 ohm" resistor is probably 470,000 ohms or something. If you still see 4.7 volts, the resistor is either shorted or by-passed somehow.
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N333M
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Joined: Tue Feb 20, 2018 1:58 am
Location: Hangar at Shelby County Airport (2H0)

Re: Fuel level sender scales and offsets

Post by N333M »

Thanks Bob, this all makes good sense to me and I’m going to follow
N333M
Don G.
Central Illinois
RV9A 160 hp O-320 carb.
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