Fuel flow sensor accuracy

There is a way to correct it, by adjusting the Auxoff setting. I don't have the manual in front of me, but the directions are in there. It's a trial and error thing. If you're reading 17% high, try adjusting the Auxoff 17% low as a starting point. IIRC, the recommended range is 180-220 and the default is 200, but I ended up with a 170 setting. I am consistantly within a few % when I top off, but it took a lot of playing around to get it that accurate.

You have to adjust the accuracy following the instructions in the manual. It took me about 5-6 top offs to get the adjustment. I still track the accuracy and over about 38 gallons, my FF will read about 1/2 gallon high.

My experience

On climbout under full power, full rich, I see 16 gph or higher. My fillups, my tank gages and my fuel flow are all very close and I didn't need to adjust it. (also GRT EIS/EFIS). I am reporting this because while you may be reading high, the high readings on climb are not so high.

Superior says:
if running 50-75 deg. ROP at 75% you should get about 0.5 BSFC. That would give you 11.25 gph. At peak you should see about 0.43 BSFC and LOP according to "Walter" should get you down to 0.40 BSFC. These numbers assume 180 HP.

I hope this helps.

Scott,

Does GRT make the pressure sensor, or is it another commercially available unit that they resell? Presumably it is a powered transducer...have you checked the voltage going to the pressure sensor?

Thanks,
Scott

The fuel flow senders are powered devices that are usually made by FloScan. They do take voltage in, but this doesn't change the accuracy of the devices because they send pulses out to represent flows, not voltages.

These senders always need to be tweaked some due to installation errors. You will need to go in and adjust some setting in your unit to make it accurate. Once you get it tuned in, it's pretty common to have 0.1 gallon accuracy over a 50 gallon tank.

The FloScan sensor that all the fuel flow meter manufacturer's use is 2.5% accurate. (according to FloScan 9 years ago when my airplane first flew) This accuracy is adjustable by changing a constant in the unit. Try not to get the urge to change it every time you fill up. Take an average of readings over several tanks.

My fuel flow is only 100% accurate at one given power setting. If I take the hobbs hours flown over a year and the fuel purchased over a year, I end up with a system that is 3.5% accurate. It tells me that I burned 3.5% more fuel than I did. I prefer to have more fuel than I think I do.

Typically my fuel is 100% accurate when the electric boost pump is OFF and the flow indicator reads 7.0 GPH. That is about a 23 square power setting or full throttle and 2300 RPM.

Hope this helps.

I'm also running an EIS4000. The calibration setting defaults to 200 which GRT says will need adjusting once establishing a history of approx 100 gals.
Keep track of your top offs and adjust accordiingly.
I've still not completed my calibration but FF reads:
WOT climb out = 15.5-16 gph.
ROP 75% cruise = ~9.0
ROP 65% cruise = ~7.0

Always kept track of my ins and outs. Consequently, I have adjusted my FloCal down to 180 about 10 hours ago. It's been reading very good lately. I still have 3 fill-ups to enter into my spreadsheet but I think I'm within 1 - 1.5 gallons now. And the L/R tanks are reading within .5 - 1.0 gal.

But lately my left fuel tank sender is acting up. It sporadically goes all over the place in level flight. On the ground it seems OK.

Happy my fuel totalizer is starting to be more accurate.

Scott Will said:

22+ hours into my flight test period and my fuel flow sensor is still reading high (GRT EFIS w/ EIS 4000). It consistently reads over 10.5-11 gph in cruise on an IO-360. Been keeping track of my "ins and outs" as far as fuel goes. Looks like it's reading about 17% high. Sometimes I see 16 gph like on climbout.

I placed the sensor in the same place I've seen it on other RV's, in the cockpit just beyond the boost pump with plenty of "straight" line on both sides.

Click to expand...

In the cabin after the boost pump is virtually the worst place to mount a fuel flow transducer. All the available literature says so. People mount it there because they see other builders putting it there...it's the blind leading the blind.

Mine is mounted 1/2 way between the selector valve and the boost pump. Been very accurate for 14 years. I have the "K" factor set to 2.5% conservative. That gives me about 1 extra gallon out of 35.

Available literature v. LAME advice.

Captain Avgas said:

In the cabin after the boost pump is virtually the worst place to mount a fuel flow transducer. All the available literature says so.

Click to expand...

Because of all the available literature I took the Flexible Fuel Line, which runs from the Engine Pump to FI, to a LAME and asked him to cut it in the middle and fit couplings, so that I could put my FLOSCAN in the middle of the hose.

He explained that in his experience (scores of GA machines operating in the hot and dusty Outback) , vibration had more detrimental effect on Floscan units than bends in the pipe either side of it. He recommended that I mount it solid, on the firewall and made up a short section of Stainless Steel tube with flares to connect it to the Firewall bulkhead fitting. (Pictures available from [email protected])

So my Floscan is preceded and followed by two elbows.

On refuelling I find my GRT EFIS Calculated Fuel Remaining is within 1% of the actual Fuel Used. ie. One litre in 100 litres.
As the say in America; "Go figure!"

So I suspect the FLOSCAN manufacturer is being over conservative.
Pete

fodrv7 said:

Because of all the available literature I took the Flexible Fuel Line, which runs from the Engine Pump to FI, to a LAME and asked him to cut it in the middle and fit couplings, so that I could put my FLOSCAN in the middle of the hose.

He explained that in his experience (scores of GA machines operating in the hot and dusty Outback) , vibration had more detrimental effect on Floscan units than bends in the pipe either side of it. He recommended that I mount it solid, on the firewall and made up a short section of Stainless Steel tube with flares to connect it to the Firewall bulkhead fitting. (Pictures available from [email protected])

So my Floscan is preceded and followed by two elbows.

On refuelling I find my GRT EFIS Calculated Fuel Remaining is within 1% of the actual Fuel Used. ie. One litre in 100 litres.
As the say in America; "Go figure!"

So I suspect the FLOSCAN manufacturer is being over conservative.
Pete

Click to expand...

All of the major manufacturers of engine monitors (JPI, EI, etc) recommend installing the fuel flow transducer after the mechanical fuel pump. Whether there are bends in the line seems to be less problematic than the actual location. Builders with fuel injected engines who instal the transducer between the boost pump and the mechanical pump almost universally report that they have very large errors of fuel flow readings when the boost pump is switched on. Perhaps you have noticed this also.

In addition it is considered that there is a greater possibility of vapour lock when you locate the transducer upstream of the mechanical pump due to the drop in pressure caused by the fuel pump having to suck fuel through the relatively narrow aperture of the transducer.

Injector Pulse Time Fuel Flow

The most accurate fuel flow numbers are available if injector pulse time can be measured.

Flow impellers work, but not as well as injector pulse time. I've used the impellors. The last system had 2, one for incoming fuel and 1 for bypass fuel from the injector system, with a computer that subtracted one from the other. It worked but not that great.

dd

Captain Avgas said:

Builders with fuel injected engines who instal the transducer between the boost pump and the mechanical pump almost universally report that they have very large errors of fuel flow readings when the boost pump is switched on. Perhaps you have noticed this also.

Click to expand...

Not on my install I don't. I have the transducer in the tub between my legs, downstream of the boost pump and prior to the engine-driven pump. On a 35-45 gal refill of the tanks, my fuel computer is less than 1 gallon off, on the high (safe) side. I was careful to keep the flow into and out of the transducer as straight as the space would allow.

Missed understood.

Captain Avgas said:

All of the major manufacturers of engine monitors (JPI, EI, etc) recommend installing the fuel flow transducer after the mechanical fuel pump.

Click to expand...

You missed my point.
Perhaps I wasn?t clear.
The point I was making was that, YES! ALL THE LITERATURE is against what I did.
BUT IT WORKS IN THE REAL WORLD.
The instructions that came with my Floscan ?advised? to put it in a straight section of fuel line, with no bends either side for six inches. There is NO such place on the fuel line of my RV-7. The straightest section is between the Engine pump and the Fuel Injection unit, RIGHT ABOVE THE EXHAUST. Putting it there means removing the Fuel Line Fire Sleeve.

So, what I did was to take the advice of a very experienced LAME (Licenced Aircraft Mechanical Engineer) who runs a company with over 80 GA aircraft operating in the Hot (Vapour lock country) Australian Outback.
He ALWAYS puts the FLOSCAN on the firewall, as in HIS EXPERIENCE it gives the best accuracy there as it is away from HEAT and Vibration.

Now, I am not advocating anyone else does this. I am simply passing on a REAL WORLD experience.
YES! I do get an increase in Fuel Flow when the Boost Pump is turned on; for about one second.
NO! I have never seen vapour locks (or engine hesitation) with the Boost Pump turned OFF even under full take-off power.

And the reason I stuck my nose in here (this subject NOT being my Forte) was that some were putting the Floscan in an even worse place; before both pumps.

Pete.

fodrv7 said:

YES! I do get an increase in Fuel Flow when the Boost Pump is turned on; for about one second.
Pete.

Click to expand...

Are you saying that if you switch on the boost pump for an extended period, say during take-off or landing, the fuel flow reading will be inaccurately high... but it only surges for "about one second"....after that it settles down and reads correctly with the boost pump still on. Many others with fuel injected engines and the transducer before the mechanical pump have reported that the fuel flow reads high for as long as the boost pump is on. I'd just like to clarify this point.

Momentary Rise

Captain Avgas said:

but it only surges for "about one second"....after that it settles down and reads correctly with the boost pump still on.

Click to expand...

Yes!
The Fuel Flow on my IO-360M1B with Precision Fuel Injection rises momentarily when the Boost Pump is turned on and then returns to the reading that existed before it was turned on.

Pete.