Flow Scan F/F errors

[ghostriver]

Flow Scan fuel flow shows a fuel flow of 25 L/hr with the boost pump off and 34L/hr with the boost pump on, both after reaching a steady state (about a minute) with no power change. (Similar results at different power settings) The boost pump is about 25” before the Flow Scan.
I don’t quite understand this as the boost pump is before the Flow Scan, which is before the gascolator and engine driven pump. The engine is an 0-320 carbureted. There is no fuel leak either. To me, the actual fuel flow should not be changing since there are no changes in any of the engine parameters- cht, egt, rpm mp etc.
This would affect the “calculated” burn and “calculated” fuel remaining.
Do you have any ideas as to why this is happening? And how to remedy this.

 

[dynonsupport]

You're the first customer of ours that has reported a phenomenon like this and we're not exactly sure what to make of it.

This builder had what sounds like the same problem and describes his troubleshooting on his website.

 

[aceflyingservice]

The boost pump can cause a pulsing in the fuel line causing the vanes in the flow meter to pulsat back and forth. The meter interprets this as positive fuel flow regardless of which way the fuel is flowing. Voyager had the same problem, causing a lot of head scratching as to how much fuel was actually on board.

 

[ghostriver]

I am wondering if a mini-surge tank after the boost pump might solve the problem. Anyone with any ideas?

Bob

 

[ghostriver]

Or a check valve? I would hate to mess with the fuel system without knowledge as to what I am doing, so any comments are welcome.

Bob

 

[meljordan]

Matronics makes a "Fuel Pulsation Dampener" ( http://www.matronics.com/fuelchec/dampener.html ). I don't have any direct experience with this, but it appears to be designed specifically to address the issue you are experiencing. It would be nice to hear from anyone that has used one to see if it helped correct this sort of problem.

Regards,
Mel Jordan, RV-6A
Tucson

 

[ghostriver]

Thanks for the info. It costs $45 with on line discount less shipping. I've ordered one so will post results after I fly with it.
Bob

 

[Richard Talbot(Guest)]

To keep things simple, why not use one of the sensor inputs on the EMS to detect when the fuel pump is on? That way you could allow another K value to be calibrated to the device in this configuration.

Better still write a simple calibration routine that can be used by the builder to automatically determine the correct calibration amount whilst in flight.

Richard

 

[dynonsupport]

Richard,
The K factor with the fuel pump on/off isn't a bad idea, but I think that the real issue is that the fuel flow fluctuates so much that may not be accurate. When it's acting funny like that, there probably isn't a K factor that is correct at that point.

As for calibrating in flight, the only way to calibrate a K factor properly is to flow a known amount of fuel through the flow sensor. The best way to do this is to put a LOT of fuel through it, like 50-100 gallons, and then look at the difference betwen real and measured fuel flow. If there was a way for us to automatically set up the K factor without requiring the pilot to keep track of fuel for a few flights then we would do it for sure.