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dwilson

Well Known Member
I am flying a GRT system with a flowscan flow meter inserted between the gascolator and the mechanical fuel pump. My install has a piece of aluminum tubing out of the gascolator then 180 deg into the flowscan sensor just above the gascolator.

The electric fuel pump is located in the usual position on the cabin side of the firewall with the usual plumbing to the gascolator.

When I activate the electric fuel pump my reading jumps about 3 gph. When I turn off the electric fuel pump it settles back down.

Anyone else experience this?

Is the gage inaccurate with the e-pump on or is it really using that much extra gas????? (seems like the engine would flood)

Duane
 
Duane,

Yep, mine does the same and it was something I was warned about when I did this installation.
 
dwilson said:
I am flying a GRT system with a flowscan flow meter inserted between the gascolator and the mechanical fuel pump. My install has a piece of aluminum tubing out of the gascolator then 180 deg into the flowscan sensor just above the gascolator.
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Duane, for a truly accurate reading you need to have your sending unit mounted after your mechanical pump. Basically you are interested in reading what your flow is right before it goes into the carb or in the case of an injected engine, the fuel servo or the flow divider.

Your boost pump in this case is just providing the mechanical pump some extra head pressure in the inlet line, but your sender is reading that as true flow, which it isn't. I wouldn't be alarmed by it, but it will throw off your fuel flow calculations a bit for the amount of time you have the e-pump on.

Rob
 
When I activate the electric fuel pump my reading jumps about 3 gph. When I turn off the electric fuel pump it settles back down.
Anyone else experience this?
Is the gage inaccurate with the e-pump on or is it really using that much extra gas????? (seems like the engine would flood)
Click to expand...

Very common. The explanation I have seen is in respect to the design of the popular fuel flow sensors. These sensors have a little turbine wheel in the fuel stream and an optical sensor that "counts" the turbine vanes. This electrical pulse is sent to the engine monitor which yields a solution that displays fuel flow.

The electric diaphragm pump creates interruptions in the steady fuel flow which cause the turbine wheel to oscillate which in turn generates "extra" pulses. Consequently, the optical sensor counts higher than the actual flow would dictate, and we see an erroneously high reading on the engine monitor.

As long as we understand the mechanics of the fuel flow sensor there is no reason to worry about the higher reading with the pump running. Actual fuel flow has not increased, just the number of pulses the optical sensor is seeing.
 
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Mine does the same thing, but only adds about 1.0 gph. One of the reasons I like my "Fuel Pump ON" light is that it reminds me not to leave the fuel pump on when I don't need to, or I'll pollute my Totalizer readings. It's not a huge deal if you use the fuel pump for take-of and landing only - it will cause the totalizer to read just a little more than you have actually used, which builds in a little reserve. I sure wouldn't want the totalizer off in the OTHER direction!

Paul
 
Avatar

OT, I know, but I have to say, Sam you get the best Avatar of the year award. "Beating a Dead Horse" and animated too.

that's genius!

Rob
 
N908RV said:
OT, I know, but I have to say, Sam you get the best Avatar of the year award. "Beating a Dead Horse" and animated too.

that's genius!

Rob
Click to expand...

Thanks guys, but I can't take credit for it.....I lifted it from another forum somewhere. :)
 
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