Fuel Flow while on Aux Pump

[BobTurner]

Originally Posted by Bevan

If the red cube measures fuel with a wheel turning in the flowing fuel, and some device to count the paddles of the wheel as they go by, and if this red cube is located in the fuel line between the two pumps, it would seem to me that the increase in readings would be caused by a hydraulic interaction between the two pumps. One would think that the boost pump is a more constant flow/pressure than the diaphragm type mounted to the engine. This would cause the fuel at the measuring device to have a component of back and forth movement as well as pure forward movement. This back and forth movement would be picked up by the counter as and extra "count" at certain points in its rotation hence the larger reading.

My thoughts only.

Bevan

Following the above theory Matronics sells a damper (vertical tube with trapped air at the top) which they claim eliminates the error. Has anyone tried it? I see the same thing, although not as large as reported above.

[Bevan]

I solved this and another problem by relocating the flow sender (Floscan) to the line between the fuel servo and the spider. Now my readings don't change with application of aux pump.

A pressure damper may work but where do you put it? before or after the measuring device or both?

This would add more fittings, weight and possibly more volume of fuel in the engine area which could be subject to absorbing heat depending on location.

Bevan

[Sam Buchanan]

Originally Posted by Bevan

If the red cube measures fuel with a wheel turning in the flowing fuel, and some device to count the paddles of the wheel as they go by, and if this red cube is located in the fuel line between the two pumps, it would seem to me that the increase in readings would be caused by a hydraulic interaction between the two pumps. One would think that the boost pump is a more constant flow/pressure than the diaphragm type mounted to the engine. This would cause the fuel at the measuring device to have a component of back and forth movement as well as pure forward movement. This back and forth movement would be picked up by the counter as and extra "count" at certain points in its rotation hence the larger reading.

My thoughts only.

Bevan

That is precisely the explanation I've heard numerous times since 1999 (RV-6, Facet pump) when this question arises.

[Lenny Iszak]

Had the same problem on the 10. Moved it to between the servo and the spider. As rightrudder mentioned, now I forget to turn off my fuel pump

One negative of having the flow sender between the two pumps is the slightly inaccurate fuel totalizer numbers. Not a big deal though unless you use the aux pump for longer periods. Even then you're still on the safe side cause you'll have more fuel in the tank than the totalizer reports.

The sender I had in the tunnel was a FloScan and it was jumping around too much for it to be accurate enough to collect data for LOP injector balancing. The redcube is much smoother, probably because of its new location.

Lenny

[darata]

Hi guys,

Maybe I can offer a bit of insight. When the FT-60 is mounted between the boost pump and the engine driven pump and the boost pump is turned on, it creates the hammer effect against the engine driven pump similar to what is observed in residential plumbing. We've all heard the knocking pipes. This is what happens in a the fuel system, to a much lesser degree of course. However, when this does occur, it causes the impeller of the FT-60 to rotate forward then backward quickly. Those additional rotations are counted and displayed as higher fuel flow. That is the primary reason why we recommend the transducer be mounted after any and all fuel pumps.

[alpinelakespilot2000]

Originally Posted by Lenny Iszak

One negative of having the flow sender between the two pumps is the slightly inaccurate fuel totalizer numbers. Not a big deal though unless you use the aux pump for longer periods.

My experience too, having the sender b/w the tank selector and boost pump. Since I do a lot of pattern work when it's not cross-country season, having the boost pump on all the time makes my fuel totalizer pretty much useless. One of my next projects is to place the sender between the engine pump and carb.

[istrumit]

Originally Posted by darata

Hi guys,

Maybe I can offer a bit of insight. When the FT-60 is mounted between the boost pump and the engine driven pump and the boost pump is turned on, it creates the hammer effect against the engine driven pump similar to what is observed in residential plumbing. We've all heard the knocking pipes. This is what happens in a the fuel system, to a much lesser degree of course. However, when this does occur, it causes the impeller of the FT-60 to rotate forward then backward quickly. Those additional rotations are counted and displayed as higher fuel flow. That is the primary reason why we recommend the transducer be mounted after any and all fuel pumps.

Thank you David. That makes perfect sense. Understanding it means I won't worry about it. I plan to leave it as is and treat it as a reminder to turn off the pump !

[jay bell]

My red cube is between the electric and mechanical pumps, and it used to read acurately before I swapped my Champion mechanical pump for a Tempest pump. Now fuel flow reads high with boost turned on. It certainly caused me concern until I followed up the related threads.

There is/was nothing wrong with either of my mechanical pumps; I damaged the Champion while attempting a pump shroud installation. The replacement Tempest unit was verified by my engine shop after I noticed the erroneous flow readings.

I will replace all my hoses at 5 years, and relocate the red cube near the servo at that time. I would be curious to see if anyone else has correlated the erroneous flow readings with a specific brand of mechanical pump

Jay

[lr172]

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