Fuel Flow?

[rdockery_3]

I've noticed that if I switch my electric fuel pump off during cruise that my fuel flow will drop by 1 gph. I know that this isn't actually the case, I should get the same fuel consumption with both pumps on as I would with the electric off. I just wanted to understand why the D180 reads so much higher flow with both pumps on.

 

[seagull]

With the electric pump on there is a more pressure, and more flow through the return orifice. The Dynon reads a total.

 

[BobTurner]

Even with no return line to deal with, I see a small increase in in measured flow with the aux pump on. It’s complicated, but most likely the aux pump introduces a bit of swirl in the flow and the impeller sensor, close down stream, turns a bit faster.