MGL Avionics User Forum

Hi all. I just attached a red cube fuel flow sensor following all the protocols. One instruction I have is for a K factor of 68000. The other instruction says that’s for Liters and not gallons so the K Factor should be set at 18000. It also requires a 5k6 resistor to be jumped between the 5v on the RDAC and the FF1 terminal, which I did. I’m getting a fuel flow reading like 36GPH which impossible with the Subaru engine I have. The FF sensor did not come with a resistor so I ordered some from Amazon. It’s listed 5k6 but the rings don’t match what I think it should look like. My question is, can the wrong resistor be causing the ultra high reading using 18000 K Factor? The K Factor setting on the Extreme EMS only goes to 60000 so there’s no adjusting up for correction.
Mike

Hi.

For my Honda engine i set a K factor of 17700. Don't you have a multimeter to check the value of that resistor?

Regards. Manolo.

The resister is fine. All kind of conflicting information from both companies. EI says 68000, Michigan Avionics says 18000. At 18000 I get 36 gph. I’m not sure what I’m doing wrong.

Our systems expect a K-Factor based on liters.
K-Factor is nothing more than the number of pulses the sender creates for one liter of fuel passing through the sender (liters in our case).

The flow sender itself is usually labeled marked by the factory as to how many pluses it creates for a given volume of liquid. The red cube comes with a fixed value of 18000 (in reality the real number is up to you to figure out - it will be a bit different but close to this)

In case of the red cube it is for U.S. Gallons and not liters. Since 1.0 Gallon = 3.7854 liters it means you need to divide 18000 / 3.7854 so you get 4756 pulses per liter so that means a k-factor of 4756 in liters.

You should start with that value.

Now measuring any liquid flow is an art. Not making that up. There are many things that affect flow and how a flow meter interacts with the flow. Viscosity of the liquid varies with composition and temperature. It's fairly stable with most fuels but not 100%. A big factor is pressure of the fuel as it goes through the sender. Usually in our aircraft the pressure is not constant but fluctuates very rapidly and by a very large amount due to the way most fuel pumps work. This can cause a whole pandoras box of troubles. The most obvious one is if you have a fuel pump before the sender and after the sender there is something (anything) that expands with pressure - fuel lines, fuel filters etc. On every pressure pulse fuel is pushed through the sender and something expands to take that pressure as not everything will go into the injectors or carburetors. So the moment the pump is in its input stroke the pressure disappears and the fuel flows straight back through the sender. The sender only adds - it never subtracts. Sharp bends in fuel lines before the sender cause the fuel to rotate (spin) before getting into the sender and that can have an effect - usually that creates linearity problems as the rate of spinning varies with flow rate.
Look for leaks - air leaks on the suction side of the pump will cause a stream of bubbles - that's volume as well and will affect the reading dramatically. With fuel you can also get vapor bubbles forming at any point in your fuel system that causes low pressure like a narrowing or similar - the effect is the same.
Finally keep in mind that some fuel systems have return flows - flow through the pump is always at maximum (optimized for a constant pressure and feed rate pump) and any fuel not immediately taken up by the engine simply flows back to the tank via a separate line (via a pressure regulator in fuel injected engines). Obviously this is a challenge for fuel measurement systems.

Hi Rainier and thanks so much for getting back to me with some great information. I think maybe the math doesn’t add up and we are supposed to multiply and not divide. Per Electronics International, makers of the Red Cube FT 60: “ The FT-60 transducer generates a pulsed signal at 68,000 pulses per gallon. That converts to 17,965 pulses per litre”.
At 3.7584 liters per gallon, and a k factor of 18000 pulses per hour per litre, to find pulses per gallon multiply 3.7584 times 17965 = 67,519 pulses per gallon roughly. Is what you’re telling me about the Extreme EMS that I MUST use the per litre k factor as that is how the computer works? The lower I go on the k factor the higher my gph shows. I’m sorry I’m having trouble wrapping my head around this.
Mike