Fuel Flow - Gallons Litres.

[sunfish]

The installation manual specifies the initial calibration setting of 68000 pulses/gallon for the red cube FT-60 flowmeter (page 7 - 57).

This is not specified as USG or Imperial gallons, which is it?

Does Skyview automatically do the conversion to litres if I set units as litres or do I need to set pulses per gallon at 68000/3.78541 or 68000/4.54609 depending on the definition of the gallon?

 

[skysailor]

The 68000 pulses per gallon is for USG by my experience. Since it is a pulse per unit volume you are setting, I think if you input this in the Dynon in setup, when you put the unit in Litres it will all indicate correctly. That is my understanding but Dynon Support will have the answer and may reply to your thread as well.

 

[dlloyd]

You're making this too hard for yourself. Set your display to gallons or pounds or liters and Dynon is already set up to do the math.

 

[vlittle]

I have found that the gallons setting is more precise than the liters setting. Gallons is in tenths, while liters are in whole numbers. Makes leaning and calbrating injectors more difficult.

V

 

[RobertHamilton]

The accuracy of both is the same, and is the accuracy of the fuel flow sensor. While the resolution is 2.5 times finer for Gallons, Liter units still matches accuracy pretty well.

 

[GlennB]

Another point, which I learnt along the way:

Even if you select litres/hr as the default fuel flow display, the GAMI spread when using “lean find” will still be depicted in USG.  Thus you get the higher resolution of tenths of a gallon for that parameter...

..and consistent units of measure when claiming bragging rights over others. Our spread is 0.1 gph.  Game on!   

 

[sunfish]

The linearity of the FT60 is quoted as 1% and the repeatability is quoted as 0.25%.

As a general measurement rule you would expect to see a resolution of 10% or less of full scale reading.

The Gal/hr flow rate does that with its one decimal point, but to my mind for a Rotax 912 iS in cruise at around 15 l/hr +- half a litre is a little coarse - and even then I'm assuming that skyview rounds up or down at the .49 -.50 mark which it might not do.

To put that another way, if the reading is "14" that means somewhere between 13.50 and 14.49, which is a bit too big a gap for a 912 iS fuel flow.

The FT60 is accurate enough to support one decimal place of litres and that would allow more accurate trend monitoring and fuel flow setting instead of +- 0.5litres.

I know a tenth of a litre is nothing for a big Continental or Lycoming that swallows juice by the gallon, but to us folks with nothing bigger than a sewing machine up front 0.1 litres might be signifigant.

 

[GlennB]

The linearity of the FT60 is quoted as 1% and the repeatability is quoted as 0.25%.

As a general measurement rule you would expect to see a resolution of 10% or less of full scale reading.

The Gal/hr flow rate does that with its one decimal point, but to my mind for a Rotax 912 iS in cruise at around 15 l/hr +- half a litre is a little coarse - and even then I'm assuming that skyview rounds up or down at the .49 -.50 mark which it might not do.

To put that another way, if the reading is "14" that means somewhere between 13.50 and 14.49, which is a bit too big a gap for a 912 iS fuel flow.

The Red Cube is indeed highly accurate, once calibrated.  For me, then, its main utility is to indicate how much fuel has been used, and how much is left.  Since I’m leaning to an EGT ROP/LOP, the indicated flow is a result, not a target.  As long as it’s somewhere near the expected value, and all cylinders are performing equally, I’m happy that all is well.

Since there’s no manual leaning on your 912iS, may I suggest that the resolution of its indicated flow is even less relevant?  If the number is a litre more or less than expected, is any action required or even possible?  I’d expect that, should you suffer a blockage or leak, the flow will change by a lot more than that, and you’ll notice it.

The FT60 is accurate enough to support one decimal place of litres and that would allow more accurate trend monitoring and fuel flow setting instead of +- 0.5litres.

I know a tenth of a litre is nothing for a big Continental or Lycoming that swallows juice by the gallon, but to us folks with nothing bigger than a sewing machine up front 0.1 litres might be signifigant.

For trend monitoring, there’s a lot of value in establishing a SavvyAnalysis account, and keeping an eye on your datalogs over a period.  That will likely tell you a lot more than indicated flow during the flight, as you’ll benefit from the high internal resolution of the Skyview and you can correlate it with other data such as RPM, cruise alt, OAT etc.

Checking fuel uplift after a flight against the expected number from the Skyview will confirm that the Red Cube calibration hasn’t shifted.