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Adjusting K-Factor with new fuel pump

seagull

Well Known Member
I have changed the fuel pump to the 40135 which has more pressure than the previous pump. My fuel remaining on the Dynon with the old pump was spot on. At the end of any flight the tank had exactly the amount the Dynon said it had.

Now with the new higher pressure pump the numbers are still perfectly accurate.

I would have thought that with more pressure there is more volume and since we only have one fuel flow sensor the K-factor would need to be adjusted.

Where am I wrong in my thinking?
 
The fuel is not compressible at these pressures and the sensor simply reads the volumetric rate and calculates as a running total. At 5-40 ksi it is compressible but that is another matter.

So, why would you think higher pressure would affect the fuel sensor?
 
The fuel is not compressible at these pressures and the sensor simply reads the volumetric rate and calculates as a running total. At 5-40 ksi it is compressible but that is another matter.

So, why would you think higher pressure would affect the fuel sensor?

Higher pressure should flow more fuel through orifice back to the fuel tank so overall fuel flow through the transducer should be higher with higher fuel pressure.

Seems to make sense when you say it fast...
 
Jim,

Bill seems to be saying the accuracy is not affected, not that the flow rate is unaffected by the higher discharge pressure.

Rich
 
I think Bill has a RV-7 and I don’t think his fuel system recirculates the fuel. May be the basis for his answer.
 
I think I answered my own question. This might explain why there is little to no difference on the Dynon.

Based on the specification page at https://www.facet-purolator.com;

Part Number: 40105
Max/Min: 4.5 - 3.0
GPH: 30

Part Number: 40135
Max/Min: 7.0 - 4.0
GPH: 32

They have increased the pressure without a significant change in flow rate.
 
Higher pressure should flow more fuel through orifice back to the fuel tank so overall fuel flow through the transducer should be higher with higher fuel pressure.

Seems to make sense when you say it fast...

This would be true if the sensor is measuring both engine usage and bypass flow. A less than desirable way to plumb the system, it would seem, although it could work nicely with internal calculations instead of a direct reading panel result, or the bypass is very small, like to prevent vapor lock or for vapor collection and purge.

Jim,

Bill seems to be saying the accuracy is not affected, not that the flow rate is unaffected by the higher discharge pressure.

Rich

Yes Bob, I was referring to the accuracy of the measurement through the device. I was assuming it was the red cube of the impeller type.

I think Bill has a RV-7 and I don’t think his fuel system recirculates the fuel. May be the basis for his answer.

Correct - - Back to what the sensor measures. If the sensor truly measures total flow and there is an orifice bypass, the bypass could be quite small. In that case the fixed offset of FF relative to total flow could be a small error. I am not familiar with your fuel system schematic or the splits in flows between consumption and bypass.

edit - I see this is a linear shuttle pump and with the pressures must be a carb. I don't understand where this orifice bypass is. Sorry for confusion.
 
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The red cube doesn’t have an impeller.. it is optical.

The red cube DOES have an impeller, though you are correct that it uses an optical sensor to count the impeller revolutions. The K factor is used to convert fractional impeller revolutions to GPH.

Larry
 
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This would be true if the sensor is measuring both engine usage and bypass flow. A less than desirable way to plumb the system, it would seem, although it could work nicely with internal calculations instead of a direct reading panel result, or the bypass is very small, like to prevent vapor lock or for vapor collection and purge.

This is exactly how the system is designed, and operates.


edit - I see this is a linear shuttle pump and with the pressures must be a carb. I don't understand where this orifice bypass is. Sorry for confusion.

You are posting to a discussion in the RV-12 forum. The RV-12 has two carbs actually, and because of that, a fuel system that is a bit different that what is common on the rest of the RV models.
 
So, several flights now with the new 40135 Facet electric fuel pump. Looks like 0.3-0.5 add'l fuel flow showing in cruise flight. I'll adjust "K" factor at some point - just not now.
 
So, several flights now with the new 40135 Facet electric fuel pump. Looks like 0.3-0.5 add'l fuel flow showing in cruise flight. I'll adjust "K" factor at some point - just not now.

I am seeing similar in fuel flow but will trend it for a few more hours to be sure. I do not see the fuel remaining changing though.
 
The red cube DOES have an impeller, though you are correct that it uses an optical sensor to count the impeller revolutions. The K factor is used to convert fractional impeller revolutions to GPH.

Larry

I love it when I learn something! You are right! I thought these were purely optical.. Here’s a Bearhawk guy taking apart his FT-90 gold cube.. I assume they are similar in construction..
Thanks!

https://youtu.be/HXJMLcpcl10
 
In the end the value of the fuel flow rate is more significant for its consistency than the absolute value. When I see a value I’m not used to it raises an eyebrow.
 
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