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  #1  
Old 05-17-2021, 04:47 PM
seagull seagull is offline
 
Join Date: Dec 2019
Location: San Bernardino
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Default Adjusting K-Factor with new fuel pump

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?
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  #2  
Old 05-17-2021, 06:16 PM
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Stansuski Stansuski is offline
 
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I found myK factor to also not need changing with new pump.
Stan
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  #3  
Old 05-17-2021, 06:23 PM
BillL BillL is offline
 
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Location: Central IL
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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?
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  #4  
Old 05-17-2021, 06:47 PM
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Piper J3 Piper J3 is offline
 
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Quote:
Originally Posted by BillL View Post
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...
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  #5  
Old 05-17-2021, 08:51 PM
RFSchaller RFSchaller is offline
 
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Jim,

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

Rich
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  #6  
Old 05-17-2021, 09:00 PM
seagull seagull is offline
 
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I think Bill has a RV-7 and I dont think his fuel system recirculates the fuel. May be the basis for his answer.
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  #7  
Old 05-17-2021, 11:23 PM
seagull seagull is offline
 
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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.
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  #8  
Old 05-18-2021, 07:06 AM
BillL BillL is offline
 
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Quote:
Originally Posted by Piper J3 View Post
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.

Quote:
Originally Posted by RFSchaller View Post
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.

Quote:
Originally Posted by seagull View Post
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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Lord Kelvin:
I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.

Last edited by BillL : 05-18-2021 at 07:13 AM.
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  #9  
Old 05-18-2021, 07:36 AM
Taltruda Taltruda is offline
 
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Quote:
Originally Posted by BillL View Post


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.

The red cube doesnt have an impeller.. it is optical.
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  #10  
Old 05-18-2021, 07:56 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Taltruda View Post
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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Last edited by lr172 : 05-18-2021 at 07:59 AM.
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