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Odd readings from fuel capacitance plates

david.perl

david.perl

Well Known Member
Plane has just passed the 200 hour mark.

I noticed a couple of months ago whilst refueling the tanks, they were overeading about 20 ltrs (4-5 gallons) per side when about 1/3 full. This is on both sides so that would eliminate a bad electrical join. I've only used Avgas so am pretty sure its not a fuel mismatch issue.

When full, the readings are accurate so the capacitance plates are putting out the correct voltage at that point, however below that, the misreading somewhere starts and then continues.

Im keeping track of fuel burn with my red cube which is pretty accurate.

I plan on draining both tanks and undertaking a recalibration on both tanks.

Before doing that I wondered if anyone might shed some light on this odd behaviour?
 
I drained both tanks and recalibrated the sensors.

The problem hasn't gone away which i dont understand.

Anyone got any other ideas as to why the readings should be different?

I plan to take the voltage readings from the capacitance plates inflight in 10 ltr incremental drops on each side and then adjust the voltage readings as per each drop on each side to try and get more accurate readings from the plates
 
Mine are erratic but I never trust the Vans units anyway. I have digital FF indicator which is accurate to within 1 Ltr per full tanks (pretty good) that plus an accurate wooden calibrated dip stick I know my fuel Qty very accurately at anytime, comforting👍
 
I'm sure my setup is different; I have the Vans plates, dynon converters and GRT eis4000, mine is only accurate at full and empty. I can only calibrate an offset and scaling factor, not specific points in between.
I've had fuel leaking through the bnc connector cause a shift in the fuel readings. I don't remember the exact number, but I think it was reading 5 to 10 gallons high.

I'm not sure if it was the fuel wicking through the connector or fuel getting into the converter that was the problem.
 
jcaplins said:
...
I've had fuel leaking through the bnc connector cause a shift in the fuel readings. I don't remember the exact number, but I think it was reading 5 to 10 gallons high.

I'm not sure if it was the fuel wicking through the connector or fuel getting into the converter that was the problem.
Click to expand...

If you didn?t seal BOTH ends of the wire from the inboard capacitance plate to the center terminal of the BNC connector, it WILL leak. Fuel has a very low surface tension and will wick between the conductor and insulation inside the wire.
 
longranger said:
If you didn?t seal BOTH ends of the wire from the inboard capacitance plate to the center terminal of the BNC connector, it WILL leak. Fuel has a very low surface tension and will wick between the conductor and insulation inside the wire.
Click to expand...

Agree 100%. For 1 tank I either forgot to seal the wires or I just didn't do a good enough job.

I put a tiny o-ring in the bnc connection, and a year later seams to still be working great.
 
I haven't flown an airplane yet that had an accurate fuel level reading in flight using a method that measures the level directly. Floats stick, or don't read the full tank. Capacitive senders can be confused by different fuels or additives.

By far the most accurate is the dip-stick before flight. Equally accurate has been the red cube fuel flow sender installed firewall forward. If I were building new, I wouldn't put any senders in the tanks. I would put a red cube in each wing root (or on each side of the fuel selector) and measure flow out of each tank directly. No senders in the tanks = less places for leaks, and more volume in the tank for fuel.
 
Snowflake said:
I haven't flown an airplane yet that had an accurate fuel level reading in flight using a method that measures the level directly. Floats stick, or don't read the full tank. Capacitive senders can be confused by different fuels or additives.

By far the most accurate is the dip-stick before flight. Equally accurate has been the red cube fuel flow sender installed firewall forward. If I were building new, I wouldn't put any senders in the tanks. I would put a red cube in each wing root (or on each side of the fuel selector) and measure flow out of each tank directly. No senders in the tanks = less places for leaks, and more volume in the tank for fuel.
Click to expand...

Agree with using a dipstick. I time my flights and the fuel computer is a backup telling me how much is remaining. I never look at the fuel gauges other than to note they are there and working.

Unfortunately FAR 91.205 says you will need fuel gauges regardless if you want them or not (for the US anyway).
 
I really like the idea behind the Belite sensors, but unfortunately they don?t seem to work well with vented fuel systems. They measure tiny pressure changes and equate that to the fuel remaining, but the vent system changes the pressure above the fuel which completely disrupts the process. There are other threads in vaf addressing this problem.

Gordon
 
Waiex-guy said:
I really like the idea behind the Belite sensors, but unfortunately they don?t seem to work well with vented fuel systems. They measure tiny pressure changes and equate that to the fuel remaining, but the vent system changes the pressure above the fuel which completely disrupts the process. There are other threads in vaf addressing this problem.

Gordon
Click to expand...

Hmmm... I?m disappointed to hear this. Can you point me in the direction of those threads that talk about this??
 
jcaplins said:
Unfortunately FAR 91.205 says you will need fuel gauges regardless if you want them or not (for the US anyway).
Click to expand...

Actually...no...unless you are flying night or IFR, in which case, most modern Op Lims require you to equip per 91.205. But if you read the introductory paragraph to 91.205, it requires all of that equipment for an aircraft with a Standard Airworthiness Certificate. Experimentals have Special certificates, so 205 does not apply - unless specifically called out in your Ops Lims.
 
h&jeuropa said:
The problem with the Belite is that it measures gage pressure, which is pressure relative to ambient pressure. If you have a typical fuel tank vent which is pointed into the airstream, your tank is actually pressurized. The pressures we are trying to measure for fuel quantity are quite small and this pressurization will cause large errors.

I know this because I originally tried a gage pressure sensor and could not get it to work.

I made my own system using a differential pressure sensor so I measure the difference in pressure between the top of the tank and the bottom. I used a Freescale sensor which outputs 0-5 volts which I feed directly into my GRT EIS. It works well.

You could accomplish the same thing by using two Belite probes, one at the top of the tank and one at the bottom. The pressure difference of the two would be fuel quantity.

Jim Butcher
Click to expand...

Hey Jim,
I know your post is from an old thread but I?m reviving it in hopes that you?ll be able to expand on this a little bit. Are you suggesting that if two of the Belite probes were used in each tank... one at the top and one at the bottom of the inboard bulkhead, and then tying the wires together where they connect to little computer interface, that the differential readings would give the proper fuel level reading? Also, would there be any differences in readings if you were sitting on the ground VS. flying... or flying at different speeds when the pressure in the thanks would vary depending on speed and or different density altitudes? Thanks!
 
jcaplins said:
Agree 100%. For 1 tank I either forgot to seal the wires or I just didn't do a good enough job.

I put a tiny o-ring in the bnc connection, and a year later seams to still be working great.
Click to expand...

When you say "in" the BNC connection, where exactly to you mean? Inside the nylon inner "sleeve", or inside the bayonet fitting and outside of the central sleeve itself?
 
RV7A Flyer said:
When you say "in" the BNC connection, where exactly to you mean? Inside the nylon inner "sleeve", or inside the bayonet fitting and outside of the central sleeve itself?
Click to expand...

I installed the o-ring around the pin of the Male BNC connector.

Hopefully this image will work:
jYTq3jCVmmJvt4dsYJlRio2xac92ZaJTG6pYh_pEqOtdwZuN1YD-n5DSceecQtlMQrE73V1Zv7XdYz1DKHiWfmPSLpqyeIO5CcXhAQ3Mb-u0Lk_MA-wwizTJmvWvFgh3obrI-i9K0A=w2400


https://photos.app.goo.gl/nTrK1WSQJdyhEYTP7
 
Ironflight said:
Actually...no...unless you are flying night or IFR, in which case, most modern Op Lims require you to equip per 91.205. But if you read the introductory paragraph to 91.205, it requires all of that equipment for an aircraft with a Standard Airworthiness Certificate. Experimentals have Special certificates, so 205 does not apply - unless specifically called out in your Ops Lims.
Click to expand...

I stand corrected. Thanks.
 
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