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Vans fuel gauges

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lstinthot

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I have a set of Van's fuel gauges in a Murphy. They are not accurate at all. I'm in the process of filling the tank one gallon at a time and taking resistance measurements as I fill. It appears the sending units are working correct when I drained the tank. Resistance went up from around 30 ohms to 250 ohms.

I've verified the wire from the sender is good and the sender is grounded good.

When the sending unit wire is hooked up to the gauge and power is off, resistance is good through the sender. When I turn power on the resistance goes to zero.

I did not build this, in the process of buying it.
 

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lstinthot said:
I have a set of Van's fuel gauges in a Murphy. They are not accurate at all. I'm in the process of filling the tank one gallon at a time and taking resistance measurements as I fill. It appears the sending units are working correct when I drained the tank. Resistance went up from around 30 ohms to 250 ohms.

I've verified the wire from the sender is good and the sender is grounded good.

When the sending unit wire is hooked up to the gauge and power is off, resistance is good through the sender. When I turn power on the resistance goes to zero.

I did not build this, in the process of buying it.
Click to expand...
are they not accurate, or are they not linear? the only tank that will show a linear response is a rectangular tank. those senders and gauges are a simple voltage divider that derive the indication from arc of movement vs ohms. if the shape of the tank does not allow for a perfectly linear drop in level (which a wing tank never will) you will never get a linear movement.
 
I believe the float is moving based on the resistance readings going up as I drained the tanks. It's when I hook the wire up I'm not understanding the gauge response.
n82rb said:
are they not accurate, or are they not linear? the only tank that will show a linear response is a rectangular tank. those senders and gauges are a simple voltage divider that derive the indication from arc of movement vs ohms. if the shape of the tank does not allow for a perfectly linear drop in level (which a wing tank never will) you will never get a linear movement.
Click to expand...
I went through and added a half a gallon at a time today. I measured resistance at each step. The resistance started to change after 2.5 gallons in the tank, however the indicators did not move till 8 gallons were in the tank. They were pretty linear the rest of the way up. Basically says when I hit zero, I have 8 gallons in that tank.

Took plane up for a 40 minute flight with the fuel mid band of level and they seemed more stable today. Maybe I cleaned some corrosion off the connectors when I had the wires off.

I also took measurements with a dip stick so I can have accurate dip measurements for pre-flight.
 
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