Capacitance Fuel Probe / Sender

Tim Terry

Supporter
I've had some trouble with my OEM fuel sender and would like to switch to a capacitance type. Any experience here you can share? Brand? Source? Tips and tricks? Pitfalls?

Thanks!
 
The problem with capacitance systems is that they are calibrated for a specific fuel dielectric. If you get race gas, ethanol mix, or water in the fuel, the gauge will no longer be accurate. This has been a big discussion in the homebuilt aircraft community. Here is a quote from a popular forum (100LL is aviation fuel):

The output of a capacitance fuel quantity measuring device depends on a property of the fuel called dielectric constant, which is a measure of how much electrical charge the fuel can hold per volt of an applied voltage, compared to air. The dielectric constant is dependent on the chemistry of the fuel. Once you calibrate for 100LL, you it's not likely you'll see much difference between different name brands of 100LL, as all brands must meet a spec which doesn't allow much variation in the chemistry.

I'm not sure how the dielectric constant of non-ethanol mogas compares with 100LL, but I think both are in the vicinity of 2 (that is, fuel holds about twice charge per volt than does air). The dielectric constant of ethanol is 24.3 (per wikipedia) so it greatly affects the dielectric constant of the fuel. The effect of ethanol-laced fuel on a sender calibrated for 100LL would be to show a significantly higher fuel level than is actually present.
 

Jasper

Supporter
I have one of the reed switched VDO's in my Cobra (Backdraft). I like them, simple and robust. However....
There are a finite number of 'reed switches' long the travel of the float. In my 180mm length that I got for the Cobra, there are only 4(maybe 5?) different steps defined. That means your fuel gauge moves in 4 (or 5) integer steps, not linear.
Not ideal...
 

Clayton

Supporter
I have one of the reed switched VDO's in my Cobra (Backdraft). I like them, simple and robust. However....
There are a finite number of 'reed switches' long the travel of the float. In my 180mm length that I got for the Cobra, there are only 4(maybe 5?) different steps defined. That means your fuel gauge moves in 4 (or 5) integer steps, not linear.
Not ideal...
Well that’s disappointing.
You would hope the tubular type is more linear.

Clayton
 
I've got the centroid ones in my car - They have adjustment screws to calibrate with your gauges. I need to add a couple of resitors to get them within range. I'm happy with them.
 
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