Almost every fuel-tank sender I know of is resistive and
runs electricity thru the tank.  Except for that floating
stick protruding out of the tank on a Piper J3 cub.

For the most part, they work fairly well on an automobile,
though not very accurate -- both because of the meter
innacuracy and the fuel being uneven in the tank at most
times.

AFAIK, it's a very simple circuit with the voltage source
in series with the resistance and the meter.

I'm slowly converting most of my gauges to digital (using
PIC's of course!), and for the fuel gauge, I intended to
just run that resistance thru a simple voltage divider
and into an analog port on the 16F872.  Calibration would
be done within the code.

Cheers,
-Neil.





-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Benjamin Bromilow
Sent: Tuesday, June 25, 2002 11:20 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: Fuel tank sensor / real world solutions


Hi all....

I've just taken delivery of a fuel tank sensor and display for the car I'm
building next week :)
However, the sensor has got me thinking.... Many moons ago, there was a
long thread on this list about sensing fuel levels in containers. I seem
to remember optical solutions where popular. Most people shyed away from
anything involving electricity within the fuel tank. So when I took receipt
of my sensor I was rather surprised to find an open potentiometer- a float
arm actuates the wiper (all this under petrol or in the vapours). This leads
to believe me that
1) the company that produces this is heavily insured :)
2) I may have pre-warmed petrol and/or a free afterburner function
3) the meter limits the current going down the sensor.

I tested the resistance and it was fairly linear 0-300 ohms. The meter
appears
to be moving coil/iron but it's sealed so I can't peak inside. Any ideas
on how the internals of the meter work? I'm also curious how they stop the
petrol guage from being adversely affected by the car battery voltage
(9-15v)....


I'd probably be tempted to use the R as part of a RC oscillator circuit
going into a F/V converter. Hopefully that way I'd limit the current through
the potentiometer. I'm not sure what heat dissapation I'd be happy with
in my petrol tank though!! If I was going to invent it from afresh I'd use
a platic bowden cable off the float arm and have it actuate an external
potentiometer.... But any ideas how they do it in the real world??

Ben

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