Fuel Level Sensor design 1

[CarlFST60L]

Hi guys,

I need to come up with an easy way to measure the fuel in a set of vehicle for telementry that is accurate to around 1L.

I dont know enough about how these cars actually mesure the fuel level, i believe most use a float and resistor wheel of some sort?

Most cars, i believe, do have access to the fuel level via the OBDII inerface.

I would much prefer to design an OP amp with a very high impedence input, measure the 'signal wire' and have a proportinal voltage output which i can use.

Any advice, tips, or even products that already do this would be appreciated.

Carl.

 

[RajeevCell]

Arguably the best method may be thermal sensing. Take a physically small thermistor and drive a small current through it. If it is located in air (or fuel vapor), it will self heat and the electrical resistance will significantly change. If it is submerged in fuel, the fuel will very effectively carry away the heat, and the electrical resistance will remain in the "normal" range.

This micro-RTD array system is indeed interesting. However, supplying heat to a fuel container doesn't sound very safe. I have not done the math, though. There must be some restrictions on it.

 

[Warpspeed]

The amount of heat required is very small (milliwatts). And the required temperature rise for reliable operation would be perhaps a few tens of degrees celsius above ambient. Well below the flash point of fuel.

The fuel passing through the fuel rail of your EFI engine is heated up to almost full engine operating temperature, maybe 85C without creating any problems.

 

[cbarn24050]

You wouldn't need nything like tens of degrees however circuit failure could result in a spark so it wouldn't easily pass safety regulations.

 

[itsmoked]

Naw this is done all the time! Think of the existing noisy garbage rheostat that is immersed in your gas tank right now! It probably sparks regularly. Gasoline in a gas tank is NOT flammable. Gasoline is flammable only in the range 1.4 o 7.6 percent mixture ratio of gas to air which does not exist in gas tanks.

But this is all irrelevant since the OP wanted something that used "existing vehicle sensors" as he cannot/will not modify existing vehicle tanks. Furthermore you must remove most vehicle tanks these days to even see the top of the sensor port. In my car you must remove the third member,(rear axle), too!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Warpspeed]

That depends. Vehicle ambient operating temperature might need to reach from below freezing to perhaps 50C (or more?) in the tropics. The thermistor wet/dry temperature differential needs to be sufficient to ensure reliable operation over the whole range. There also needs to be allowance for slight differences between individual thermistors, including aging effects.

The usual wire wound fuel tank potentiometer has a far higher operating voltage and current than any thermistor would require. It would seem to be potentially far more dangerous, especially with a constantly moving wiper.

And don't overlook that there is also a certain minimum energy discharge requirement to produce either a spark or an arc.

 

[cbarn24050]

itsmoked I never took a gas sensor apart I just assumed the resistor was in a sealed compartment. I accept the fumes are not flammable in the tank normaly but what happens if someone leaves the filler cap open? Warpspeed you dont run the thermistor hot. You measure its resitance, pump in some power, measure again. A 2 degree rise is more than suffcient for reliable operation whatever the ambient is.

 

[Warpspeed]

Thank you cbarn for that excellent suggestion. That is a much more elegant solution, and fairly easy to implement.

The rate of scanning need not be that fast, so temperature cycling each thermistor individually is an excellent idea.