Which Thermister???

Hi Gerald,

I do not pretend to know very much about fuel tank sensors but I am having trouble imagining how a thermistor could be used as a low level sensor. For example in most situations the fuel and the air above it will have the same temperature. I am interested in your question though and if thermistors are actually used in this application it would be nice to learn the physics involved.

John

Thermistors are non-linear, I think their transfer fn is a fairly high-order poly.  If the sensor is to detect the binary low-fuel condition, if a little electrical energy is input, and it is in air it will self-heat into a certain regime.  If it is immersed, it will stay cool

Hi Don,

Yes I can see that this could be done. Do you know if this is an actual technique?

John

http://www.ti.com/lit/ds/snis160e/snis160e.pdf  (fig 32)

You should know better than to ask me about the nature of reality at this hour by now, John

I know that it is useful in liquid chromatography, but IDK about bulk quantities of fuel.  I do think that's how they do it, even in airplanes.  Probably the trick is to scrupulously avoid the flash point.

Does our hero have the repair literature for his gadget?

Hi Gerald,

Sorry if we haven't been exactly on subject. I know that regular epoxy is impervious to gasoline. I once patched a puncture in the gas tank of my 1967 Plymouth Fury III with regular mix epoxy and it held without problem for the 5 or 6 years that I drove the car. If you feel the above mentioned Thermistors will not handle gasoline perhaps you can put a coating of epoxy on them. Personally I would probably go with original parts if I was dealing with a fuel tank sensor. No sense putting yourself in any danger if there is a malfunction and a potential spark.

John

Level Sensing: Analog Dialogue: Analog Devices

you can just google 'thermistor' and 'fuel'

Classically, the circuit was the battery, a light bulb and an NTC thermistor in series

I realize that this is a simple question for experienced electronics people but it was a puzzle to me.

The discussion has been enlightening and informative. I have found the correct thermister for this system.

The simple answer is that the thermister serves as the ground leg for the light and little if any current is

present in the fuel tank.  Thanks for the help.

Old Okie/Gerald  McGarvin