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Posted on Monday, September 19, 2016 - 11:48 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Custodian/Admin Only) Ban Poster IP (Custodian/Admin only)

I ended up buying the stock part.

The part number is P0789.KA, and is $15.

But out of curiosity for how the thing works (it is quite clever) I spent some time doing some forensic engineering. There are some good threads on it, here are the details in a nutshell.

This is a custom assembly, a small canister of about 9mm diameter with holes that allow fuel to flow into it. It looks like aluminum with a pressed in fiberglass PCB base, leading to an insulated wire pigtail going to a female spade connector.

Internally, it is an exposed element (not epoxy coated) that I believe is simply a larger 1.8K or so NTC (Negative Temperature Coefficient) thermistor (temperature sensitive resistor).

The low fuel light will go on whenever the thermistor is below 850 ohms.

So how does a temperature sensitive thermistor tell if fuel is there or not? It does so by taking advantage of an annoyance, one I discovered the hard way when working on my closed loop heated grip controller.

The problem is that to read the resistance of a resistor, you have to put current through it. But current + resistance = heat. So by reading the thermistor, you also heat it. Not by much in terms of power, but you heat exactly the tiny bit that is the temperature sensitive bit.

This is an annoyance when you are trying to read temperature, and you just try and put as little current as possible through the thermistor for as little time as necessary.

But for a fuel tank level sensor, it can be your best friend. You put "too much" current through the thermistor. That tries to heat it. But you then dunk the whole mess in liquid, which is a great cooling system.

So basically, if you are submerged, you can never heat the thermistor, because the liquid is conducting heat away too quickly for the very limited power you are putting into it.

But if it is not submerged, air is a great insulator, so the power will heat up the thermistor.

So to use a thermistor as a fluid level sensor, you just put X amount of power into it, and see if it heats up or not as a result. If it does, it isn't submerged. If it doesn't, it is probably submerged. The temperature sensor is also the heating element.

Very clever.

You could probably get a ten cent 1k NTC thermistor, and put it in a bit of fuel line, and mount it in there with the right pigtail, and I bet it would work fine as a low fuel sensor. But you have to worry about mounting it, and make sure it is fuel resistant, and putting the pigtails on, etc. And you probably have to order the 1k thermistor anyway, and pay shipping.

And, um, you are building an electrical thingy you power up and submerge in your gas tank which you then put between your legs. Yeah.

So you will probably just order the $15 factory part, like I did, and be done with it.

(I had 10k thermistors laying around, but no 1k ones, or I probably would have built my own just for fun and bad judgement...)
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