Oil temperature sender

I'm reading a book right now that among other things has a lot to say about oil. One of the things it says is:

"An engine should never be driven hard until the oil reaches 70°C". 

And later:

"In a race engine, bearing failure is a possibility any time the oil temperature goes past 130°C, and generally hp is lost over 115°C".

That is interesting. The first quote goes without saying but the second one makes me concerned of something I've neglected.

The CSR already has a 1-pol oil temperature sender on the oil return line from the dry sump pump, but it goes nowhere, at least on my car. Now when I have this fancy logger I can easily log the oil temperature and display it on my Dash.

As the sender is 1-pol I used a 2.2kΩ pullup resistor to the reference +5V. So far so good, now it only needs to be calibrated.

Right...

As I'd already done the wiring I thought I do it in the garage right next to the car. I put some engine oil in a stainless steel cup and heated it on my camping stove while watching the voltage on the dash and the oil temperature using a kitchen thermometer. The idea was that it would give me real world values despite all tolerances and the wire's resistance. The problems were that the oil temp sender did respond much slower on temperature changes than the thermometer. Also the temperature differed very much between different locations inside the cup. I ended up heating in steps of about 20°C and then waiting a few minutes until the temperature has settled before reading the voltage and temperature. (No, the smell of hot engine oil was not very pleasant). Maybe a better approach would be to measure the sensor's resistance in the oven instead.

°C	Volt
11,7	3,260
37,6	1,882
57,7	1,070
79,2	0,600
97,7	0,358
116,5	0,204
136,5	0,126

I entered the values in the Analysis software that came with the logger and it gave me the formula:

-12,2 * x^3 + 74,2 * x^2 - 155 * x + 149

It is quite cool that the logger can calculate this real time and display it on my dash! But is it accurate?

Well not quite. It will give me a hint within maybe 5-10 °C and maybe that is enough. Regular interpolation would probably give better results. But why stop there?

Steinhart–Hart equation

(aka I got too much time because it is holidays and the children are sleeping)

The Steinhart–Hart equation is a model of the resistance of a semiconductor at different temperatures. The equation is:

After testing some different values I found that A=1,47E-3, B=2,40E-4 and C=8,77E-8 gave me a curve that looked more like my measured values.

Logarithmic chart of temp vs voltage.

Temp in °C = 1/(A+B*LN(R)+C*(LN(R))^3)-272,15
Where R = V*2200/(V-5)

So the final equation I use for my DL1 logger is:
1/(0,00147+0,00024*LN(-x*2200/(x-5))+0,0000000877*(LN(-x*2200/(x-5)))^3)-272,15

I think it works.