Ok, I think the oil test, after a water test to calibrate the 100º C is the way to go.
No plastic around the sensors this time. Just immerse them in the "vegetable cooking" oil..
Here is the first measurements I took, but I did something a bit wrong. I set the cooker at full power, and I think one sensor responds faster than the other at temperature changes.
Multi --- Radio
40 ------------ 39
50 ------------ 48
57 ------------ 55
66 ------------ 63
75 ------------ 73
92 ------------ 90
113 -----------109
126 -----------120
140 -----------134
150 -----------143
158 -----------150
170 -----------164
As you can see as the temperature was going up quite fast, there was a bit of difference starting at about 120º until the 170º mark.
But then while cooling down, both seem to match better. Also removing the sensors from the oil and putting them there again showed similar temperatures on both sensors, as you can see on this video
https://www.youtube.com/watch?v=p-PImqCqkmg
While the oil was cooling down, I took some resistance measurements. Comparing them to the sensor datasheet I posted somewhere above, it seems that the resistance values match the table. Here are my measurements:
Temp in degrees ºC and resistance in Ohms
190º --- 730
179º --- 883
165º --- 1270
150º --- 1675 - Expected 1770
130º --- 2780 - Expected 2940
120º --- 3667 - Expected 3850
110º --- 5040 - Expected 5070
102º --- 6170
98º --- 7100
90º --- 9160 - Expected 9100
As you can see, the values are quite close to the datasheet values. They are approximate because the numbers were always changing, making it difficult to take an exact measaurement..
Now i just have to flash the other oxs that is connected to the sensor on the engine and run it.
Thanks to this testing I found out that I was probably overheating the engine due to a wrong propeller size.
Comparing the results from this test and the soldering iron test, I have learnt that the sensor has to be very well attached to the cylinder head, making good contact with it..
João