Engine block or head temp?

[PeeBee]

Thanks Phil, I'm more than happy with +/-1 deg, and I trust your professional opinion. . I honestly thought losses through different materials there would render the reading completely irrelevant.
Unless somebody comes up with a better idea for an accurate-ish reading for oil temperature, that is what I will do.

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If you want the highest accuracy you would go direct contact between the fluid and the instrument simply as there are no heat losses at the boundary faces over and above the conduction of the heat thru the metal probe body, ie heat from motor transferred to oil, oil transfers heat to the outer casing of the direct contact probe, heat migrates thru the probe metal, likely thru a conductive paste inside the probe head to warm the thermocouple tips which has a small electrical voltage, and the resistance changes as the metal probe wires heat up, and you get your variable scale output read as DegC.. With the clamp on config, you go engine oil to filter housing body, thru the metal thickness - there are insulating temp drops depending on the thickness, then out the other side of the filter housing, where you strike an insulator, being the paint of the filter, then you bright metal clamp probe, heats up and transfers the heat to the thermocouple head which is soldered to the clamp ring, and the whole voltage/resistance thing happens as above. So there is some efficiency to be gained with direct contact, however you only get the benefit if you have calibrated and matched instruments of a high standard and your physical operating environment is stable. For automotive applications outside of test rigs, the calibration would be relatively ordinary compared to industrial and certainly miles off lab level. This is why there can be such a wide range of results with even two instruments sitting side by side - calibration variance, quality of components and the physical locations could experience het cold, water/cold air etc.
Years ago I was chasing accuracy on a steam jacketed heating kettle I had designed and it needed to hot a sauce at a preset temp for a 5 minute period to ensure all the bugs were killed. The product was super sensitive to over temp conditions due to the fat content, and would break down and seperate in a horrible blob of circa 800kg, all lost product. If not heated high enough, the kill step was not achieved so the process was delayed until it was, thus impacting production lines with no product. Anyway, I engaged an instrumentation tech to help me and we used all highly calibrated instruments with very high sensitivity, still could not get the accuracy. Turned out the paddle design had generated an un stirred pocket right over where the probe was and was reading 3 deg out. Took 2 weeks to find that. The instruments were fine, the accuracy was exceptional, but the physical location turned out to be crap due to this quirk within the kettle stirring design. I added a deflector to the stirrer arm and moved the probe 25mm away and it suddenly all worked beautifully. Once again, the best instruments can't compensate for a bad installation. In theory it should have work from the get-go, but this one product snookered me fair and square - another design lesson.

[Seluntiy]

I bought a digital temperature sensor on Ali Express. I disassembled the standard sensor and inserted it inside, added thermal paste and sealed it.

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[Winnie]

That's neat.
I have got my engine guard up and running with one probe in the lower thermostat housing and another clamped onto the oil filter.
I had to call Phil from Engine Guard to go through a couple of things and he was great to talk to over the phone.
Overall happy with the unit so far but it will get a good test this weekend towing the caravan.


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[Winnie]

Any suggestions on roughly where you would expect the oil temperature to be at a steady coolant temp of 80-83C?
I clamped the sensor onto the primary oil filter and feel like it is reading low. It might be fine but I would have expected the oil temp to run hotter than the coolant. It is reading significantly lower.

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[PeeBee]

My 'hot oil temps' match the hot/off engine water temps' within 2degC. at normal running temps, so 86water = 88 oil. I have my temp sensor for HOT clamped onto the outside of the filter body.

[Winnie]

Thanks Phil. My oil temp is reading 10-20C lower than the coolant temperature. Occasionally they will come to a parity but the oil temp will never surpass the coolant temp.

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[PeeBee]

That sounds a bit weird. I assume it's a good connection and no cooling effect? You could always take the sender off and dunk it in either hot or cold with a comparison thermometer to ensure its not the thermocouple issue and then gauge accuracy

[Winnie]

That sounds a bit weird. I assume it's a good connection and no cooling effect? You could always take the sender off and dunk it in either hot or cold with a comparison thermometer to ensure its not the thermocouple issue and then gauge accuracy

I did check it in hot water before installing it. The lugged sensor is clamped directly to the filter closer to the threaded end, maybe it should be closer to the end?

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[PeeBee]

No that should be fine

[Winnie]

I'll keep an eye on it over our new year holiday and note back.

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