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When I made my own engine monitor for my RV-3 back in 1999 I found it easiest to use sensors that output linear 0 to 5V.
So when I got an EM3 from Tracy, I modified it to use such sensors (LM35, TMP35, TC03 and similar ICs). My reasoning was that they were spec'd to accuracy of a couple of degrees over temperature range of interest.
I wondered why the EM3 had 8 EGT and 8 CHT inputs. (Actually in the EM3 the 8 CHT inputs also go into the same MAX6675 chip designed for Type K thermocouple (EGT) wire.)
After several failures and weird readings of LM35's I'm now beginning to see the wisdom of Tracy's approach. And the LM35CAZ is not cheap either.
If you buy a roll of type K thermocouple wire (less than $1/foot), even if it's off a bit compared to an ideal Type K wire, all measurement points should be accurate relative to each other. And you just need to calibrate one input for absolute accuracy and use those calibration factors for all the other inputs.
I've spent a lot of time trying to figure out why the coolant output from my right radiator is so much lower than the left's. I even made 3/4" cu tubing inserts with 1/4" wells in them for the ICs to make sure I was measuring actual fluid temps.
After swapping the hoses to the radiators, the coolant out (out from engine) sensor now inexplicably reads lower by more than 20 deg F than the in-engine sensor. Thought it might have been air in coolant near sensor, but lower readings remain after multiple engine runs,
For the CHT inputs, I had bought Type J thermocouple wire, reasoning that it'd be more sensitive than Type K. Well, 50uV/C is really not much better than 41uV/C, and being fed into a chip that expects Type K really makes it much harder to re-correct (the MAX6675 chip corrects for ambient temp for Type K not J).
So, I think I'll rip out all sensors, order a roll of Type J wire, make and install new sensors: 1/8NPT plugs drilled for 5/32" brass tubing with wire twisted and soldered to the pinched-shut brass tubing. (My old sensors were 1/8NPT plugs drilled for 1/4" cu tubing that the LM35's could fit in.)
Quite a bit of work, but hopefully will pay off in terms of reliable temperature readings. Without those I don't think I can optimize the cooling system.
Hope that others may learn from my mistakes.
Finn
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