From: "Finn Lassen finn.lassen@verizon.net" <flyrotary@lancaironline.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Temperature sensors
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Date: Fri, 21 May 2021 15:43:13 -0400
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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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