From: "Finn Lassen finn.lassen@verizon.net" <flyrotary@lancaironline.net>
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Subject: Re: [FlyRotary] Re: Temperature sensors
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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Date: Fri, 4 Jun 2021 12:25:54 -0400
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I got the digital meat/cooking thermometer I ordered. It actually 
indicates 212°F in boiling water.
Strapped a home made thermocouple (Type K wire with ends twisted and 
silver soldered) to the tip and put it in boiling water.
Had to change the EM3 scale factor by 4% to make the EM3 read 212°F.

The MAX6675 chip does have max cold junction error of +-3°C and +-2.25°C 
temperature error. I guess just my bad luck that it adds up to 4°C or 
more at 100°C. Not quite the accuracy I was going for.

Finn

On 5/21/2021 9:47 PM, Charlie England ceengland7@gmail.com wrote:
> That should make it fairly simple to roll your own; you can silver 
> solder the junction into the fitting after you weld the junction.
>
> On 5/21/2021 8:27 PM, Finn Lassen finn.lassen@verizon.net wrote:
>> The EM3 likes grounded thermocouple wires, however not really sure 
>> it's required.
>> They go into balanced inputs. Actually there are four 8x1 
>> multiplexers in the EM3. They have clamping diodes to protect inputs.
>>
>> Finn
>>
>> On 5/21/2021 4:36 PM, Charlie England ceengland7@gmail.com wrote:
>>> Don't forget to check on whether the EM3 wants grounded or 
>>> ungrounded probes. I forget which is which, but one type of 'meter' 
>>> will accept either style; the other won't.
>>>
>>> On 5/21/2021 2:43 PM, Finn Lassen finn.lassen@verizon.net wrote:
>>>> 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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>>>>
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>>>
>>
>>
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