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 17:49:33 -0400
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I guess I should repeat the test with just twisted ends.

Yes, the EM3 is set up for eight Type J (CHT) inputs in addition to the 
eight Type K (EGT) inputs. However, with considerable error if the EM3 
input module is not kept at a constant temp (e.g. 25°C).
The is because the MAX6675 chip, that is designed to convert the small 
voltages from thermocouple wire into a digital signal, is designed for 
type K. It measures the voltage on the wire, then adds 41uV/°C for the 
temperature on its pins before converting the input to 4 bits/°C, using 
41uV/°C. Called cold-junction or ice-bath compensation. For a Type J 
thermocouple it should instead be compensating with about 52uV/°C. So 
the hotter the input module, the less the CHT channels will show. At 
150°F input module ambient, it'll show about 20°F too low if calibrated 
at 80°F. Unfortunately I located my input module in my left cowl cheek 
extension where it's getting that hot in spite of just have added a 
blast tube. Anyway, Type J is really only 28% more sensitive than Type 
K. So that's why I have Type K wire on order and will replace my Type J 
wire.

Finn

On 6/4/2021 5:19 PM, Charlie England ceengland7@gmail.com wrote:
> Well, if all the probes have exactly the same error and they're 
> operating in the same environment, maybe it isn't that bad, especially 
> if you know the approximate error.
> Any chance that the silver solder is adding a junction? My engineer 
> neighbor talks about just twisting the wires and hitting them with a 
> TIG torch (no filler)
>
> Also, can you set up the EM3 for type J? Lower temp range so they 
> might be more accurate down around 200 degrees F than K probes.
>
> Charlie
>
> On 6/4/2021 11:25 AM, Finn Lassen finn.lassen@verizon.net wrote:
>> 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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>>>
>>
>>
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