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From:
pat goodyear <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, pat goodyear <[log in to unmask]>
Date:
Tue, 8 Mar 2016 20:32:08 -0800
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Trying to get the idea as to what was asked and not answered.   As an instrument and controls technician I have spent many moons playing with thermocouples so I will give my nickels worth..
I don't believe the question was how can you make a thermocouple but.
I believe the question asked was can type K thermocouple wire be soldered to a circuit board.  The answer is yes but should not be.   If one thermally bonds the thermocouple wire to a pad the EXACT amount of material must be deposited on both junctions and they must be in close proximity to one another.   The junction must be thermally stabilized at all times to minimize the error induced, typically this is done inside a RTD referenced and controlled oven.   The two wire materials involved are extremely hard to solder, I have had luck silver brazing.   Now the ideal method is to connect the thermocouple wires to a terminal strip and run copper to the circuit board.   Keeping in mind the two junctions must be kept at the same constant temperature as noted above, and the two (copper-chromal) and (copper-alumil) junctions need to be mathematically removed from the monitored voltage to provide an exacting reading, given the fact that they are stabilized this involves a standard reference offset.   

I would bring the thermocouple wires to a terminal board on the required circuit board and form a mechanical bond at that point, with the terminal board soldered to the circuit board and not the thermocouple wires.    We typically used reference junctions stabilized at 32 F for the copper connections to the TC wire when TC wire was not used the full run.  We use solid state cooling elements to stabilize the reference, monitored and controlled by a RTD circuit.   Check Omega references for ideas.

Pat Goodyear, Retired Control Tech, Pacific Gas and Electric Co.
Sent from my iPad

> On Mar 8, 2016, at 4:30 PM, Guy Ramsey <[log in to unmask]> wrote:
> 
> Me too. heat sink adhesive. or buried in a hole and held in place with kapton tape.
>> On 3/8/2016 3:28 PM, John Burke wrote:
>> Given the data and other considerations on this thread I am really really glad I stick these things down with epoxy......ROFL
>> 
>> Best regards,
>> 
>> John Burke
>>  
>>> On Mar 8, 2016, at 11:53 AM, Robert Kondner <[log in to unmask]> wrote:
>>> 
>>> Bhanu,
>>> 
>>> Wow " invariant transformation temperature for liquid to Ni3Sn4 bimetallic transition alloy". Are you saying the thermocouple wires disintegrate or get dissolved?  That bead is solid metal. When I form a bead here I use an oxygen acetylene torch.
>>> 
>>> If you get a shift in material composition at the solder point yes that point has a potential shift, but you also introduce two additional junctions , one on each thermocouple wire. Once you sum up all the thermo-electric potentials you will find the summation is exactly the original of the two thermocouple wires.
>>> 
>>> For type K you have chromel – alumel.
>>> 
>>> ------------| Anything You Want |---------
>>> Chromel                                                  Alumel
>>> 
>>> Anything You Want can be a solder pot or PCB solder spot or intermetallic compounds ..., Anything You Want.
>>> 
>>> You can have Anything You Want in between the Chromel and Alumel but the end enf will sum up to the chromel to alumel emf. Now measuring this emf is difficult (regardless of the Anything You Want) because here we often insert copper connections. You want those copper connections at the same temperature and at a known temperature as they will generate an emf.  That is why folks use ice references. And the copper emfs will be different for each the chromel and alumel wires!
>>> 
>>> My Understanding, please verify!
>>> 
>>> Bob K.
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>>> -----Original Message-----
>>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Bhanu Sood
>>> Sent: Tuesday, March 08, 2016 12:44 PM
>>> To: [log in to unmask]
>>> Subject: Re: [TN] Soldering a K Type TCouple to FR4 Board
>>> 
>>> Bob,
>>> 
>>> Here's what I know by looking at the Sn-Ni (the k type t/c is loaded with Ni) alloy phase diagram, around 231C which is the invariant transformation temperature for liquid to Ni3Sn4 bimetallic transition alloy, the composition of the thermocouple junction changes. This shifts in the expected temperature v/s e.m.f. characteristics or loss of functionality. I will read the document, thanks.
>>> 
>>> Bhanu
>>> 
>>> 
>>> 
>>>> On Tue, Mar 8, 2016 at 11:48 AM, Robert Kondner <[log in to unmask]> wrote:
>>>> Bhanu,
>>>> 
>>>> 
>>>> 
>>>>  You gotta read a book on this. See:
>>>> http://www.pyromation.com/Downloads/Doc/Training_TC_Theory.pdf
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> Myth: There should never be a third metal in the hot junction
>>>> 
>>>> 
>>>> 
>>>> To create a thermocouple junction, all that is needed is to
>>>> electrically short the ends together. Butting the wire ends against a
>>>> metal surface will create a junction. Remember, that the thermocouple
>>>> signal is generated over the entire length of wire.
>>>> 
>>>> 
>>>> 
>>>>  Lets say you have two side of the thermocouple, side A and side B
>>>> and another material C.
>>>> 
>>>> 
>>>> 
>>>> The welding of side A to a material C results in a thermocouple being
>>>> formed, The welding of side B to the material forms another thermocouple.
>>>> The final sum of the two thermocouples equals the junction potential
>>>> of A welded to B.
>>>> 
>>>> 
>>>> 
>>>> Tell me if I am wrong! Been known to happen!
>>>> 
>>>> 
>>>> 
>>>> Bob K.
>>>> 
>>>> 
>>>> 
>>>> -----Original Message-----
>>>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Bhanu Sood
>>>> Sent: Tuesday, March 08, 2016 10:59 AM
>>>> To: [log in to unmask]
>>>> Subject: Re: [TN] Soldering a K Type TCouple to FR4 Board
>>>> 
>>>> 
>>>> 
>>>> If you weld each side to a third material, its no longer a junction,
>>>> no thermo-"couple"..
>>>> 
>>>> 
>>>> 
>>>> On Tue, Mar 8, 2016 at 10:54 AM, Robert Kondner
>>>> <[log in to unmask]>
>>>> wrote:
>>>> 
>>>>> Hi,
>>>> 
>>>>> I do not think this is correct.
>>>> 
>>>>> My recollection of the physics is you can cut the bead off the
>>>>> thermocouple, weld each side to a 3rd material, and get the
>>>>> temperature of the material.
>>>> 
>>>>> What this does is introduce additional junctions but the final output
>>>>> is always equal to the characteristics of the original bead. Even if
>>>>> you manage to diffuse other materials into the thermocouple wire
>>>>> materials it is the same case.
>>>> 
>>>>> Anyone else out there have any thoughts?
>>>> 
>>>>> Bob K.
>>>> 
>>>>> -----Original Message-----
>>>>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Bhanu Sood
>>>>> Sent: Tuesday, March 08, 2016 8:43 AM
>>>>> To: [log in to unmask]
>>>>> Subject: Re: [TN] Soldering a K Type TCouple to FR4 Board
>>>> 
>>>>> Are you sticking the thermocouple head in SAC solder? Be careful!
>>>> 
>>>>> Once you stick the t/c in SAC solder and melt to 250C (which bwt is
>>>>> above the 185C curie point) the chromel which contains 90-percent
>>>>> nickel and 10-percent chromium and alumel which contains
>>>>> approximately
>>>>> 95 percent nickel, with aluminum, silicon, and manganese making up
>>>>> the other 5 percent will alloy with the tin, changing the temperature
>>>>> e.m.f relationship. You may not have a working thermocouple afterwards.
>>>> 
>>>> 
>>>>>> On Tue, Mar 8, 2016 at 8:14 AM, Bob Wettermann <[log in to unmask]> wrote:
>>>>>> Morning All:
>>>> 
>>>>>> Has anyone had success in soldering Type K Thermocouples to a PCB.
>>>>>> The assembly call for a SAC305 no clean alloy....
>>>> 
>>>>>> There are 2 THole connections that will need to be hand-soldered as
>>>>>> part of this assembly.
>>>> 
>>>>>> Thanks!
>>>> 
>>>> 
>>>>>> ____________________________________________________________________
>>>>>> _
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>>>>>> [log in to unmask]
>>>>>> ____________________________________________________________________
>>>>>> _
>>>>>> _
>>>>>> --
>>>>>> Bob Wettermann
>>>>>> BEST Inc
>>>>>> [log in to unmask]
>>>>>> Cell: 847-767-5745
>>>> 
>>>> 
>>>> 
>>>>> --
>>>>> Bhanu Sood
>>>>> Tel: (202) 468-8449
>>>> 
>>>>> _____________________________________________________________________
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>>>>> _____________________________________________________________________
>>>>> _
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> --
>>>> 
>>>> Bhanu Sood
>>>> 
>>>> Tel: (202) 468-8449
>>>> 
>>>> 
>>>> 
>>>> ______________________________________________________________________
>>>> 
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>>>> ______________________________________________________________________
>>> 
>>> 
>>> --
>>> Bhanu Sood
>>> Tel: (202) 468-8449
>>> 
>>> ______________________________________________________________________
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