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Author: Subject: Proper Use of Thermocouple
Db33
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[*] posted on 12-12-2016 at 18:09
Proper Use of Thermocouple


i bought this on ebay because i saw Nurdrage or someone on youtube using one. Its a thermometer probe. Am i correct in assuming that i can use this in place of a normal thermometer for distillations and stuff by putting it in the thermometer adapter on top of the condenser? I think Nurdrage covered the end of his probe with a piece of glass from a pipette or something, should i do that? Any other tips or hints i REALLY appreciate because i am very new to this stuff. Thank u.


s-l1600.jpg - 76kB

Edit: Gave thread a descriptive title

[Edited on 12-13-2016 by zts16]
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diddi
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[*] posted on 12-12-2016 at 19:09


the thermocouple metals may be attacked by your distilate so YES always protect with glass. ensure that the thermocouple is is direct contact with the glass (you could consider using silver thermal paste) also make sure you seal the top of your glass tube



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PeterC
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[*] posted on 13-12-2016 at 00:20


You can either put the thermocouple into a sealed pasteur pipette (preferably one that fits your apparatus) or buy a thermometer shield like this: https://www.aliexpress.com/popular/ground-thermometer.html
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Db33
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[*] posted on 13-12-2016 at 07:40


so the tip of the thermocouple always has to actually touch the glass? is that hard to do?
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PeterC
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[*] posted on 13-12-2016 at 08:23


For best result it should so I would either tape it to the glass or cheat and use some thermal transfer paste + tape (won't be too accurate but should hypothetically work).
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[*] posted on 13-12-2016 at 08:59


I've always used a small amount of silicone oil as a heat transfer medium in thermocouple wells.

For GMP manufacture, the solvent used in the reaction is also used as a heat transfer medium in the thermocouple well to minimise/eliminate the potential for contamination with silicone oil in the event of a breakage.
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Db33
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[*] posted on 13-12-2016 at 09:04


Quote: Originally posted by DJF90  
I've always used a small amount of silicone oil as a heat transfer medium in thermocouple wells.

For GMP manufacture, the solvent used in the reaction is also used as a heat transfer medium in the thermocouple well to minimise/eliminate the potential for contamination with silicone oil in the event of a breakage.


so you pour some silicon oil in the glass tubewith the thermocouple?
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[*] posted on 13-12-2016 at 09:15


Read but not tried
Sn : Pb, 63 : 37 solder melts at c183 C
Sn melts at c232 C
Pb c327 C
so can be used to embed thermocouples in glass tubes.

[Edited on 13-12-2016 by Sulaiman]




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[*] posted on 13-12-2016 at 09:56


Quote: Originally posted by PeterC  
For best result it should so I would either tape it to the glass or cheat and use some thermal transfer paste + tape (won't be too accurate but should hypothetically work).


I challenge the claim that this would impair accuracy in some way. Are you thinking perhaps that it is a bare thermocouple junction? If so then I would agree, and indeed insist that only chromel-inert insulating liquids be considered.

Otherwise, why would that be? At worst any arrangement would simply slow the heat transfer and cause a lag in response in achieving thermal equilibrium. The purpose of the thermal paste is to speed up the transfer and thus improve the response time.

Accuracy should be unaffected. Thermal equilibrium is thermal equilibrium.

[Edited on 13-12-2016 by careysub]

[Edited on 13-12-2016 by careysub]




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[*] posted on 13-12-2016 at 10:05


Quote: Originally posted by Sulaiman  
Read but not tried
Sn : Pb, 63 : 37 solder melts at c183 C
Sn melts at c232 C
Pb c327 C
so can be used to embed thermocouples in glass tubes.

[Edited on 13-12-2016 by Sulaiman]


And then there is Wood's Alloy, mp 70 C. It contains cadmium, but this is a tiny amount in a sealed glass tube (or that matter, pure gallium or indium, or galinstan, or mercury).

One thing to keep in mind when considering an electrically conductive (and a metallic) thermal coupling agent is how the thermocouple is packaged. If a metal comes in contact with the thermocouple junction then its sensing properties are probably ruined.

If you have a bare thermocouple stick with silicone oil or other insulating liquid of high BP which won't react with the chromel alloy.

If it is in a stainless steel jacket then anything should be OK.




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[*] posted on 13-12-2016 at 10:12


Quote: Originally posted by careysub  

And then there is Wood's Alloy, mp 70 C.

Alloys containing bismuth, like Woods, Cerrosafe, Cerroshield and similar tend to expand on (and for sometime after) solidification. I tried casting Cerroshield (Bi/Pb/Sn) in a thick walled test tube once, and it broke appr 30 seconds after solidification.

Quote:
If a metal comes in contact with the thermocouple junction then its sensing properties are probably ruined.

You'd think so, but solder is fine. At my old job we routinely made K-elements from wire, they were simply twisted together and soldered with regular lead/tin solder. Worked perfectly every time.




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[*] posted on 13-12-2016 at 10:13


in theory I disagree,
it should be possible to have any number of metals mixed/joined/alloyed
and the thermocouple reading should be accutate -
provided that all are at the same temperature.
https://en.wikipedia.org/wiki/Thermoelectric_effect




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[*] posted on 13-12-2016 at 11:52


Quote: Originally posted by Db33  
so you pour some silicon oil in the glass tubewith the thermocouple?


Yep. More oil means more thermal mass which means slower response to change in temperature, so just use a small amount (about a centimetre depth).
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[*] posted on 13-12-2016 at 12:16


OK then - mercury or galinstan would be good to go, as well as silver thermal paste. Nothing that solidifies.



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[*] posted on 13-12-2016 at 13:12


Cooking oil as a heat transfer medium, anyone?
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[*] posted on 13-12-2016 at 13:28


Quote: Originally posted by unionised  
Cooking oil as a heat transfer medium, anyone?


That'd work equally well too. I mentioned silicone oil because thats what is usually found in the lab...
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[*] posted on 17-12-2016 at 08:33


i recieved it today in the mail and there is a weird issue, if you look in the photo at the very end of the wire is 2 tiny wires poking out. When i received it it has these 2 little wires that are hanging out about 1cm out but there is a ball on the end, some kind of glue to keep them together? its silver. Anyways do i just put this in the bottom of something like this
s-l500.jpg - 42kB and that will take the temp of the distillation?
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[*] posted on 17-12-2016 at 08:38


or could i use this instead?


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[*] posted on 17-12-2016 at 09:55


I use a probe like that pictured and do not jacket it or anything, but then I only use it where metals are non reactive. For other things I use a glass thermometer. I'd love to be able to use it all the time as it's much more accurate. In your 2nd to last pic I have no idea what you're talking about though, there's nothing but glass in that picture, no wires.
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[*] posted on 17-12-2016 at 10:31


selection is very important as Thermal couples can be a degree or so off, better gear will be more accurate, but the cheaper chines stuff so be aware of that.

Mercury can not be used with the thermal couples as it will short it out, they are a voltage source, so any thing conductive will cause it to read grossly inaccurate.

So use care full selection on the temp ranges, do not use it for things that will require careful accuracy less it is a higher quality unit with a narrow range thermal couple.

So as said must be in a sealed cell like the one you have, use a drip of white thermal grease, or arctic silver Ceramique (http://www.newegg.ca/Product/Product.aspx?Item=N82E168351000...) as it is not conductive with low inductive qualities.
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[*] posted on 17-12-2016 at 13:45


Quote: Originally posted by XeonTheMGPony  
Mercury can not be used with the thermal couples as it will short it out, they are a voltage source, so any thing conductive will cause it to read grossly inaccurate.


I wouldn't use mercury, but not for the reason stated. Mercury alloys with a lot of things, and might not be compatible with a particular thermocouple's metals. Same with any molten metal system. Molten solder gradually dissolves copper, for example.

I understand your reasoning, but I'm not sure that electrical conductivity is a factor like you think it is. It should be possible to bridge the two thermocouple wires together with a third metal, so long as both junction areas remains the same temperature.




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[*] posted on 17-12-2016 at 16:16


would the probe, the long metal one, if it was put inside a tube like this, would it read the internal temp correctly threw the glass?

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[*] posted on 17-12-2016 at 16:29


Quote: Originally posted by Db33  
would the probe, the long metal one, if it was put inside a tube like this, would it read the internal temp correctly threw the glass?



It would if the thermal conductivity along the length of the probe is negligible compared to that through the glass to the probe junction. Air has poor thermal conductivity, so minimizing the air path with heat sink compound, solder, etc., helps greatly.

A lot of probes are designed with stainless steel jackets, as this metal has low thermal conductivity, relative to something like copper.




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[*] posted on 18-12-2016 at 08:06


I'm planning on putting a stainless steel probe inside of a glass tube to use as a thermometer. If it's a snug fit, would I have to worry about different coefficients of thermal expansion cracking the glass? I know stainless steel is almost certainly higher than borosilicate glass, but with a 5 mm diameter probe, the differences could be negligible for the 20-200C range.
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[*] posted on 18-12-2016 at 09:05


I will read correctly but there will be a larger lag time from reaction temp to reading due to the mass, so if you need to monitor fast temps this will not be a good system, but for slow temp changes this will be fine.

Some thing to keep in mind. Now if I where to do that I would dope the tipe with thermal grease then use a spacer such as a o-ring or some care fully wound paper at the top to use the air as an insulator, the actual junction will be closest to the tip of the probe.

@ WGTR: Being this is morning and first cup of coffe still. I may be in error, but I do not think so, thermal cupels can be shorted out. They are essentially a potential generator, you are reading the voltage that they generate then mapping the voltage to the temperature table. Any thing that can interfere with the voltage signal will adulterate the temp readings.

I can care fully tack solder the ball to a pipe (HVAC in this case) to measure the fine temp changes accurately, but any thing going to the outlet wires will short the generator out and cause a failed reading.

Alloying is the lesser issue to the fact mercury can coat the ball and short out the signal, as will conductive thermal greases
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