Sciencemadness Discussion Board - Proper Use of Thermocouple

Proper Use of Thermocouple

Db33 - 12-12-2016 at 18:09

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.

Edit: Gave thread a descriptive title

[Edited on 12-13-2016 by zts16]

diddi - 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

PeterC - 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

Db33 - 13-12-2016 at 07:40

so the tip of the thermocouple always has to actually touch the glass? is that hard to do?

PeterC - 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).

DJF90 - 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.

Db33 - 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?

Sulaiman - 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]

careysub - 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]

careysub - 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.

Fulmen - 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.

Sulaiman - 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

DJF90 - 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).

careysub - 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.

unionised - 13-12-2016 at 13:12

Cooking oil as a heat transfer medium, anyone?

DJF90 - 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...

Db33 - 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

and that will take the temp of the distillation?

Db33 - 17-12-2016 at 08:38

or could i use this instead?

alking - 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.

XeonTheMGPony - 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.

WGTR - 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.

Db33 - 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?

files.php.jpg - 14kB

WGTR - 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.

Melgar - 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.

XeonTheMGPony - 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

unionised - 18-12-2016 at 09:26

Quote: Originally posted by XeonTheMGPony

@ 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.

Cool!
If you are right we just solved the energy crisis.
Get 2 thermocouples and put them in 2 glass tubes in a bucket of water.
Put a drop of mercury in one of them.
If you are right you will get a different voltage (compared to the one without Hg)
Build lots of them and use that voltage to run the world. No energy supply needed.

Alternatively, you are mistaken.
Shorting out the junction won't matter- nor does soldering it. As long as the metal doing the shorting is at the same temperature as the tip of the probe, it doesn't affect the output voltage.

What happens is that, instead of getting a chromel/ alumel junction (or whatever) you get two junctions:, chromel/ mercury and mercury alumel.
But the sum of those two junction's voltages is the same as the voltage from the chromel alumel one.

XeonTheMGPony - 18-12-2016 at 09:47

No I am not wrong thermal couple piles (Hooked up in series) are well known VOLTAGE generators, they produce micro amps current, a tiny bit of self education will have taught you this, they are used in furnaces for a VERY long time to run the safety shut off valve on pilot failure. And lets not ignore the utter stupidity of your statement of not needing an energy source?!?! What the hell do you think heat is! and ya lets run the world off lemons while we're at it, or better yet Peltiers! ya that's it! (If one took the time you could even use pelts as a temp sensor!)

I said WETTING is coating it so nothing but mercury there will be no junction (IE Dead short), using your head goes a long way here, even still in morning mode I made that part clear!

http://www.msm.cam.ac.uk/utc/thermocouple/pages/Thermocouple...

Adding crap to the standard thermal junction requires remapping again being a smart ass made you look rather bit of a fool As I said it will throw out the ACCURACY of your readings, (Depends when they switch to the copper legs given but people have been known to vastly over fill things and mercury is very good at coating!).

Keeping your sensing gear clean, and well maintained and free from contaminants is vital for good reliable repeatable readings

So befor being a smart ass try reading and thinking then reply.

[Edited on 18-12-2016 by XeonTheMGPony]

unionised - 18-12-2016 at 11:22

Ho Hum "a tiny bit of self education will have taught you this"
Lets start with this
"thermal couple piles (Hooked up in series) are well known VOLTAGE generators, they produce micro amps current,"
Nope, they can produce very large currents as long as the resistance of the circuit is small.
Then there's
" they are used in furnaces for a VERY long time to run the safety shut off valve on pilot failure."
Yep they are; and the thermocouple generates enough current to hold open a solenoid valve. That's not something you do with microamps.
The time they run for is irrelevant- essentially they are powered by the burning gas.

"And lets not ignore the utter stupidity of your statement of not needing an energy source?!?! What the hell do you think heat is! "
Indeed, lets not ignore it.
I think heat is a form of energy.
I also think that the laws of thermodynamics mean that you can't extract energy- as work- from heat, unless you have a temperature difference.
But that's my point.
The two thermocouples (which you say produce different voltages) are in the same bucket of water (and I remind you- nobody said it was warm water).

If they produce differing voltages at the same temperature then you have a free energy machine.

I agree with this bit
"using your head goes a long way here, even still in morning mode I made that part clear! "
and with this
"So befor being a smart ass try reading and thinking then reply."
except I'd have tried to spell the words correctly.

[Edited on 18-12-16 by unionised]

XeonTheMGPony - 18-12-2016 at 12:43

That was my point the one would produce NON it would behave as a short 0v, or at best will mess with reading accuracy. You could have explained things with out the smart assery! hence you being a intentionally disingenuous Not to mention mashing two different modes of operation we are not nor ever where talking about TWO sensors, we are talking about ONE TEMPERATURE sensor and a fowled up sensor does not do its job accurately, Modern thermal couple readers use a biased circuit as the fixed cold side reference with ruffly 1M ohm impute resistance, they measure the voltage change, so some thing that can effect the signal will cause deviation, it is one more variable that one does not need in life! I have had this happen befor, care to miss represent any thing ells?

As a standard to measure things Aim for a non conductive (electrically) and non inductive sensor with low thermal mass for fast responsiveness to changes in temp.

And for any one interested I dug through my big box of sciency electrical stuff and found my big spool of shielded K type cable with glass insulation (Read higher temp range then your average cheapy) So if any needs a few feet. Forgot I even had that.

As for what you would have don, the message is the important part so I could care less! I wouldn't have tried being a smart ass in making a point or a correction first!

[Edited on 18-12-2016 by XeonTheMGPony]

aga - 18-12-2016 at 13:44

Db33, why are you asking these things with stock ebay photos ?
(take a photo with your Samsung smartphone instead)

It's a thermometer. Measure the temperature of your planned reaction with it, which surely is why you bought it.

What, exactly, have you tried already ? Anything at all ?

Same for goes for the Legion of supposedly New 'members' who post endless opinions and googled weblinks (to get a high post count) yet have never shared even a shred of the things they have done themselves, not even a single account (never mind photos) of even one thing they themselves have done.

For me, IF i get a procedure to work, i feel i Must share it here, mostly due to excitement, partly due to a debt to SM for all the chem the Real members have taught me, partly ego.

This used to be a Chemistry forum, for people who are interested in Doing Chemistry.

Seems we just have to talk/link in google searches these days.

[Edited on 18-12-2016 by aga]

unionised - 18-12-2016 at 13:59

Quote: Originally posted by XeonTheMGPony

That was my point the one would produce NON it would behave as a short 0v, or at best will mess with reading accuracy. You could have explained things with out the smart assery! hence you being a intentionally disingenuous Not to mention mashing two different modes of operation we are not nor ever where talking about TWO sensors, we are talking about ONE TEMPERATURE sensor and a fowled up sensor does not do its job accurately, Modern thermal couple readers use a biased circuit as the fixed cold side reference with ruffly 1M ohm impute resistance, they measure the voltage change, so some thing that can effect the signal will cause deviation, it is one more variable that one does not need in life! I have had this happen befor, care to miss represent any thing ells?

As a standard to measure things Aim for a non conductive (electrically) and non inductive sensor with low thermal mass for fast responsiveness to changes in temp.

And for any one interested I dug through my big box of sciency electrical stuff and found my big spool of shielded K type cable with glass insulation (Read higher temp range then your average cheapy) So if any needs a few feet. Forgot I even had that.

As for what you would have don, the message is the important part so I could care less! I wouldn't have tried being a smart ass in making a point or a correction first!

[Edited on 18-12-2016 by XeonTheMGPony]

And my point is that "none" (Note the spelling, btw) is different from that of a normal thermocouple.
And if they produce different voltages then you an connect them in series and still have a net voltage. and you can get that to do work.
And that's a breach of the conservation of energy.

You are making the slightly absurd claim that a thermocouple in mercury can't generate a voltage because there's a metal across it.

Do you not realise that the same is true of a normal thermocouple?

The reason I'm talking about two sensors is simply to show that you are wrong about what one of them (the one in Hg) would do.

Sulaiman made the same point.
So did WGTR.

You think it would produce no voltage. If that was true, you would have a free energy source.
Stop ranting, and accept that you might be wrong.
Think about what happens if you have, as you say mercury "shorting out" the chromel/alumel junction.
In effect you have a chromel/ mercury junction and a mercury/ alumel junction (as I said before)
Andeach of those two new juctions produces its own EMF
And the sum of those EMFs must- by the conservation of energy- equal the emf you would get from the chromel/alumel junction.

wg48 - 18-12-2016 at 17:42

Unionised and the others are almost totally correct. For modern electronics with high input imedances any shorting out of the original junction with any combination of conductors simple produces a parallel sensor that produces the same voltage as the original junction provided both are at the same temperature.

That’s a principal of thermocouple theory. Though I did not realise it has to be that way for the thermodynamic reasons already given.

For the almost part: If any current flows from the thermocouple it means heat is flowing into or out of the thermocouple and that implies deferential temperatures and voltage drops in the junction/junctions. Which could create differences in the voltage between one junction and a muti junction. I have not done the calculation but I suspect with modern high input impedance (compared to the junction resistance) electronics the error is insignificant compared to other errors.

Db33 - 19-12-2016 at 08:18

Quote: Originally posted by aga

Db33, why are you asking these things with stock ebay photos ?
(take a photo with your Samsung smartphone instead)

It's a thermometer. Measure the temperature of your planned reaction with it, which surely is why you bought it.

What, exactly, have you tried already ? Anything at all ?

Same for goes for the Legion of supposedly New 'members' who post endless opinions and googled weblinks (to get a high post count) yet have never shared even a shred of the things they have done themselves, not even a single account (never mind photos) of even one thing they themselves have done.

For me, IF i get a procedure to work, i feel i Must share it here, mostly due to excitement, partly due to a debt to SM for all the chem the Real members have taught me, partly ego.

This used to be a Chemistry forum, for people who are interested in Doing Chemistry.

Seems we just have to talk/link in google searches these days.

[Edited on 18-12-2016 by aga]

1. because the ebay image shows exactly what mine looks like and so if i can save the hassle why bother?

2. Samsung? ive never owned a samsung phone, i own an iPhone? I tried taking photos with it but the darkness of this area was not letting me get a good photo so i just used an ebay image. .

The reason i asked the question is obvious i wouldve thought. I want to know BEFORE i do a reaction so that i can minimize and damage or risk i cause. I like to learn as much as i can before doing a reaction or something.

aga - 19-12-2016 at 09:05

OK. Sorry if i was being a bit harsh.

Look forward to seeing your experiment with this device in the near future, in glorious technicolour.

Used my cheapo 1300 C thermocouple today - registered 720 C in a charcoal fire with the wind blowing.

P.S. photos come out fine when Light Sources are available, such as the Sun, a candle or a torch.

unionised - 19-12-2016 at 12:35

Quote: Originally posted by wg48

Unionised and the others are almost totally correct. For modern electronics with high input imedances any shorting out of the original junction with any combination of conductors simple produces a parallel sensor that produces the same voltage as the original junction provided both are at the same temperature.

That’s a principal of thermocouple theory. Though I did not realise it has to be that way for the thermodynamic reasons already given.

For the almost part: If any current flows from the thermocouple it means heat is flowing into or out of the thermocouple and that implies deferential temperatures and voltage drops in the junction/junctions. Which could create differences in the voltage between one junction and a muti junction. I have not done the calculation but I suspect with modern high input impedance (compared to the junction resistance) electronics the error is insignificant compared to other errors.

"Unionised and the others are almost totally correct. For modern electronics with high input impedances any shorting out of the original junction with any combination of conductors simple produces a parallel sensor that produces the same voltage as the original junction provided both are at the same temperature. "
It's a thermodynamics thing - and it's the conservation of energy so it's not "almost totally correct"; it's mathematically exactly correct.
The input impedance of the meter makes a difference to the accuracy- but the voltage from the thermocouple is exactly the same even if you dunk it in mercury.

Xenon,
I will have another go at explaining it.
Imagine I "short circuit" a chromel/ alumel couple with a wire made of chromel (all kept at the same temperature).
Do you see how that won't make a difference to the output? it just moves the junction slightly.
What about if I use alumel to short it out?
Do you see how that also won't change the output?
And I could do that with any two metals- the outcome wold be the same. It doesn't matter what the thermoelectric effects are- the effect is zero

Well, the next question is what's so special about mercury that it does change the output?
Well, if you still don't accept it, I suggest you set up a power station made of lots of thermopiles which work without a temperature difference.

yobbo II - 19-12-2016 at 12:46

One problem with soldering or putting in mercury or even brazing is that when the temperature gets high the metals may 'melt' ie. dissolve into one another and destroy the junction. Even brazing K type is not allowed if the temperature is going to be over approx. 800C as the braze will melt and then the wires will dissolve into the braze and the junction will fail (literally break). It is similar to how a copper tiped soldering iron will dissolve into the solder after some time and you need a new tip or you need to redo the tip.

Thermocouples should be welded together under an inert shield using the metal for the couples themselves as the 'filler'.

unionised - 19-12-2016 at 13:08

If your mercury is at 800 degrees C, you already have a problem. :-0

But you are right, this sort of practical issue is something you need to consider.

XeonTheMGPony - 19-12-2016 at 19:38

that is what I was attempting to say but seemed to have failed, mercury will coat the wire moving the junction up reading a different temp (IE Adds a factor of error). (Perhaps shorting was the wrong word, but again if the mercury or other conductive material coats the wire it will read zerro or the wrong temps)

at same temp then yes it would read the same but in what dynamic system do you have no thermal gradient. ( We do all remember we're talking about a dynamic system, yes?)

I all ready conceded I was in error in thinking about shorting aspect as I said wasn't all there yet, had you started off with just the facts would have saved allot of aggravation.

and I have made Thermal couples and made some just now as well and going to put it in a bit of mercury and heat. Mercury has a habit of coating things in a sealed vessel when heat is applied it can wet metals if not as others stated Amalgamate with it.

I have all ready observed temp deviation due to contamination of thermal couples! Got some cheap Chinese ones on order too will test one of them along with the heavier temp/gauge that I have.

So well see after some tests, will be a few days as getting my power system online for the house.

but lets see what the results are.

And will have some fun tests once my Vacuum chamber is don.

[Edited on 20-12-2016 by XeonTheMGPony]

XeonTheMGPony - 19-12-2016 at 19:54

Quote: Originally posted by yobbo II

One problem with soldering or putting in mercury or even brazing is that when the temperature gets high the metals may 'melt' ie. dissolve into one another and destroy the junction. Even brazing K type is not allowed if the temperature is going to be over approx. 800C as the braze will melt and then the wires will dissolve into the braze and the junction will fail (literally break). It is similar to how a copper tiped soldering iron will dissolve into the solder after some time and you need a new tip or you need to redo the tip.

Thermocouples should be welded together under an inert shield using the metal for the couples themselves as the 'filler'.

for oven brazing a Dry O2 free Nitrogen with 5% hydrogen as it reduces any oxides at such temps.

Need to get an HHO torch built.

unionised - 20-12-2016 at 12:04

Quote: Originally posted by XeonTheMGPony

that is what I was attempting to say but seemed to have failed, mercury will coat the wire moving the junction up reading a different temp (IE Adds a factor of error). (Perhaps shorting was the wrong word, but again if the mercury or other conductive material coats the wire it will read zerro or the wrong temps)

[Edited on 20-12-2016 by XeonTheMGPony]

Is there something that stops you understanding the fact that - whether the mercury "shorts" the junction or not, you won't get zero volts from it except under conditions where a "clean" thermocouple would give zero volts?
Zero isn't an option here.

Also, you may remember that the point of the mercury was to reduce temperature gradients.

[Edited on 20-12-16 by unionised]

unionised - 20-12-2016 at 12:06

Quote: Originally posted by XeonTheMGPony

Quote: Originally posted by yobbo II

for oven brazing a Dry O2 free Nitrogen with 5% hydrogen as it reduces any oxides at such temps.

Need to get an HHO torch built.

Oxidation isn't the problem for a brazed junction; melting is the problem (unless it's "brazed" with termocouple wire as the filler).

Fulmen - 20-12-2016 at 14:11

Xeon: Unionseed is correct. By dipping a chromel/alumel junction in mercury you simply get two junctions in series; chromel/mercury and mercury/alumel. Together they will produce the same voltage as the chromel/alumel-junction.

unionised - 21-12-2016 at 03:25

Incidentally, it seems* that mercury does not wet chromel or alumel so in reality (rather then on planet Xeon) this doesn't happen "mercury will coat the wire moving the junction up ".

*
https://books.google.co.uk/books?id=cEhLDePEMk8C&pg=PT26...

Fulmen - 21-12-2016 at 03:53

Interesting, but I don't think wetting is required for an electrical connection.

unionised - 21-12-2016 at 03:56

Quote: Originally posted by Fulmen

Interesting, but I don't think wetting is required for an electrical connection.

True, but it is required for Xeon's post to have made much sense.

XeonTheMGPony - 21-12-2016 at 04:02

Quote: Originally posted by unionised

Incidentally, it seems* that mercury does not wet chromel or alumel so in reality (rather then on planet Xeon) this doesn't happen "mercury will coat the wire moving the junction up ".

*
https://books.google.co.uk/books?id=cEhLDePEMk8C&pg=PT26...

See again with the smart assery, had you simply said that then it removes the concern, which is what I called it, a POTENTIAL issue

But I will still run a test based off their observations non the less to see what it does

[Edited on 21-12-2016 by XeonTheMGPony]

XeonTheMGPony - 21-12-2016 at 04:14

Quote: Originally posted by Fulmen

Interesting, but I don't think wetting is required for an electrical connection.

Well it is not electrical we're after in this situation it is thermal, and wetting isn't all ways wanted but happens with some materials with out care full isolation, which was the point of my damned post from the get go!!!!!!!!!!!!!

Keep good isolation while maintaining good thermal conductivity to keep the sensors clean and accurate. There are materials that do with that removes any POTENTIAL issues that can happen with mercury, wetting was one issue that occurs with mercury, but apparently not with the alloys in thermal couples but this then creates a secondary issue as stated in the book posted.

But some one got hung up on smart assery then keeping to the simple data.

[Edited on 21-12-2016 by XeonTheMGPony]

XeonTheMGPony - 21-12-2016 at 04:16

Quote: Originally posted by unionised

Quote: Originally posted by XeonTheMGPony

Quote: Originally posted by yobbo II

for oven brazing a Dry O2 free Nitrogen with 5% hydrogen as it reduces any oxides at such temps.

Need to get an HHO torch built.

Oxidation isn't the problem for a brazed junction; melting is the problem (unless it's "brazed" with termocouple wire as the filler).

I was simply telling those who may do brazing a good reducing atmosphere, it was nothing to do with thermal couples in of its self.

Hmm wonder what this may be use full for when fusing thermal couple junctions. I am starting to think you are going out of your way to misrepresent what I am saying!

[Edited on 21-12-2016 by XeonTheMGPony]

[Edited on 21-12-2016 by XeonTheMGPony]

unionised - 21-12-2016 at 04:52

Your point at the "get go" was this
"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."

And it's wrong.
When I pointed that out- and proved it- your response was
"No I am not wrong ... a tiny bit of self education will have taught you this,"
and "I said WETTING is coating it so nothing but mercury there will be no junction (IE Dead short), using your head goes a long way here, even still in morning mode I made that part clear! "

Thus far Fulmen, Sulaiman WGTR, WG48 and I (perhaps others too- sorry if I missed anyone) have all pointed out that you are flat out wrong.

Firstly, do you realise that you were wrong?

Secondly do you plan to apologise for saying things like these?
"a tiny bit of self education will have taught you this,""
and
"using your head goes a long way here, even still in morning mode I made that part clear! "

Also, I apologise for not recognising that your post about brazing was nothing to do with thermocouples and only served to help those who have controlled atmosphere brazing equipment, but don't know how to use it. (I suspect that's a small group which may have been part of the cause of my confusion)
Perhaps it will help me if, in future, you don't post irrelevant stuff that's off topic.
Thanks

I assure you that I don't feel any need to misrepresent what you say.

[Edited on 21-12-16 by unionised]

biomechem - 5-1-2017 at 21:07

I thought about wrapping thermocouple in thin aluminium foil, then covering all the aluminum with silicone (heat resistant one will be the best), and then allowing it to dry. In the result we have a passive shield with low thermal capacity (lower than glass), and what's more very cheap and easy to replace. Additionally it is possible to get rid off aluminium foil that is glued to a sillicone by putting it in a NaOH or KOH solution.

[Edited on 6-1-2017 by biomechem]

unionised - 6-1-2017 at 02:33

The silicone sounds like a good idea- though it would need to be suitably chemically resistant if it was in contact with anything but glass.
I don't really see the point of the aluminium. Why would you want a heat shield?

biomechem - 6-1-2017 at 10:34

Quote: Originally posted by unionised

The silicone sounds like a good idea- though it would need to be suitably chemically resistant if it was in contact with anything but glass.
I don't really see the point of the aluminium. Why would you want a heat shield?

Wrapping TC in aluminum foil is optional but if you don't do this silicone will stick to thermocouple and it will be impossible to take it off. As I said before when you take off this sillicone shield KOH will react with foil but sillicone will be untouched.
Why it is important to have removable sillicone shield? Because sometimes you need to distill acids and sillicone will not withstand it.

Second idea that just came into my head is to wrap TC with Teflon tape - more chem. resistant than sillicone and easier to use.