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Thermocouple

D

Danny

Jan 1, 1970
0
Hi

I am making a thermocouple from 28swg copper and constantan wire, i plan
to twist the ends together and solder the joints, will this solder have
any affect on the characteristic of the thermocouple? I only chose
soldering because a spot welder is a little bit out of my league.

Danny
 
S

Spehro Pefhany

Jan 1, 1970
0
Hi

I am making a thermocouple from 28swg copper and constantan wire, i plan
to twist the ends together and solder the joints, will this solder have
any affect on the characteristic of the thermocouple?

No. Be sure to get the right alloy if you expect type T output.

Best regards,
Spehro Pefhany
 
M

Mac

Jan 1, 1970
0
Hi

I am making a thermocouple from 28swg copper and constantan wire, i plan
to twist the ends together and solder the joints, will this solder have
any affect on the characteristic of the thermocouple? I only chose
soldering because a spot welder is a little bit out of my league.

Danny

You could probably fuse the wires with a car battery. I once turned a
piece of 9-strand 12 gauge cable into single strand cable by accidentally
shorting it, very briefly, across a car battery's terminals. Come to think
of it, you could use a large capacity slow-blow fuse to limit the total
energy. Just a thought.

Of course, these kinds of hit-and-miss techniques are not for everyone.
But the kind of person who makes his own thermocouple seems a good
candidate.

Mac
--
 
R

Roger_Nickel

Jan 1, 1970
0
Danny said:
Hi

I am making a thermocouple from 28swg copper and constantan wire, i plan
to twist the ends together and solder the joints, will this solder have
any affect on the characteristic of the thermocouple? I only chose
soldering because a spot welder is a little bit out of my league.

Danny
Ordinary solder will not work. If you must solder, use silver solder.
You will end up with two thermocouples (copper-easyflo,
easyflo-constantin) but even though technically dodgy it works O.K at
low temperatures. Spot welding is definitely best , maybe you can
improvise. I once worked with a thermocouple welder which used a
microswitch to switch power to the primary of a low output voltage
transformer. IIRC it had about a dozen turns of no. 8 copper wire on the
secondary. The microswitch was tripped by a weight falling down an
adjustable inclined rail and holding the switch closed as it passed.
Simple and effective, necessity is the mother of invention.
 
S

Spehro Pefhany

Jan 1, 1970
0
Spot welding is definitely best

Attempting to spot weld round wires together with an improvised welder
is a good way of making a sensor both amateurish and unreliable. Inert
gas shielded welding is the "right" way, BTW.

Best regards,
Spehro Pefhany
 
W

Walter Harley

Jan 1, 1970
0
Spehro Pefhany said:
Attempting to spot weld round wires together with an improvised welder
is a good way of making a sensor both amateurish and unreliable. Inert
gas shielded welding is the "right" way, BTW.


I've heard that chemical impurities in the junction affect the accuracy of
the thermocouple. (I know nothing about it, myself.)

Does anyone have a quantitative sense of the extent of the effect? Like,
how much worse is a hand-soldered thermocouple from a professionally-welded
one?
 
S

Spehro Pefhany

Jan 1, 1970
0
I've heard that chemical impurities in the junction affect the accuracy of
the thermocouple. (I know nothing about it, myself.)

No, not if they are confined to the *junction*.

Best regards,
Spehro Pefhany
 
W

Walter Harley

Jan 1, 1970
0
Spehro Pefhany said:
I've heard that chemical impurities in the junction affect the accuracy of

No, not if they are confined to the *junction*.


Which fits what John Woodgate said, too.

So why not always just solder? Is it a matter of dealing with extreme
temperatures (past the melting point of the solder)?
 
S

Spehro Pefhany

Jan 1, 1970
0
Which fits what John Woodgate said, too.

That's reassuring. ;-)
So why not always just solder? Is it a matter of dealing with extreme
temperatures (past the melting point of the solder)?

(except for a few application areas, mostly T/Cs *are* used above the
melting point of solder or at cryogenic temperatures)

* Many (most, actually) T/C alloys are not easily solderable or "wet"
very poorly
* Temperature range severely limited (most base metal alloys can go
to 500-1000°C and precious metal alloys to maybe 1750°C.)
* Poor mechanical strength of the joint
* Cost is not really lower (just capital cost of equipment)
* Lead-based solders are not food-safe or are not chemical resistant


Best regards,
Spehro Pefhany
 
J

John Popelish

Jan 1, 1970
0
Walter Harley wrote:
....
So why not always just solder? Is it a matter of dealing with extreme
temperatures (past the melting point of the solder)?

That is what I understand. Any soft solder, by definition, has a
lower melting point than the wires being joined. A weld is rated for
as high a temperature as the wires.
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Walter Harley
Which fits what John Woodgate said, too.

So why not always just solder? Is it a matter of dealing with extreme
temperatures (past the melting point of the solder)?
Well, somewhat below the actual melting point the tin migrates out of
the solder and the joint becomes bad.
 
S

Spehro Pefhany

Jan 1, 1970
0
I read in sci.electronics.design that Walter Harley

Well, somewhat below the actual melting point the tin migrates out of
the solder and the joint becomes bad.

But it will still work for a while even with the solder melted,
assuming something like surface tension holds it in place, so the
exact limits are kind of inexact.

Similarly, the upper limits for base-metal thermocouples (even when
manufactured optimally) may be much lower than the melting points of
the alloys because of service life considerations.

Best regards,
Spehro Pefhany
 
B

Boris Mohar

Jan 1, 1970
0
No, not if they are confined to the *junction*.

Would arc welding work? The method I have in mind is to use the TC junction as
one electrode and arc directly to it from another (carbon?) electrode
( AC? DC?) until a bead forms.


Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
Aurora, Ontario
 
S

Spehro Pefhany

Jan 1, 1970
0
Would arc welding work? The method I have in mind is to use the TC junction as
one electrode and arc directly to it from another (carbon?) electrode
( AC? DC?) until a bead forms.

That's pretty much how it's done, but you need the whole kit with (the
right) inert gas and so on to do it right. The Home Depot specials
won't do the trick.


Best regards,
Spehro Pefhany
 
B

Barry Lennox

Jan 1, 1970
0
Would arc welding work? The method I have in mind is to use the TC junction as
one electrode and arc directly to it from another (carbon?) electrode
( AC? DC?) until a bead forms.

Yes, but only with the correct scale of equipment, otherwise it would
be a nightmare.

There's several commercial systems that either dump a cap (usually
variable for power control; about 35-75 volts) through the junction,
or use a step-down transformer as spot-welders. A previous employer
had one for the former, and they work very well.

For my own use, I use a modified microwave oven transformer with 3
thick turns on the sec. The primary side is controlled by an SSR,
variable from 0.05 to 1.0 secs. A copper anvil and small knife bar on
the sec hold the TC wires together, and splat! it's done. I have made
perhaps 200+ TCs this way without any problems whatsoever.

Barry Lennox
 
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