Welding thermocouples

B

Bob Masta

Guest
Greetings, all.

I'm looking for a better way to weld thermocouples than
my present car-battery-and-carbon-rod method. The TCs
are 24 gage type K (chromel-alumel). I'd like to keep
this inexpensive, preferably from junkbox parts. (I only
need to do this about once every month or so.)

Extensive Googling hasn't turned up any circuits, etc, but I gather
that a capacitor discharge arrangement might be just
the ticket. Anyone have any idea on voltage and charge?
And, how to actually do it? I'm afraid that if I just poke
the twisted wires at the capacitor terminal, I'll either
weld everything to the terminal or spatter hot metal
everywhere.

Or would I be better off to build a high-current supply
and emulate the car battery? My main complaint about
the car battery is that it is inconvenient to go out and
mess around with jumper cables, etc, if it happens to
be raining or cold.

Or, any better ideas?

Thanks!




Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
 
You're right, Mr. Popelish. This is a better and more obvious "easy"
solution to the OP problem than a homebrew CD welder. If there's any
oxide on the surface of the T/C wires, it helps a lot to manually
remove that with fine sandpaper before silver soldering.

I still like my jeweller's welder, though. If you're sweating limits
of accuracy, a weld will give you slightly better results than silver
solder, I think.

Chris
 
On Thu, 27 Jan 2005 11:39:24 -0500, John Popelish <jpopelish@rica.net>
wrote:


I usually just silver solder the ends together with a small butane
torch.
Click to expand...
John, I forgot to mention that this is for use in a pottery kiln
at temperatures around 2200F. Too hot for silver solder!

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
 
"Chris" <cfoley1064@yahoo.com> wrote in message
news:1106844032.711689.226850@z14g2000cwz.googlegroups.com...

A lot depends on your setup, but I'd start out with one or two 10,000uF
caps for C.

...

Try to set up something so you have a consistent pressure on the weld
nugget through the weld tips (possibly a weight). And don't twist the
wires before welding. Try to just cross them in an "X", so you only
have one weld nugget. Adjust your voltage (1.2V to 6.25V will give you
a 25:1 power range) so you get good welds.
Click to expand...

Unfortunately 20,000uF of aluminum electrolytics at 6.25V will yield a
really pitiful snap upon short circuit. That only corresponds to 390mJ of
energy, with the majority of it going up in the ESR of the capacitor(s) and
the contactor used to complete the circuit.

Now if you crank the voltage up to say 35V, perhaps crank the capacitance up
to 47,000uF, use low resistance wiring all around, and replace the
mechanical contactor with an IRF2804 MOSFET, you might have something that
produces spot welds. Or perhaps you will have just made yourself something
that makes a big mess or destroys itself.

Datasheet for MOSFET here:

http://www.irf.com/product-info/datasheets/data/irf2804.pdf
 
Bob Masta wrote:
Greetings, all.

I'm looking for a better way to weld thermocouples than
my present car-battery-and-carbon-rod method. The TCs
are 24 gage type K (chromel-alumel).
Click to expand...
At a lab I worked we used the arc of a carbon-arc lamp fitted to an
extremely elderly plate-camera microscope. Also worked for Pt/Pt-Rh. :lol:
 
On 27 Jan 2005 08:40:32 -0800, "Chris" <cfoley1064@yahoo.com> wrote:

<snip>

Try to set up something so you have a consistent pressure on the weld
nugget through the weld tips (possibly a weight). And don't twist the
wires before welding. Try to just cross them in an "X", so you only
have one weld nugget. Adjust your voltage (1.2V to 6.25V will give you
a 25:1 power range) so you get good welds.

Chris, I haven't had any luck locating a dental welder, so I want
Click to expand...
to have a go at the "junkbox special". But I'm still not
clear on the details of using your nice circuit. You
mention crossing the wires instead of twisting, and
you mention "welding tips". Is the crossed-wire junction
supposed to be pinched between the tips, as in a spot weld?

Thanks, and sorry to be so dense. This is all really new
to me and I haven't been able to find any good practical
info yet... besides yours!

Best regards,










Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
 
Bob Masta wrote:
Greetings, all.

I'm looking for a better way to weld thermocouples than
my present car-battery-and-carbon-rod method. The TCs
are 24 gage type K (chromel-alumel). I'd like to keep
this inexpensive, preferably from junkbox parts. (I only
need to do this about once every month or so.)

Extensive Googling hasn't turned up any circuits, etc, but I gather
that a capacitor discharge arrangement might be just
the ticket. Anyone have any idea on voltage and charge?
And, how to actually do it? I'm afraid that if I just poke
the twisted wires at the capacitor terminal, I'll either
weld everything to the terminal or spatter hot metal
everywhere.

Or would I be better off to build a high-current supply
and emulate the car battery? My main complaint about
the car battery is that it is inconvenient to go out and
mess around with jumper cables, etc, if it happens to
be raining or cold.

Or, any better ideas?

Thanks!
Click to expand...
I usually just silver solder the ends together with a small butane
torch.

--
John Popelish
 
Bob Masta wrote:
Greetings, all.

I'm looking for a better way to weld thermocouples than
my present car-battery-and-carbon-rod method. The TCs
are 24 gage type K (chromel-alumel). I'd like to keep
this inexpensive, preferably from junkbox parts. (I only
need to do this about once every month or so.)

Extensive Googling hasn't turned up any circuits, etc, but I gather
that a capacitor discharge arrangement might be just
the ticket. Anyone have any idea on voltage and charge?
And, how to actually do it? I'm afraid that if I just poke
the twisted wires at the capacitor terminal, I'll either
weld everything to the terminal or spatter hot metal
everywhere.

Or would I be better off to build a high-current supply
and emulate the car battery? My main complaint about
the car battery is that it is inconvenient to go out and
mess around with jumper cables, etc, if it happens to
be raining or cold.

Or, any better ideas?

Thanks!




Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Click to expand...
Hi, Bob. Thanks for all your perceptive posts -- glad to be able to
help.

If I were in your shoes, I'd be thinking about finding a small dental
welder at a garage sale or on ebay. They're practically
indestructible, already have the hardware built in, and will give you
consistent welds. Also, you can get 'em cheap. They're just about
perfect for welding T/Cs.

There's more than just applying voltage in making an adequate
resistance weld. Voltage losses and pressure on the wires to be joined
are just as critical. When you get a small dental welder, you buy a
consistent solution to these problems.

However, if you're using a car battery and carbon rods now, you could
probably do better with a home brew setup. Get a good sized 12VDC wall
wart and a big automotive 12VDC coil relay (like the ones used to drive
the starter solenoid) and do something like this (view in fixed font or
M$ Notepad):

| Ersatz C-D Welder Control
| ____
| + | | ___ +
| o---o-o--|317 |--o-|___|-----. .----------o----.
| | | |____| | 1K | | |
| | | | .-. | | |
| | | | | | o /o |
| - C| | | |240 CRY1 / |
| ^ C| | '-' / |
| | C| | | o V
|12VDC | | o-----o | Welding Tips
| '-o | | +| X
| | | | C ---
| | o .-. | --- ^
| |=|> | |<---' | |
| | o | |1K | |
| | '-' | |
| - | | | - |
| o---o-o----o-----------------o--------------o----'
|
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

A lot depends on your setup, but I'd start out with one or two 10,000uF
caps for C.

As far as it goes, I would really recommend using the money you saved
and stopping by a welder supply house for a couple of small welder
tips. They're not too expensive, and many shops will sell them in
small quantities if you ask nicely.

Try to set up something so you have a consistent pressure on the weld
nugget through the weld tips (possibly a weight). And don't twist the
wires before welding. Try to just cross them in an "X", so you only
have one weld nugget. Adjust your voltage (1.2V to 6.25V will give you
a 25:1 power range) so you get good welds.

Of course, most caps aren't meant for crowbar-type discharge, but you
said you were looking for a junkbox-type solution. Keep an eye on your
caps, and periodically test them for damage. And remember to keep your
welding tips clean.
Glad to be of help to you for a change. Thanks again.
Chris
 
Chris wrote:
You're right, Mr. Popelish. This is a better and more obvious "easy"
solution to the OP problem than a homebrew CD welder. If there's any
oxide on the surface of the T/C wires, it helps a lot to manually
remove that with fine sandpaper before silver soldering.

I still like my jeweller's welder, though. If you're sweating limits
of accuracy, a weld will give you slightly better results than silver
solder, I think.

Chris
Click to expand...
Yes, especially if you heat a significant length of wire above the
bead. I clamp both wires in a heat sink and just cross them. Then
heat the extended ends till the solder bonds the cross. Then clip the
ends off. Heating to red heat can slightly change the exact
properties of the wires, but as long as this is kept very near the
ends, it doesn't change the output voltage which develops along the
wires as the temperature changes from the sensed temperature to that
at the measurement circuit.

But if the thermocouple will measure a temperature that has a large
gradient along the wire, it is best not only that the couple is
welded, but that it is welded to the surface being measured. The two
wires can even be welded to separate spots on the surface being
measured.

--
John Popelish
 
Bob Masta wrote:
On 27 Jan 2005 08:40:32 -0800, "Chris" <cfoley1064@yahoo.com> wrote:

snip

Try to set up something so you have a consistent pressure on the
weld
nugget through the weld tips (possibly a weight). And don't twist
the
wires before welding. Try to just cross them in an "X", so you only
have one weld nugget. Adjust your voltage (1.2V to 6.25V will give
you
a 25:1 power range) so you get good welds.

Chris, I haven't had any luck locating a dental welder, so I want
to have a go at the "junkbox special". But I'm still not
clear on the details of using your nice circuit. You
mention crossing the wires instead of twisting, and
you mention "welding tips". Is the crossed-wire junction
supposed to be pinched between the tips, as in a spot weld?

Thanks, and sorry to be so dense. This is all really new
to me and I haven't been able to find any good practical
info yet... besides yours!

Best regards,
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Click to expand...
Hi, Bob. If you're welding the two wires, you want one and only one
weld nugget. That would lead you to cross the wires like an "X", and
place the junction of the "X" between the welding tips.

Fritz' advice above is good. When I wrote the first post, I was
thinking for some reason about welding 30AWG wire (the most common at a
prior job). Obviously you'll need a lot more energy for 24AWG wire.
His advice of higher voltage and more capacitance is a good one, with
either a series resistor or a voltage regulator to supply 12VDC for the
automotive relay coil.

I'm not quite as enthusiastic about the MOSFET, first because of added
circuit complexity, and also because of the possibility of exceeding
maximum current. Of course, you can do that with a relay too, but in
the event of failure you'll probably just end up with a spot-welded
relay contact. Since we're talking about junk relays, you should be
able to pop and swap easily and inexpensively. The series resistance
of a homebrew setup, along with the inherent sturdiness of the larger
automotive relays, will probably be enough to prevent this from
happening too soon. If you pick a common type of car, you can snag
junkyard automotive relays for a buck a pop and have as many usable
spares as you want.

Whether you eventually find a dental/jewellers' welder (first choice,
and you may also look for the smallest handheld or tweezer spot
welders) or go homebrew, you should set up the weld schedule by looking
at the welds produced. Your weldment should be a little more than half
the wire diameter, with no signs of arcing or splashing. Tear the
wires apart to examine the weld nugget. You'll have to experiment to
find the correct voltage and pressure settings. Once you're close,
tweak the pressure for final adjustment.

As to the welding tips, they're also called spot welding electrodes.
Go to a welder supply house and get a couple with 1/16" flat tips.
They're only a couple of bucks apiece, and they'll open a package if
you ask nicely.

Good luck
Chris
 
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