The_Davster
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Needed: Thermocouple resistance vs temperature chart
I have a platinum rhodium thermocouple embedded in a small furnace. However I need the chart to convert the resistance of the thermocouple to the
temperature, google turned up no results.
EDIT: You guys will never guess how I got the furnace with a thermocouple in it 
[Edited on 16-6-2006 by rogue chemist]
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Marvin
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You may find the thermocouple is a Pt/PtRh and that the surve depends on the amount of Rh in the alloy.
If its 10% which I think is usual then you'll find a tabulation in CRC handbook of chemistry and physics.
I'm unsure if there is an electronic version floating around I think there might be.
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Magpie
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Rogue: I suggest you look around on Omega's website. I think they have what you are looking for. You do mean mv vs temperature, don't you?
Yes, I do want to know how you got the furnace. 
[Edited on 17-6-2006 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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The_Davster
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Yeah, they do have one, but I am uncertain on the interpretation of it. It has temp vs microvolts, wheras I am trying to find one with a resistance
in ohms instead of microvolts. Is there an easy method of converting between the two?
EDIT: Magpie, Omega's website?
[Edited on 17-6-2006 by rogue chemist]
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Magpie
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A thermocouple's output is a voltage.
A thermistor, like those commonly made of Pt, would have a resistance that changes with temperature.
Edit: I meant to say "RTD" instead of "thermistor." But they are the same idea.
[Edited on 17-6-2006 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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The_Davster
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Really? Then I have been under some real misconceptions about thermocouples, I had always thought that their resistance changed with temperature, not
that they actually produced electricty.
Unfortunatly I do not know whether what is inside is a thermistor or thermocouple.
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Magpie
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I think it most likely is a thermocouple.
Omega Engineering has a lot of info. Google "Omega thermocouples."
I meant to say "RTD" instead of "thermistor" on the earlier post. But the principle is the same.
[Edited on 17-6-2006 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magpie
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With Pt-Rh, you have a very expensive TC good for very high temperatures. It would be a type R, S, or B. You need to know the percentages of each
element to determine which you have.
I found the tables on Omega's site under technical references.
The single most important condition for a successful synthesis is good mixing - Nicodem
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The_Davster
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Thanks for all your help Magpie, I gotta find the percentages now. I supose Ill need some sort of really sensitive voltmeter for this, the table in
the CRC uses microvolts. I am also kinda unsure on how to read the table in the CRC.
Now for my story which will make some very jealous:
There is a building at my university that is going to be torn down soon, mostly physics, but a little geology and one chem lab. Now everything is
pretty much moved out and some doors are left open and lights on overnight. So I am thinking, hmm I wonder if all this physics crap is junk to the
university, so I go in and take a look around. Hey...A little miniature furnace, hole is only one inch. Now 'stealing' this was in the forefront of
my mind, but I decided against it. Instead I ask every physisist who walks by "Is this room full of junk, or is it being kept?" After asking several
dozen I get put in touch with the owner of the lab....everything left is junk
I specifically ask about the furnace and it is junk, they just dont have the manpower to move everything, so whats left I can take. I walk out of
there with 4 of these furnaces, all good till 1250C and all with a thermocouple. I show my proff(I am doing solid state research, and we do lots of
high temp synthesis) and he takes two, I should have realized the value of these first.
Also got some clamps, glass bottles from that room, much else was physics junk, although I did eye the giant broken induction furnace...but there was
no way I could ever get that home.
On another note, I did some dumpster diving on the universities dumpsters after I realized how much was being gotten rid of. And HOLY....It was
ORGASMIC
Highlights:
Vaccuum furnace (1250C max)
Arc melter, needs current supply, argon supply and vaccuum pump, its just the body
10 of those platinum loop thingies for flame tests( I have heard nichrome works as well, I must do test!)
multimeter
Hot plate/magnetic stirrer, dirty but good
Hot plate
Big ampoule of xenon(surprised it dident break)
unused glass bottles
Gotta test everything out tomorrow, and go back for a vaccuum pump and some junked microwaves so I can get the arc melter going.
[Edited on 17-6-2006 by rogue chemist]
[Edited on 17-6-2006 by rogue chemist]
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Magpie
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Rogue you have really scored on the salvaging! Good work! I'm sure you'll be hanging around that building like a vulture until everything is out.
I ran across a similar situation at my local junior college a while back. I saw some junked lab sink hardware laying out in the construction yard
that I was tempted to "steal." I also saw a lot of old glassware being boxed up. I thought about asking the building manager (a technician) if I
could have it but I was too paranoid so didn't. I had had a few sparing matches with this character and thought he might somehow hold this against
me. I was going to go over his head but reconsidered as, again I didn't want the publicity and just knew they would have "liability concerns."
At some point I want to build a tube furnace good for 1300C. I think I can use a type K thermocouple, which I can get for around $50. Those you have
are worth several hundred $ IIRC.
The single most important condition for a successful synthesis is good mixing - Nicodem
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waxman
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Thermocouples are best used with the instrument specifically designed to convert their micro/milli volts to temperature. You won't accurately measure
those voltages with any other voltmeter. $100 or so from Omega.
Treasure those Rh T/C's! Very spendy!
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hinz
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Here is a good link to various thermocouple mV/temp tables:
http://instserv.com/rmocoupl.htm
Note that each thermocouple has a individual colour that implies it's alloy. I would amplify the very low voltage of the thermocouple with an
operation amplifier and feed this voltage to a normal voltmeter. Operation amplifier have in the best case an accuraty of 200 microvolt this is appr.
20°C with a Type S (10% Pt content) thermocouple. Another solution is that you feed the operation amplifier outpt to an analog digital converter of
some kind of microchip (AVR for example) This microchip calculates the themperature of the furnace, but you have to programm this before  . The programming of my furnace controller takes forever  . The simpest way would be to buy some of those nice controllers, maybe even the
cheapest:
http://iseinc.com/process_and_temperature_controls.htm
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The_Davster
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I am afraid my integrated circuits knowledge is, well, pretty much nonexistant. Got a schematic? All I need to know is what pins on what chip to
connect to what.
Thanks for the advice on controllers, but all this free stuff I have been getting is making me, well, cheap.
I am currently mulling over in my mind whether to ask people here if anyone wants to trade a controller for one of these furnaces. If I were to ask
this would there be any takers? Its a good deal
And Magpie;
| Quote: |
I'm sure you'll be hanging around that building like a vulture |
OHHHH YEAAH
(could have gotten a 7kv capacitor today, it was the size of a computer cpu, but PCB era... )
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lordmagnus
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Thermocouple Vs. Thermister
Yeah, they are right.
A thermocouple produces different voltages in relation to the temp. they are exposed to. A thermister is a device who's resistance changes in
relation to the temp. it's exposed to. Thermisters come in two flavors, NTC and PTC. NTC is the most commonly used, which means NEgative Ttemp.
coefficiant, in other words, the reistance drops as the temp. increases. You can get a PTC in which the resistance goes up as the temp. goes up as
well.
A thermocouple is two or more metals welded together that produce different voltage levels as the applied temp. changes. They are far more durable
to the high temp. used in chemical synthesis than a Thermister would be, although not quite as accurate. the most commonly used kind I can think of is
a K-TYPE thermocouple.
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Texium
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