Updated thermocouple PIC webpage

Updated thermocouple PIC webpage

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This shows that measuring and logging very low temperatures does not have to cost much, provided you can use a soldering iron and run Linux. I made my own type T thermocouples for just a few Euro, those, together with the described simple hardware and not so simple software, work well from 70K to just below the melting point of 60/40 solder, about 190°C .

Reply to
Jan Panteltje
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cost much,

You'd better calibrate those thermocouples somehow. Homemade tc's aren't very accurate, especially at cryo temperatures.

You can buy type T thermocouples for a few euros too!

John

Reply to
John Larkin

On a sunny day (Wed, 13 Jul 2011 09:23:35 -0700) it happened John Larkin wrote in :

cost much,

I have calibrated those and found those to be within +-1 degree on the above freezing side. There is no reason other than sales pitch from tc sellers that 'homemade' should be different, as it is the same resistance wire and thermocouple junction, that voltage is a physical constant. And no, buying complete thermocouples here is MUCH more expensive.

Why not a sig? Some people are like cockroaches. They are all over what you do, cannot enhance its value, but degrade its value, have no clue what they are on about, just look for something that can sustain them being cockroaches, and in the process drop their excrement and multiply. For that sort of problems only the KILL(file)ing helps.

Reply to
Jan Panteltje

cost much,

How can you make a thermocouple "inaccurate", assuming you use the proper alloy wires? Soldering the junction (as opposed to proper TIG welding) doesn't affect the accuracy in most circumstances.

The wire we normally use ("special limits") is good to around +/-1.5°C at 100K, and better than +/-1°C over the range from -65°C to >250°C.

Reply to
Spehro Pefhany

cost much,

The wire is the trick. "Extension grade wire" follows the curves near room temperature. "Thermocouple grade" wires, and thermocouples themselves, are specified to some accuracy over the working range. Wire alloy purity is the key. Things get especially hairy at cryo temps, when the slope is low.

Jan used resistance wire and copper wire, so if he's getting good results at LN2 temps, he's a bit lucky.

John

Reply to
John Larkin

cost much,

Ah, I didn't see that. If he's not using proper thermocouple (not extension) grade wire, then it's all up for grabs. T is copper-constantan, for example, and there is more than one alloy called Constantan. I suspect that modern base metal thermocouple extension wire is actually as good as thermocouple wire, but it's not guaranteed, and of course extension wire for precious metal thermocouples is nothing like the same alloys.

Reply to
Spehro Pefhany

On a sunny day (Wed, 13 Jul 2011 13:06:03 -0700) it happened John Larkin wrote in :

Well, if you look it up, some month or a year ? ago I asked in this group about 2 different types of wire. No reply. I did the homework myself, and it works. Is luck involved? I think not. BTW the word 'calibrate' is incorrect in this context in my view. That could be needed in an analog system. The only calibration my system has, it to set the cold side sensor (LM135 or LM335) to the room temperature (100k trimpot). The other essential thing is to measure the reference voltage with a good DVM and set the #define in the asm source file to that, The PIC sends the reference voltage as a number, and the ADC steps as numbers so you can interchange hardware (the PC side program will use those numbers to get a true voltage from the ADC steps). Where things may vary is in the tolerance of the resistors. A precise person would use better than 1% resistors and perhaps a better opamp I guess. For this reason I state 4 K tolerance in the very low temp range, but it can be much better I think.

Reply to
Jan Panteltje

Did you google it?

Omega has some good turorials.

But how do you know it works?

If you had fairly pure LN2, you could use its boiling point as a check/cal point. But LN2 absorbs atmospheric O2 and gradually changes boiling point.

The exact chemical composition of wire used as a thermocouple affects its accuracy. It gets worse at cryo temperatures.

I meant to measure a known temperature and see what your system indicates.

John

Reply to
John Larkin

On a sunny day (Thu, 14 Jul 2011 08:11:48 -0700) it happened John Larkin wrote in :

It is not that hard, Omega has all the data.

-----------------------^ :-) Yes, and I know their producs and pricing by head almost.

Simple, I have a soldering iron with huge digital display, that tests up to about 180 C. And I can make ice, that melts at 0 C. And I can see when oxygen is condensing, same for the N.

Yea, but for type T there are actually only 2 wire types (see Omega), and this resistance wire is one of them. I dunno about oxygen free copper for the other half :-) Not so much into the HiFi temp, but I was surprised to see people use LN2 to cool their PCs.

Right.

Reply to
Jan Panteltje

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