Thermocouple question

L

little mac

Joined
Dec 3, 2005
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122
Anyone know if the output from a thermocouple is linear ? By that, for those who may not understand, If I put it in boiling water (212 F) and it has an output of (for numbers sake) 50 millivolts, can I then expect 100 millivolts at 424-150 Millivolts at 636 ect ?
 
I know very little about the specifics of thermocouples, but they aren't linear.

Some of them used for specific applications are approximately linear within their target range, but if you don't know that your thermocouple is made to measure what you're measuring with the temperature in its range, then it's definitely not.
 
Great, It's been a long time since I did that kind of math. Not in a hurry to do it again. Guess I'll have to convince the boss to write a program to make the conversion. He wants to do some experimenting with a K-type, I'll just have to volunteer my soon to be forge as a test bed for the nominal fee of the prototype.
 
My Fluke thermocouple directions read: "Set the Multimeter to the 200, 300 or 400 millivolt range. This setting will read temperatures equal to the number of millivolts the range has (ie., 320C or F if the millivolt range reads 320mV." Don't know if this helps or not.

One of the most frequently used temperature sensors is the thermocouple. Thermocouples are very rugged, inexpensive devices that operate over a wide temperature range. A thermocouple is created whenever two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermoelectric voltage is known as the Seebeck voltage, named after Thomas Seebeck, who discovered it in 1821. The voltage is nonlinear with respect to temperature. However, for small changes in temperature, the voltage is approximately linear, or
where DV is the change in voltage, S is the Seebeck coefficient, and DT is the change in temperature.

S varies with changes in temperature, however, causing the output voltages of thermocouples to be nonlinear over their operating ranges. Several types of thermocouples are available, and different types are designated by capital letters that indicate their composition according to American National Standards Institute (ANSI) conventions. For example, a J-type thermocouple has one iron conductor and one constantan (a copper-nickel alloy) conductor. A complete list of available thermocouples is shown in Table 1 below.

I didn't write this stuff, it's here.http://zone.ni.com/devzone/cda/tut/p/id/4237
 
Thanks for the link Blub. I was trained in electroincs back in the day when things were repairable and made here. Those kind of math equations were common in school and it's surprizing just how much of it I understood. Still, I'll leave the fancy software/hardware stuff to the boss, I just machine the stuff these days. Don't think I can wrap myhead around all those numbers and get the same answer twice anymore.
 
Reference tables in PDF format are available from Omega for all types in both Centigrade and Fahrenheit. Go to the following link and scroll down to Thermocouple Reference Tables and download the ones you need. Makes it a lot easier than trying to calculate them yourself.

http://www.omega.com/thermocouples.html
 
Thermocouples don't actually measure absolute temperature either...they measure the temperature difference between the hot junction (the probe tip usually) and the cold junction (typically the instrument). Just putting a DMM on the leads of your thermocouple and reading the millivolts does not correct for the variance due to the cold junction (the contact points between the leads of your meter and the thermocouple leads) being at a temperature other than the standard temperature which is used for the published data tables. A proper display unit or controller with a thermocouple input has this correction capability built in.

If you insist on building your own, look up the chips made by companies like National Semiconductor. They are made to bring in a thermocouple input, perform the cold junction correction, and amplify the signal to a more easily manageable level, all in a single chip. NS used to give out samples to anyone who asked too...at least we got a bunch when I was in college.

http://www.national.com/an/AN/AN-225.pdf

While building your own controller is fun, with the price of good, flexible, programmable controllers these days it just doesn't make sense unless you have a very unique function in mind.

Good luck!
 
I'm not real sure what is intent is. We design and manufacture chemical controllers for the circuitboard and plating industries. These controllers automatically control the chemical balance of the various baths used to plate on , etch off or develope the crcuit boards. Some are temp sensitive. He wants to incorporate the thermocouple into some of the equipment to deversify it capabilities.
He has been very supportive of my interest in knifemakeing. He will let me use the shop to machine the parts for my grinder, use the company accounts to purchase the materials, donate the scrape alum for casting the contact wheel ( casting was his idea), offered to adapt the forth-coming temp unit to operate as a controller for the casting furnace, forge, salt pots ect.
I think his line of reasoning is that if he helps out, he can live a new hobby thru me and we both learn alot that can be re-applied elsewhere to his/my advantage. Gotta love a boss like that.
 
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