I'm planning this Arduino build and have some concerns about my thermocouple plans. I'm just a hobbyist and have a very basic understanding of TCs.
Basically I will have two K type thermocouples measuring temperatures at two different places in the same heating chamber. These will lead to my project box with an lcd screen for me to adjust temps and monitor. I want to have TC cables with male ends on both sides so that I can disconnect from both the heated chamber and the project box when not in use.
Is it bad practice to have too many couplings in a TC circuit? My main concern is that the place where the cable will plug into the hot chamber will be warmer than where it's plugged in outside (it will be in a significantly cooler area, so it's safe, but it's still warmer). I've read that if the metal changes in the TC circuit, the TC couplings need to be a consistent temperature on both sides of the the link to get a consistent reading. I don't think the metal in my adapters are the same metal as the TC wire.
Is this fine? Should I just let the TC dangle from the heating chamber? Suggestions on better TC adapters? Thanks for any suggestions that can be offered.
Each metal change will generate a voltage which may either add or subtract from the thermocouple voltage. As long as you can characterize that voltage vs temperature and allow for it in your calculation it is ok. What amplifier are you using for the thermocouple voltages?
bfeick:
I've read that if the metal changes in the TC circuit, the TC couplings need to be a consistent temperature on both sides of the the link to get a consistent reading. I don't think the metal in my adapters are the same metal as the TC wire.
Yes, at the junction of different materials you get another 'thermocouple'.
Usually the thermocouple wire from the actual sensor will end in a plug, then a socket connects to either more of the same thermocouple wire (but this can be expensive in long runs) or to Thermocouple Extension Wire (which is cheaper).
You can have multiple connections as long as the connectors and type of extension wire are correct for the type of thermocouple you are using.
If you happen to have thermocouples with long enough leads, just cut 'em and insert a plug-socket pair making sure the right wire goes to the right +/- pins.
Paul_KD7HB:
Each metal change will generate a voltage which may either add or subtract from the thermocouple voltage. As long as you can characterize that voltage vs temperature and allow for it in your calculation it is ok. What amplifier are you using for the thermocouple voltages?
Paul
Paul,
I'm using 2 of these amplifiers
Right, so if the TC wire goes to a plug with a stainless steel contact then back to the same wire, the voltage should theoretically average out. I wouldn't know how the amplifier is calculating all of this. If there are errors due to links and temperature differences, do you think these errors will at lease be at lease consistently off? Meaning, will they result in the same offset at 70F as 350F?
TonyWilk:
Hi,
You can have multiple connections as long as the connectors and type of extension wire are correct for the type of thermocouple you are using.
Tony,
when you say the connectors need to be correct, do you mean the contact material within them need to be correct? Because mine SAY they're for K type, but I'm pretty sure the contacts are stainless steel which is certainly different from the wire. Would you consider these to be correct? I'll have a look at the ones you linked to to see if they're maybe a better option.
bfeick:
when you say the connectors need to be correct, do you mean the contact material within them need to be correct? Because mine SAY they're for K type, but I'm pretty sure the contacts are stainless steel which is certainly different from the wire. Would you consider these to be correct? I'll have a look at the ones you linked to to see if they're maybe a better option.
Yes, the contact material has to be correct.
Yours say they are K-Type so they should be the right material, but there is no information to confirm that.
I have seen cheap connectors (yeh, it was me, I once bought some cheap off eBay for my Kiln) and the screw terminals had ordinary steel washers and screws. They went in the bin.
Yours,
TonyWilk
P.S. Just remembered: you do want screw terminal ones, I've also seen some (these I didn't buy) with solder connections - not good at all.
(they were, in fact still are, in service at a huge glass manufacturing company on their tempering furnace. Chinese-made and all the thermocouples are connected with a random collection of soldered ends, crimp terminals and ordinary bellwire. Surprisingly it all seems to work - probably because the business end is around 750C so the other random junctions at room temperature don't affect the reading that much. Still, always makes me cringe when I go to fix anything there !)
If you bought K-type connectors and they look like steel, don't worry. But the two pins must look different. One should be a little more copper-colored than the other.
If the pins look identical, throw them away. It is better to twist the wires together than to use the wrong plugs.