adccommunitymod (AutomationDirect) asked a question.
Created Date: February 22,2008
Created By: ksmith
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We have an application where we are monitoring the temperature of several points on some equipment. We are using type T and are using the F2-04THM modules with jumpers set correctly. The thermocouple wires have been extended by 30 ' or so with shielded 2 conductor Belden cable soldered to the leads of the Tcouple wire. The temps we are sensing are from 0 to 160 degrees F, the problem is the temps seem to be off by 10, 20, 30 even 40 degrees. The PLC is in a different room and sees a different ambient temp then the sensed equipment. How much does the cold junction comp effect the readings? Have I done the wrong thing in extending the wire as I described? How much does connecting the Thcouple wires backwards affect things? New to this Thermocouple sensing. Thanks in advance for anyones help.
Created Date: February 22,2008
Created by: mmohn
Cold junctions are pretty important, but as long as the ambient temperatures are relatively close, it won't create too much error. Your big problem lies in the extension...you MUST use thermocouple extension wire to extend thermocouples; and it's best to use the proper thermocouple connectors (not soldering) to connect them together. You can't reverse the wires, either.
Created Date: February 22,2008
Created by: Bob S BN
To add to what MMohn stated, you need thermocouple extension wire and it must be of the type compatible with your type "t " thermocouple.
Created Date: February 22,2008
Created by: ksmith
Thanks for the help...I was led astray by some bad info. One other question..when bringing the Thermocouple wire into the F2-04THM module, I assume I need to go straight to the module connector instead of going through a terminal strip in the control panel first.
Created Date: February 22,2008
Created by: mmohn
It would be best to connect directly to the module; however, you can get special thermocouple connection panels if you want to have a plug-in at the control box. You still have to use the thermocouple extension wire to connect the terminal panel to the module.
A thermalcouple is created anytime two different metals are connected. A small millivoltage is produced proportional to the temperature. Some metal pairs do this really well and produce a linear output based on temperature, and those pairs are the ones used at "thermocouples " (J, K, T, etc). So, when you soldered copper wire to the thermocouple leads, you created 4 extra thermocouples (copper to solder & solder to TC wire on each lead) outside of your measurement point. The special TC extension wire and connectors are made of the same thermocouple materials (so the connections don't make extra thermocouple points), and the THM module is designed to take into account the final connection terminal block.
Created Date: February 25,2008
Created by: a agnone
Interesting reading this post. I have several oven we are measuring the air temperature. We bought rolls of J type thermocouple wire. It has a covering on it that will not burn. We simply cut the length. They are up to 34 feet long. We electrically weld the tip. Then they run all the way straight to the module. We are not experiencing any problems. Did we just luck out? The wire is quite expensive but works. The coating looks like plastic but it does not burn. I forget what they call it.
Created Date: February 25,2008
Created by: 72hdflh
a agnone,
I have been doing basically the same thing, but all I do is twist the T/C wire together real well. I am only doing this at lower temperatures (150-200 deg. F), I think that just twisting them together might become a problem at higher temps though.
You just can't mix different types of wire or terminal blocks, there are special terminal blocks for T/C's that are made of the same material as the T/C is made of (type J = iron and constantin).
Created Date: February 25,2008
Created by: mmohn
If you can afford it, running a one piece thermocouple wire from the thermocouple end all the way to the THM module would be most accurate. The reason to use extension wires and connectors is for wiring convenience or to save cost. You can actually make thermocouples for low temperature use out of extension wire; the biggest difference and cost savings is in the insulation. The non-burning insulation on the thermocouple is usually fiberglass or teflon, which makes the thermocouple wire expensive. The extension wire just has pvc. Thermocouples are just as accurate either twisted or melted together. If you put a long twist, just remember that it will measure at the first place the wires touch, not at the very end. I 've dipped "twisted wire " J-thermocouples into molten aluminum without any problem.
Created Date: February 29,2008
Created by: donrobson
I have changed to using RTD temperature sensors for these sorts of temperature measurements instead of thermocouple. You can just run standard wire between the sensor and the receiver, and junctions are less finicky-standard terminal blocks work fine. Usually it's three wires, but some sensors have four, I believe. My customers find it much easier to work with and less expensive, especially for long runs, plus they don't have to go find the proper wire.
Perhaps for future projects you might consider it - your module would be F2-04RTD and your sensor would of course need to be RTD as well.
Created Date: February 29,2008
Created by: Brian123
Originally posted by mmohn:
the biggest difference and cost savings is in the insulation.
Not necessarily true. Check out the materials used for types R and S thermocouples. Platinum for one lead and a mostly platinum alloy for the other. I can guarantee you that the biggest difference in cost between the extension and thermocouple wire is _not_ the insulation used. http://forum1.automationdirect.com/board/smile.gif
But for standard thermocouple types like J, K, and N, yes, the insulation is a big difference.
Created Date: March 07,2008
Created by: mmohn
I 've never had enough $$$ to use anything but J and K types...