Thermocouple Wiring Revisit, Part 2
March 2, 2007
Last month, I started a brief series about the inaccuracies and instabilities caused by misuse of thermocouple extension cable on temperature control systems. I’ll continue by looking at other traps you can fall into when color confusion leads to the wrong thermocouple wiring use.
As a reminder, in Traps Part 1, I explains what happens when you use copper instead of thermocouple extension cable. Because the copper fails to generate the 20oC (36oF) of signal that the proper extension cable contributes, the controller will call for more heat than needed. Worse yet, the controller will indicate the desired temperature -- 200oC (392oF) -- all the time that the temperature is slowly climbing above 200oC.
Trap 2. You Use the Correct Extension Cable but Cross the Pair of Wires at Both Ends. This is even worse than using copper. The temperature will settle out higher than indicated, by about twice the head-to-controller temperature difference.
This error can vary during the day, from not noticeable to some 100oC (180oF) too hot, depending on how near the thermocouple head is to the hot process and how long the process has been running.
This double reversal wiring error is common and understandable because the negative wire is color-coded red on all North American extension cables and can be mistaken for positive. Red positive was indeed used by Japan and Germany before they adopted the IEC world-standard colors. The figure shows the many color coding schemes used for thermocouple wires throughout the world, including the IEC specifications.
Trap 3. You Use the Wrong Extension Cable. Remember that extension cable makes a contribution to the millivolt signal and is interpreted as temperature by the controller. The proper cable will match the thermocouple in respect of its microvolts per degree. Wrong cable can make the controller indicate too high or too low, depending on how much its microvolts per degree differ from that of the thermocouple.
As noted in Case 1 (Heating Highlights, February 2007, page 23), the Type J extension cable delivered an error some five times the magnitude of that in the correct Type R cable and injected negative errors of some hundreds of degrees. There are even worse combinations than this one.
The figure covers the six most commonly used thermocouple types. It shows five varieties of cable color codes used in different countries, plus the IEC codes. The IEC codes represent a single world standard and are now showing up on imported European equipment. They could be specified on North American built equipment destined for overseas. They will eventually replace the currently used codes. Note that the negative is always white and the positive matches the cable sheath color.
Tips for Cable Identification
- Look at the colors of the cable all the
way from the thermocouple head to the controller.
- Check for wires crossed at both ends.
- If you don’t trust or cannot see the colors, perform a heat test for
polarity; that is, disconnect the extension cable from the thermocouple head
and twist or clamp the extension cable bare wire ends together. The controller
should now show room temperature at that location, and the indication should
increase the right amount when you heat this junction.
- A magnet test is useful with Types J and K wire. On Type J, the
positive conductor is iron and therefore magnetic. On Type K wire, the negative
conductor is Ni/Al and slightly magnetic.
- Note too that thermocouple connectors are color-coded to agree with
the corresponding cable sheath.