One thing I’ve learned in this industry: Even though there are standards to thermocouple color codes and types, the most basic rule for installation (In the US, when you’re wiring a thermocouple, RED is always negative) is non-intuitive to anybody who’s ever done electrical wiring.
It’s pretty easy to tell when you’ve done it wrong: You wire the thermocouple directly into an instrument, and as the process gets hotter, readings say it’s cooling down. As your process cools down, the readings show a rise in temperatures. If you reverse the wiring at a junction box, it’ll read in the right direction, but you’ll have errors because of the false junctions.
Here are some basic wiring diagrams from the reference section of the Lesman catalog, with rules to follow, and some suggestions on specifying the right thermocouple wire.
There are three basic thermocouple wiring diagrams listed:
- How to wire one thermocouple to one instrument
- How to wire one thermocouple to two different receiving instruments
- How to wire multiple thermocouples to a single instrument through a switch
As you can see, none of the diagrams are really complicated, but there are some rules you need to follow to do it properly.
- In the United States, when you’re connecting thermocouple wires to instrumentation, RED is always negative. The other color-coded wire is always positive.
- Match the thermocouple wire to the thermocouple type you’re using.
- Use thermocouple wire to make thermocouple elements, or to connect thermocouples to instrumentation. Thermocouple extension wire should ONLY be used to connect thermocouples to instrumentation, and copper wire should never be used.
- If you’re wiring a thermocouple into a switch or junction box, the contacts do NOT need to be the same as the thermocouple materials. Just know that any difference in temperature between the positive and negative contacts will become an error in the signal.
- Do not run thermocouple leads in conduits that carry power wiring. And do not run conduit carrying thermocouple leads parallel to electrical buss bars or heavy power-carrying conduit. Cross them at right angles.
There are also a few tips to consider when you’re buying thermocouple wire. It starts with matching the wire type to the thermocouple type, but to get the best performance, also consider these:
- When you’re buying thermocouple wire, choose wire insulation that’s compatible with your application environment. For applications that require moisture resistance, use Teflon, PVC, Kapton or Tefzel. For high-temperature applications, use fiberglass, vitreous silica, and ceramic fiber.
- If there’s going to be frequent flexing of the leadwire, use a stranded conductor wire to connect the thermocouples.
- To provide protection against physical abuse to the wiring, use metal overbraids and leads in flexible armor.
- To connect sensors to computers and protect against EMF stray signals, use leadwire with aluminum Mylar sheids and drain wires.
Lesman catalog reference section (Includes the diagrams shown here plus unit conversion tables, media properties, ratings and standards, and a glossary.)