Archive for category Thermocouples
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.
If you use thermocouples in high-temperature applications, you’re aware of the issues thermocouple drift can cause. Thermocouples drift. It’s not a question of IF, it’s a question of WHEN. And thermocouple drift costs processors time and money in processing errors, waste, downtime, and lost production.
Thermocouple drift occurs due to metallurgical changes of the metal alloy elements over the extended use of the sensor. Thermocouples can drift by as much as several degrees per year.
There’s no such thing as a fail-proof thermocouple. Over time, thermocouples fail. To compensate for that, a temperature controller will normally go into upscale burnout mode, and drive the furnace burner to low fire or turn down the SCRs. But then, you have to deal with the downtime, rework, or even the potential of losing product.
Not long ago, a plant operator called to see if there we had a way to work around this burnout mode, so he wasn’t wasting time and materials.
His heat treat load had almost finished its final soak when the control thermocouple broke open. The controller, as expected, drove the furnace burner to low fire. The operator then popped the controller into manual mode, so he could nurse the load through the remainder of its soak cycle. He used the temperature reading on a recorder, fed from a second, unbroken thermocouple in the protection tube as temperature indication for the load.
If the situation had happened in the middle of the night, it may not have been handled with the same attention the day-shift operator had provided.
So, he asked if there was any way to have the controller automatically “fail over” to a second thermocouple.
Ever wonder if you can mix brands of thermocouple heads, transmitters, or junction blocks? The other day, I was trying to figure out if I could, and I ran into a spec I didn’t understand – DIN Form B. So, what exactly is DIN Form B, and how does it relate to thermocouple heads, transmitters and junction blocks?