Posts Tagged temperature transmitter
RTDs are great temperature sensors – accurate and easy to install. But they are not friendly when it comes to trying to get a single RTD to go to two places, like when an RTD temperature measurement has to go to both a controller and a recorder. People call and ask, “How do I split an RTD signal?” The short answer is, “You can’t.”
An RTD cannot be wired in parallel or in series to a second device. Any RTD input supplies a known, regulated ‘excitation’ current to the RTD. Mixing RTD inputs would mix currents and that’s a Big No-No.
There’s also a lead wire compensation circuit for 3- or 4-wire RTDs that would create problems if a single RTD were connected to two different RTD inputs. There’s just no feasible means of making two RTD analog inputs play nice together.
But all is not lost. There are several ways to achieve your goal.
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?