New tempering oven at JT Custom Knives

[@the_sharp_carpenter]

What are the main differences between RTD and thermocouple systems?

[@jtcustomknives]

Rodney Philmon A thermocouple sends a voltage to the controller. That voltage is measured and applied to a formula or database which then spits out a temp. As the thermocouple gets hotter the voltage output goes up. It has to do with a temp difference between the tip of the thermocouple and the controller. An RTD works different in that it does not generate voltage but resistance. Industry standard is 100 ohms at 0°c. So the controller acts like an ohm meter and measures the resistance of the RTD. There are different kinds but the platinum RTD is most common. These have 3 wires from the RTD to the controller. One is power from the controller. This connects at the top of the RTD to both of the return wires. Between one of the return wires and the power wire is the platinum resistor. The other wire has a direct connection to the power wire. What this does is let’s the controller compare the resistances of both the return wires. This allows the controller to compensate for the resistance of the wires themselves. This lets the controller calculate what the actual resistance is of the platinum resistor. And calculate the resistance to temp in a formula. RTDs are much more accurate and don’t have a problem drifting like thermocouples do. There only problem is thy are for lower temps so thy don’t work in forges or heat treating ovens that go over like 1,400° I think.

[@the_sharp_carpenter]

@@jtcustomknives wow thanks for the detailed explanation, I did a quick search to see price etc and did notice a much more specific temperature range than thermocouple, ie. 200-500 1200-1700 ect. As compared to thermocouple which just have a max temp rating. Will have to dig deeper and see if I can make use of these in a gas powered heat treat setup. Cheers!

[@WireWeHere]

@@jtcustomknives I would like to add a little info about TC's, the, and mention extension mistake I've seen often. Thermocouples are just a pair of conductors that are made of dissimilar materials. They are normally joined at both ends making a hot and cold junction. The PID controller is doing a lot of work we normally don't have to think about. Thermocouples must be selected very carefully. Too much to mention other than to say read until you find one that fits your situation. You must not extend thermocouple wires with any other materials. This includes the crimp on fittings too, cut them off if they arrived installed. TC's can be run in parallel with as many as you like all connected to one pair of sensor inputs. The measured temperature will be the average of ALL the thermocouples. Putting a pair, or more, in parallel to find the average temp of a zone can be useful if you can think of anything. It's nice having two working together on those days when a TC gets damaged or when a programmed ramp and soak is underway, simply unplug the toasted one and on you go. The tip where the two conductors are joined can have a third metal bonded directly on for protection or to add mass and buffer rapid changes in temp. Thermocouples with stranded conductors have proven to outlive the tiny thread of the nonstranded versions. A few years ago it was common to pay over $100 for a SST TC 1/4" x 3" tip with a 6 foot length. Now they're nearly free. That all that popped out right now. Best regards, Peace