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My Fluke pyrometer reads 1280C and my kiln controller reads 1233C. What is wrong with the kiln?

It's normal for a digital controller to be out of adjustment by at least a few degrees. So it is not surprising that your kiln reads 1233 while the Fluke pyrometer reads 1280.

The Fluke could be off, too. A pyrometer or controller is only as accurate as the thermocouple connected to it. You are probably using Type-K thermocouples, which drift in temperature as they age. (The more expensive Type-S thermocouples last much longer without drift.)

Using witness cones on the shelf is the best way to check the accuracy of your digital controller. It would be a good idea to place the cones in several locations to get an average cone reading.

If you look through the controller manual, you will find instructions for one or two types of adjustments: "Cone Offset" and "Thermocouple Offset."

Adjusting Cone Offset affects only the cone you are adjusting. The other cones in Cone Fire mode remain unaffected. Thermocouple Offset affects all cones in Cone Fire mode and all temperatures in Ramp Hold mode.

Example: Adjusting Thermocouple Offset so the kiln fires 10 degrees hotter affects all firings. Making that adjustment for cone 05 in Cone Offset affects only cone 05 firings.

If you don't want to make a controller adjustment, you could also fire to a lower cone number or add a hold time.

If the controller is off by more than 50 degrees F, it is probably because the thermocouple is failing. Or it may not be pushed into the firing chamber far enough. A 1/4" diameter thermocouple should protrude into the firing chamber by 1". Since your controller is off by around two cones, the thermocouple may be failing. Another possibility is that a shelf is too close to the thermocouple. That can change the reading.

One way to check the thermocouple for wear is to reach inside the kiln while it is cold and move the thermocouple tip with your fingers. If the temperature display on the controller jumps, the thermocouple is about to fail.

At any rate, replacing a thermocouple takes just a few minutes once you've done it a couple of times. Most likely the controller is operating normally.

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