I don’t think my digital kiln is firing to an accurate temperature.
It is not unusual for a digital kiln to be a little off in firing results from the cone number you enter. But once you know how much it is off, you can adjust the controller with a feature called "thermocouple offset."
If the witness cone is over-fired, estimate how many degrees it over-fired. Follow the instructions in your manual to change thermocouple offset.
Once you have adjusted thermocouple offset, you should get amazingly repeatable firing results for a long time. But eventually, the thermocouple (temperature sensor) will begin to age. When that happens, the kiln will gradually fire to a slightly lower temperature for a given cone number. This is normal with Type K thermocouples (the standard type used in ceramic kilns). By using a witness cone in your firings, you will know when the thermocouple begins to drift.
Type S thermocouples ordinarily fire for years without drift. They are more expensive than Type K, but if you fire to cone 6 and hotter, they are definitely worth the money.
Before you buy a Type S thermocouple, be sure your controller is designed for it. Ask your supplier. Also, when you install the new thermocouple, you will have to program the controller for Type S. Controllers are set at the factory by default for Type K. If you install a Type S thermocouple on a controller programmed for Type K, your kiln will probably over-fire. So it is important to read your instruction manual.
Be careful not to jar the thermocouple during loading, especially if you have a Type S. They are expensive, and when they need replacing, it is usually due to breakage during loading and unloading.