Bending is caused by the weight of the sensing rod. A Self-supporting witness cone will check whether the controller is firing accurately. Place the cone in a location near the thermocouple. After the firing, determine if the Firing Cone has bent properly. There should be no more than a half cone difference.
Controllers depend upon accurate measurement of temperature and proper programming to fire properly.
Most controllers use a Type K thermocouple, which may not give an accurate temperature and which will change after being used. It is not unusual for a Type K thermocouple to have an error of more than 25 F when fired to Cone 6 repeatedly. This is more than a full cone. By using witness cones, you can determine whether the firing was to the right cone number. Many people use witness cones every firing and the 3-cone system periodically to check for changes in the kiln.