The kiln sitter is a mechanical device, and therefore has the possibility of not working. If any parts of the sitter stick, then it won't shut off. That's why they started putting backup timers on the sitters. Of course, if you set the timer too long then it doesn't do any good, and it's also possible for the timer to fail since it is a mechanical part as well. The only way to be 100% sure that a sitter kiln shuts of properly is to be there in person to check it. If you're not willing to do that then you have to accept that at some point your sitter will possibly fail and you will ruin some pots and/or the kiln itself.
The worst I have ever seen was a firing where a post or pot fell over onto the sitter tube and prevented the sensing rod from dropping and shutting off the kiln. It was an old sitter and didn't have a backup timer, so it kept on firing until the pots melted, then kept on firing some more until the shelves warped like potato chips and collapsed into a pile at the bottom of the kiln. The kiln was destroyed. The whole bottom half of the kiln was a melted mass of shelves and pots fused to the wall bricks. Luckily it was a really old kiln that needed to be replaced anyway.
For a digital kiln to over fire, a relay has to stick in the 'on' position, and the kiln has to be packed in such a way that enough heat is trapped in the section controlled by that relay that it can over fire. A section probably can't over fire in a cone 6 firing, but it is theoretically possible that in an earthenware firing a section of the kiln could over fire a little bit. In a two section (18" high) kiln where there are only two relays, one sticky relay can cause the entire kiln to over fire a little. I've seen that happen. I have never seen more than one relay stick 'on' at a time, however I have seen two relays fail 'off' at one time. Again, the only way to be sure is to be there to check that it has shut off.
- Mudmann65 likes this