Limit timer for kiln

[Terry Gallentine]

I have relatively simple firing schedules and therefore do not use digital kiln controllers but I would like to add some sort of limit timer to the kilns for extra safety. The limit timers that I find for pottery kilns all operate in conjunction with a kiln sitter that works with cones. I just need an electric timer that I can set to turn the kiln off at a time slightly past my expected firing time. The advantage to this is that if I were to have an accident of some sort (like a heart attack) and not be able to shut the kiln down manually, the kiln would still shut down before a meltdown occurs and burns my studio down around me.
Does anybody out there know where I can get something like this or have an idea of how it could be rigged? All of my kilns are 240v with some operating off of 30 amps and others operating off of 60 amps.
Please don't just tell me to get a digital controller. I have no ramping or soaking requirements so multiple controllers would be an expense I do not want at this time.

[Bert Weiss]

There is a code worthy way to make a kiln safe, called a high temp limit controller. The system has 3 components, the controller, a double pole normally open relay, and a thermocouple. http://www.duralite.com/store/scripts/p ... product=92

The heart of that system is the double pole normally open relay. When powered, the relay is closed. As soon as the power is removed, the relay opens. I like to put the controller on a light switch. Turning this switch off has the effect of opening the relay. I suppose you could rig this relay with a simple light timer turning off the power and opening the relay. To make a kiln safe for combing, you put a switch on the door so when you open the door, all the power goes off.

Most kilns are controlled with a single pole relay. This effectively turns the elements on and off. However even when the kiln is off, and elements cold, if you contact the element to ground, it will draw 120v and heat up.

[Kevin Midgley]

http://www.intermatic.com/Home/Products.aspx

[Brad Walker]

If you already have one, a traditional kiln sitter will also work. Just put a cone with the appropriate temperature rating in it and the kiln will automatically turn off when it reaches the temperature.

[Terry Gallentine]

Thanks everybody for the information and especially you Bert. I think that the Duralite info will help me quite a bit.

[Lauri Levanto]

here,
Far North where I live,
we use to pre-warm the car before using it.
For that purpose the car equipment stores and electrical stores provide a simple timer. It sits between the outlet and plug. It has a 24 hour clock and
with small tabs you mark when the current is permitted.
You may even provide separate times for fusing and annealing.
-lauri

[Terry Gallentine]

Thanks Lauri,
I think you are thinking of a weekly timer. I think what I need is a digital "countdown" timer that has an upper limit of 4 hours and is programmable at least in 15 minute increments if not minute increments. If I can find one, I will step it down from line voltage to 24 volts so it can control a couple if relays that will shut down the kiln.

[Bert Weiss]

The timer is a poor choice. The time it takes for a runaway relay to make the kiln really hot is far less than the time it takes for a firing cycle.

If you are subject to industrial codes, which I believe none of us are, they require a redundant hi-temp controller. This means that you can't do it by using settings inside your regular controller. I know of several strategies that might work, like redundant relays, but for a reliable safety system, the hi temp limit controller is the best solution.

As to cones, I have definitely heard stories about cones failing to shut off a controller.

[Terry Gallentine]

Actually the back up safety for a traditional kiln sitter is a limit timer. You have to take into consideration the time it would take to meltdown if a kiln relay failed but a kiln relay failure is not my primary concern. I plan on using a timer controlled relay on each of my 240v legs to add a redundancy factor to the set up.

[Bert Weiss]

Actually the back up safety for a traditional kiln sitter is a limit timer. You have to take into consideration the time it would take to meltdown if a kiln relay failed but a kiln relay failure is not my primary concern. I plan on using a timer controlled relay on each of my 240v legs to add a redundancy factor to the set up.

It's complicated. Using a second relay on each controlled leg may well stop a run away, because it is unlikely that 2 relays will fail at the same time. That is good, but how will you know when one has failed closed? If one of them fails open, it won't work at all.

Mechanical relays are a really poor choice of devices, except that they continue to work well in a hot environment. Solid state relays are thousands of times more reliable in a normal ambient temperature, but fail in a hot environment. Therefore, they are not appropriate for mounting on the side of a hot kiln. If you move your control system 18" away from the kiln to a wall, you can take advantage of the much more reliable (and quiet) solid state relays. Mercury relays are more reliable than mechanical ones, but contain mercury which harbors a potential environmental hazard. Cones, simply don't always melt as intended.

SCR solid state relays are the best choice as they can turn on and off 120 times a second, which is easier on the elements and yield tighter control. However you need a more sophisticated controller than the Bartletts and Ortons, that are commonly used.

[Terry Gallentine]

Thanks Bert,

I will be mounting the timer/transformer/relay box on the wall a couple of feet away from the kiln. That should keep the relays safe from temps. I will also be putting a couple of indicator lights on each leg downstream from the timer/relay. If the timer is off and I still have power in one leg, that should mean I have relay failure in a closed position.