Installing three zone controller on your kiln

Perhaps I misunderstand your problem, but the above doesn't make sense.

The controller does indeed switch your elements on and off, but there should be no loss of maximum performance. It's like your kitchen oven: when the current temperature is below the setpoint, there is maximum power to the element(s). Only when it reaches the setpoint does it start cutting back, in order to maintain whatever temperature you set. The difference with a digital controller is that there is not just one simple setpoint like your oven, more like a schedule of setpoints that it sets one after the other.

Yes, it's true that when you set a slow ramp-up, the kiln does not heat as fast... it's just doing what you told it! This is certainly not a fault of the controller. It's also possible to set a ramp that is so steep that the kiln can't meet it even running flat-out. So you do need to have some experience with your kiln to know what is possible as far as rise rates go.

But there should never be a problem such as you describe. You don't need any surplus power at all if you have set your schedule to be similar to your original uncontrolled rate. In that case, the controller will not be doing much of anything until you hit the setpoint. Only then does it start cutting back on the power in order to hold that temperature.

Again, perhaps I have misunderstood your problem, but my suspicion is that you have simply set an unreasonable schedule. One thing you might consider is running the kiln the old way and recording the temperatures every hour, then use that data to set a controller schedule that comes close to the "natural" kiln performance. The difference is that once you reach the cone setting, you will now be able to hold that for as long as you want, or to cut back to a somewhat lower setting for crystal growth, etc.

Once you have this base ramp -up information, you can feel free to set slower ramps over any temperature region.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

From my experience with three zone controllers, the problem could possibly occur under certain conditions.

The kiln would need to be firing very unevenly in heat distribution, and I assume this was the case to require fitting of a three zone controller in the first place. Many of the tall top loading kilns suffer from this problem - too hot at the top and too cold at the bottom. Also the elements would need to be slightly low in power dissipation for the kiln size, either through poor element design (a common problem) or age. In other words the kiln was just managing the required firing cycle before the fitting of the new controller.

After fitting the three zone controller, the top set of elements will be cycled off some of the time to try to keep the top in line with the temperature in the rest of the kiln, while the middle and lower elements are running at full blast . The net result is that the overall power dissipated during the firing is down enough to slow the firing cycle, or even prevent the completion of the cycle.

Many kilns are so poorly designed that they have very little reserve power available when new, and after a lot of firings they are struggling to reach the end of the cycle.

One answer might be to fit a kiln vent which draws a little hot air down from the top through the bottom of the kiln and can often make the kiln fire evenly. The best solution may be to fit new, properly designed, elements to compensate for the uneven firing.

A couple of questions for the original poster. What sort of kiln are you using? Did you have trouble with uneven firings? I worked as a kiln technician for many years, and I often had to redesign elements for tall toploaders to compensate for very uneven firing.

Cheers

Dave

I think you are quite correct. Simply put, a kiln controller can only work by turning off elements in the hotest zones It cannot add any additional heat in the cooler zones by turning on non-existent elements or running the current higher or something. In other words, It works by reducing power where it is high but has no way to add power beyond the design maximum where it is low. Therefore the rate of climb and max temp will always be less than the absolute maximum without a controller, thus the need for reserve. However, this is true whether it is automatically or manually controlled.

I have indeed run the kiln the old way, and tracked the temps. Running full tilt, I know that in order to reach my target temp, the heat rate is only around 60 degF per hour in the home stretch. This is running at max; I'm not even maintaining a cone rate of 108DegF. So I, respectfully, disagree with your point that "there should be no loss of maximum performance", because when the controller turns off any of the coils in an attemp to even the temps, then you must see that, in my case, my heat rate must also drop below 60.

Also, regarding your statement "This is certainly not a fault of the controller",it is certainily well taken, and, in my original post, I did not wish to imply fault on the part of the controller. My intent was to warn those who may have a kiln, such as mine, that may not perform sufficiently with such a controller.

I imag> Perhaps I misunderstand your problem, but the above doesn't

distribution, and I

(Davis Salks) Yes. When running full tilt, my final temp on the bottom zone is around 35F cooler than the middle, and the top zone is around

20F cooler.

I agree with this

I often hear coments about element age as related to poor performance. This confuses me because older elements actually run hooter than new ones. This is because their electical resistance decreases over use.

You've hit the nail right on the head, Dave!

Evenheat, 55 amp, 6 coils, inside 29" dia X 27" high, outside 34" dia X

32 high

Yes, actually that's why I thought that the 3-zone would help. But I really never considered that the kiln would not have been able to keep up. I still don't really know how I could have assessed my kiln's performance for this application.

I am open to that.

Presently I am going to backup the lid on the outside with 1-1/2" of vermiculite board and also add 1" of ceramic wool to the floor inside, hoping that the added insulation will slow down the cooling somewhat in these areas.

After testing, I will let you know what the resulting temp diffs are.

Thanks

Davis

Why would the resistance decrease with use? I'd have thought the element would slowly oxidise or vapourise, thus increasing its resistance. I can see that the element itself may run hotter (smaller surface area = less heat dissipation) but not that it will produce more heat - indeed the hotter it gets the higher its resistance and the lower the overall power output. I expect there'll be a physicist along in a minute to put me right.

Davis, here is some technical stuff for you. The resistance of elements INCREASES with age - this is the explanation.

Kanthal element wire is an alloy of several materials, including aluminium. The aluminium forms an oxide on the outside of the wire which protects the wire from further oxidation under extreme operating temperatures. However, it is not perfect, and oxidation continues slowly over the life of the element - this is why old elements have a very crusty coating. The removal of the metal from the wire reduces the effective thickness of the wire, which increases its resistance (thinner wire = higher resistance). The higher resistance reduces the current (current is inversely proportional to resistance at the same voltage) and since power is proportional to the square of the current, power developed by the elements drops by a lot more. Also, the resistance of an element at maximum temperature increases by up to

4% over the cold resistance. When I was working on kilns I would replace the element if it showed 10% or more over the design resistance when cold.

I hope this helps.

Regards, Dave

One further comment here. Adding insulation to the lid and floor may help, but never make the mistake of putting extra insulation over any external metal parts. I have seen the stainless steel casing almost melted and badly deformed by the heat trapped under added insulation.

Have you tried a kiln vent to even out the temperature, they really do work.

Dave

David - I stand corrected. Thanks.

Davis

PS - Contact me if you would be interested in helping me revise the element design. Thanks.

Dave - how does the kiln vent accomplish that?

The kiln vent draws a small amount of air out through the bottom of the kiln through holes in the lid and can correct uneven firings as well a take all the nasty gases out of your kiln room. There are many types, this is the best - lots of info here

They are not cheap, but cheaper than new elements. I could re-design the elements for you, but you would still have to get someone to make them as I no longer have a winder and I am in Australia anyway.Dave