Thursday, May 28, 2009

Element Design --- Some Basic Kiln Parameters for Potters

There are a couple of features that crystalline potters should have with their kilns and I would like to look at these as a little digression from the calculations of the elements. I would say that every potter with an electric kiln should at least consider these features. 

The first has to do with thermocouples. A crystalline potter needs an S Type thermocouple. There are other even more expensive thermocouples, but we are discussing the difference between the K Type thermocouples that normally come on kilns, and the S Type that is an upgrade if you order it when you order the kiln. The S Type thermocouple has several advantages. It will measure temperature accurately through thousands of firings. It does not wear out. However you can not just retrofit your kiln with one of these babies because you need to change a chip in the Bartlett Controller ($10.00), and you need different lead wires. It's cost is around $150.00 compared to a K Type at around $25.00. 

However, Steve Lewicki of L and L Kilns recommends that you change the K Type thermocouple every time you change your elements. He says they are accurate for only a few firings and then they begin to drift. When I got my new L and L kilns, the thermocouples were protected from the atmosphere by a porcelain tube. After about 25 firings I took the thermocouple out of the tube and then tapped the tube on the table and there was a large pile of metal shards that had spalled off the thermocouple. No way that thermocouple was accurate.

So, bottom line is that S Type thermocouples cost way more when you get them, but they do not need replacement and they are accurate throughout their life. They are essential if you need to have any accuracy at all of your kiln's temperature. 

I must say that they are easy to break and you must be careful with them. I take mine out of the kiln when changing elements, something I learned by experience. 

Another thing that Kanthal recommends is using some kind of relay that is more sophisticated than just on and off. The idea is somewhat like that of a teenager driving a car with normal relays. The element is full on and then it is full off so it's like pressing the accelerator of a car full blast and then taking your foot off the gas full blast. This has the tendency to wear out the car, and it also has the tendency to wear out the element. 

My friend Terry Fallon is designing an incredible kiln and it has solid state relays. They cycle on for 200 milliseconds and then they are off for 200 milliseconds. This has the effect of not shocking the element and also of not making the element get much hotter than the kiln. These relays are more sensitive and they need to be mounted on a heat sink so they do not burn up, but Kanthal suggests that they will increase element life.

1 comment:

Alex Solla said...

John- Tell me this. Are these type S elements exposed inside the kiln? Or is there an alumina sheath over them? How many firings does L&L predict they will last given the high temps and corrosive environs of a crystal firing kiln?