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What are Thermocouples?
What are Thermocouples?
General Information on Thermocouples
- The pyrometers and all of the electronic kiln controllers for L&L kilns work with a “Type K Thermocouple”. (Although we do make available a Type S platinum thermocouple).
- There are all different “types” in the entire range of thermocouples available to the consumer and to industry.
- All thermocouples, regardless of the "type", are made of one kind of wire on one side if the circuit, and another kind of wire on the other side of the circuit. The point at which the two kinds of wire meet is right at the end of the sensor - the thermocouple probe's tip in the kiln. This is where the temperature is read. All types of thermocouple circuits are set up this way, with two dissimilar metals making up each half of the circuit.
- Type K is just one type covering the temperature scale from 32°F to 2500°F (0°C to 1372°C).
- In the case of the Type K the metals are called Alumel and Chromel. They are usually either 14 awg (American wire gauge) or thicker 8 awg wire. The thickness of the wire is only important inside the kiln. Thicker wire lasts longer, but is more expensive and more difficult to work with.
- Our standard Type K thermocouples in the Easy-fire, Jupiter, DaVinci and Doll kilns now come with a ceramic thermocouple protection tube. (These last longer because they are protected from the kiln atmosphere).
- These have a metallic sheath on the outside of the thermocouple and are 1/4” OD.
- If you retrofit these into a kiln with larger diameter thermocouples be sure to plug up the larger hole around the thermocouple especially if a venting system is in use. Otherwise they could read cooler than the real temperature of the kiln and this could result in a disastrous firing.
- Type S thermocouples are also available with the DynaTrol (but not with the One-Touch).
- This option must normally be ordered with the kiln as the millivolt signals from a Type S thermocouple are different than the millivolt signals from a Type K thermocouple and the DynaTrol must be preprogrammed to recognize the different signals. Controls made after about 2004 can be retrofitted with Type S thermocouples.
- A Type S thermocouple will last considerably longer than a Type K thermocouple. However, it does cost considerably more money to replace it when it does fail or break.
- The dissimilar metals that make up a Type S are Platinum/Rhodium and Platinum. It is these precious metals that makes the type S thermocouple cost so much more.
- Over time the difference in the money spent using Type K vs. Type S is negligible. Type K will burn out faster than Type S but they are inexpensive to replace. Performance of one over the other is not an issue (except that the Type S will not drift as much when the kiln is fired to high temperatures).
Thermocouple extension Wire
- From the TC to the pyrometer there is insulated Type K thermocouple wire containing one very thin alumel wire, one very thin chromel wire and usually an uninsulated ground wire with aluminum foil wrapping the three wires together with the thicker plastic type insulation over that.
- The entire circuit needs to keep this same type of wire from the pyrometer to the thermocouple. There can be no sections containing other types of wire (like copper, or alumel where chromel should be).
- There is a polarity as well. The chromel side is generally referred to with the color yellow and a "+" positive designation. The alumel side is referred to with the color red and a "-" negative designation.
- If the polarity is reversed the pyrometer will read in reverse -temperatures will decrease when they should be increasing. Accidentally doing this will not damage anything if the problem is fixed promptly.
- Note that we use a special high temperature TC extension wire inside the Easy-Fire control cabinet.
Cones measure heat-work
- Cones are not temperature measuring devices.
- They measure how much heat has been absorbed by the ware in the kiln, which is the result of the combination of time and temperature.
- A particular piece of clay needs a certain amount of time at a specific temperature to properly fire it, lower temperature if the time is longer, higher temperature if the time is shorter.
- An example of this would be if you added about a 20 minute hold to the maximum temperature of a cone 6 firing, you would be able to lower that final temperature by about 20°F. An hour hold time would mean a final temperature of about 40°F lower. A two hour hold time, about 60°F lower.