|Monthly Tech-Tip |
A simple probe made from two kinds of wire (i.e. platinum, rhodium) welded together. This probe is wired to a sensitive electronic meter that displays a reading of the voltage it generates when heated.
However, the world of high-temperature measurement and thermocouples is a complex one. There are many kinds of thermocouples. Some generate a nice smooth voltage increase that bears a direct relationship to temperature increase, others require complex software to make the translation. Wire manufacturers produce detailed charts that show the mV output at each temperature, some technicians weld together their own thermocouples with confidence the output will match the charts. There are different manufacturing processes, calibration techniques, response to different atmospheres, abilities to measure different temperature ranges, different types decay in their accuracy in different ways, variations in the frequency of need for recalibration, etc.
Maintaining accurate pyrometers can be expensive and typical inexpensive type K devices used in potters kilns are not accurate at higher temperatures (most potters won't pay for the platinum/ 10% platinum-rhodium (type S) thermocouples and control systems that really should be used, and the more expensive plated switches and contacts). However, the type K are more resistant to oxidation than types E, J, and T at temperatures over 500C.
Non-contact infrared temperature measurement guns are now available on Amazon and within price-reach of anyone. Some advertise the ability to read up to 1800C. Of course, the quality needs to be assessed. For example, if they claim +/-2% accuracy on readings for 320 ~ 2120F, does that mean they can be out by 42F at 2120? That is more than a cone! Search terms like "digital pyrometer", "IR pyrometer", "infrared pyrometer".
Notice the title says "Emissivity Adjustable". That is important because for accurate measurement, the emissivity of the object you point the gun at has to be known accurately. In industry consistent results with this type of instrument depend on always aiming the gun, at right angles, at a specific target in the kiln (e.g. a firebrick), whose emissivity is known exactly for the target temperature (from the technical data from the manufacturer) and which has a surface of consistent roughness that remains stable firing-after-firing. If you are a potter you do not have calibration instruments and emissivity data. And you are probably not inclined to carefully set a piece of kiln shelf, for every firing, vertical at right angles to the peephole and blocking view of the cones! Even if you do, remember the gun calibrates to one temperature. And getting it to work is a matter of waving this plastic fun in front of an open peephole and hoping for the best before it melts! Perhaps it is better to spend this money on thermocouples instead.
The old one inside is in bad condition (a new one is sitting on top ready to install). In 2022 these cost about $35 CDN. The temperature-measuring part of a thermocouple is the join of two dissimilar metal wires, these are 8 gauge. The junction produces a temperature-dependent voltage that a pyrometer or controller can convert to a reading. Thermocouples can degrade into pretty poor condition yet still work, notice the one in this kiln is separating in two. Thermocouples generally need replacement more often than elements, they generally last about 150 firings (cone 04-06) and 50 firings (cone 6). Replacing these does not require electrical expertise.
Cones are ceramic and bend through a narrow temperature range. They used to be actively used to determine when firings were completed but now are used to calibrate electronic devices.
|By Tony Hansen|
Follow me on