Kilns and Kiln Firing
Technical Data Part II

emperature Measurement and Control

Pyrometers, controllers and programmers all are invaluable tools for potters, providing an indication of the actual heat in the kiln and performing a range of other functions. With skill, these are used to gauge efficiencies, firing rates and atmosphere control.

• Moving coil type meters
  These are generally available in 2 ranges suitable for potters, 0-1400° C and 0-1200° C. The former is the type "R" unit and must use the appropriate probe and cable and the latter type "K". The disadvantage of these meters is the inherent inaccuracy due to parallax error and the fragility of the mechanism.
• Electronic digital meters
  These units have gradually replaced the moving coil type as the cost of manufacture has rapidly decreased. Although not more accurate, it is possible to judge a small increase in temperature precisely. This is invaluable when changes are made to the atmosphere or fuel flow as a neutral atmosphere is the most efficient and will indicate the most rapid temperature rise. Conversely a reducing atmosphere will slow down the rise and may even cause a drop.
• Temperature controllers
  Temperature controllers are mostly digital but analogue styles can still be purchased. These will control the kiln at a preset temperature either soaking or to shut down the load. These can be wired to soak and shut down with a timer device but generally it can be more economical and offer far more flexibility if a programmer is used. Controllers usually will indicate the actual temperature and are useful as a safety device. Controllers must be matched to the appropriate probe type although newer digital units have the ability to be set for a variety of probes and outputs by the operator.
• Temperature programmers
  Temperature programmers are available in a multitude of configurations but generally with five or more stages. Each stage usually has a pre-settable ramp rate or soak time allowing accurate automatic control of the entire firing process. Programming can be quite involved on some of the more complicated units but generally is as simple as setting the temperature to reach and, in the case of the up or down ramp, the rate in degrees per hour. The soak stages usually have the temperature automatically preset and require a time to be entered. It is possible to create a simpler single ramp unit using a temperature controller, an energy regulator and a timer.
  Controllers or programmers are simply wired into the existing contactor or control device on electric kilns but require additional solenoids if connected to gas kilns. Most industrial kilns are controlled with programmers and in conjunction with the electronic safety units enable the kiln to be automatically started, fired up and down and shut off. A more recent innovation is the atmosphere controller. This permits the amount of reduction or oxidation to be accurately set and maintained. The burners are usually special industrial types that have a separate air supply. The cost is a little prohibitive unless the turnover of work warrants the extra degree of automation.
• Thermocouples And Compensating Wire
  A thermocouple is simply the joining of two dissimilar metals that produce a small voltage when heated. The temperature meters display this voltage on an indicator that has been designed to show this as a temperature. Most dissimilar wires will exhibit an output when joined and heated but may not be at all linear (or manageable). The three main types of thermocouples (or probes) available to potters were chosen after intensive research and exhibited the desired characteristics when heated at certain temperatures.
  The probes are joined to the indicators with compensating or extension cable. Ideally this connection should be made completely with the same thermocouple wire as the probe to minimize losses but this would be far too expensive. The compensating lead is made from cable that has similar attributes to the probe but is formed from cheaper wire. It is important to make sure you connect the thermocouple with compensating cable (not ordinary copper cable) and also use the correct type for the probe. Unfortunately the Americans, French, British, Japanese and Germans all have different color codes for the compensating cables. It is critical that the positive (plus) conductor and the negative (minus) are connected to the correct terminals. Do not expect that a red will always be a positive conductor.
  

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