TEMPERATURE 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 presettable 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 minimise 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, British, French, Germans and Japanese 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.

The following information is relevant for the most common types.

Type "R"

This is generally considered the better probe to use for the higher stoneware temperatures. This probe uses two wires with a mixture of platinum and platinum/rhodium wires that are usually very fine and fragile to keep the expense down. The voltage output from type "R" is very small and really only comes into its own at higher temperatures above about 700° C and up to 1600° C where it can be quite accurate. The correct compensating lead usually has a green outer PVC cover and either white and blue or red and black coloured inner cables. The white and black are the plus or positive sides and the blue and the red are the minus or negative cables (although the German and Japanese are the opposite).

Type "K"

This is a much cheaper probe and uses much less expensive wire. The wires are chromyl and alumel, indicating the base metals, with the addition of other metals to display the required output. The type "K" is the workhorse probe and is used from low temperatures to temperatures of around 1200° C before there can be small inaccuracies. The wire can be heated to higher temperatures. The compensating PVC cable outer is generally yellow but can also be red. The inner cable colours are either yellow and red or brown and blue with the yellow and brown the positive conductors and the red and blue the negative.

Type "N"

This is a relatively new wire that was developed to fill the gap between type "R" and type "K" and has proven to be an excellent alternative. It was not possible to obtain controllers that were suitable for type "N" until recently as type "R" would generally be preferred. The compensating cable outer tends to be orange with the inner colours either orange and red or orange and blue with the orange always the positive (remarkable co-operation!).

Some important points to consider:

• The junction where the compensating cable and the thermocouple probe join must be kept at a temperature less than 50° C (you should be able to keep your hand there for a period). If excessive heat is transferred to this joint a second temperature junction may be formed and large inaccuracies may result.

• The moving coil type meter must be kept level, ambient temperature set on the dial and the length of the compensating cable must be kept to a minimum.

• Do not expect the pyrometer to indicate the actual heat work that has gone into the piece. Understand the benefits and the limitations. Use cones to indicate the correct temperature/work rate.

• If you suspect a problem, thoroughly check the wire joints for loose terminals, oxidation, excessive twists and general deterioration. Ensure the correct polarity is observed.

• Keep the probe away from the flame path especially as long, soft reducing flames are possible.

• Try to position the probe tip as close to the work as possible. It is also usual to try to position the probe in a location that will indicate a good average temperature.

TEMPERATURE MEASUREMENT COLOUR SCALE CHART

Colourings	Celsius	Fahrenheit
Lowest visible red	475° C	525° F
Lowest visible red to dark red	475-650° C	525-1202° F
Dark red to cherry red	650-750° C	1202-1382° F
Cherry red to bright cherry red	750-800° C	1382-1472° F
Bright cherry red to orange	800-900° C	1472-1652° F
Orange to yellow	900-1100° C	1652-2012° F
Yellow to light yellow	1100-1300° C	2012-2372° F
Light yellow to white	1300-1550° C	2372-2822° F
White to dazzling white	1550° C and higher	2822° F and higher

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