and Kiln Firing
Data Part II
The following information is relevant for the most common types.
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 colored 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).
This is a much cheaper probe and uses much less expensive wire.
The wires are chromel 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 colors 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 colors either orange and red or orange and blue with the
orange always the positive (remarkable cooperation!).
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 and ambient temperature
- 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
- Thoroughly check the wire joints for loose terminals, oxidation,
excessive twists and general deterioration if you suspect a problem.
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.
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Measurement Chart > 3