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To achieve the best properties in a ceramic product, it is necessary to
ensure a controlled and uniform application of heat-work to the product
being fired. Even in sophisticated kilns with microprocessor controls, all
of the thermocouples give only 'spot' temperature readings, and cannot monitor
the effects of heat-work and other variations on the firing process. To
measure heat-work, it is necessary to make use of pyrometric devices, which
will react to the effects of time as well as temperature, in a manner similar
to that of the ceramic product being fired.
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 For
nearly 100 years, ceramic manufacturers throughout the world have used Bullers
Rings and Bullers Bars to measure the heat-work distribution in their kilns,
and thus develop optimum firing conditions for their products. Now Taylor,
Tunnicliff LTD has introduced a new range of pyrometric devices for use
at higher temperatures called Bullers Discs. Like Bullers Rings, this latest
range of devices falls into the 'shrinkage' device category, where the contraction
of the disc reflects accurately the effect of time and temperature on the
product being fired.
This contraction is measured with a special gauge, and the heat-work expressed
as a number read directly from the gauge. Skill in using a micrometer is
not required, and there is no time wasted in using tables to convert measurements
into inaccurate temperatures.
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Bullers Discs fall into two groups which cover temperatures
from 13500C to 15500C (Type HT), and from 15000C to 17500C (Type VHT). At
20 mm in diameter and 7.5 mm thick, these discs can be placed on the smallest
of kilns firing technical ceramics, or spread over the settings of large
kilns used to fire refractories. They can be used to 'map' the thermal performance
of the firing as well as being a day to day monitor of the firing consistency
and therefore product quality.
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Like all other Taylor, Tunnicliff products, Bullers
Discs are manufactured under an ISO 9002 quality management system. All
production is frequently sampled and tested before being released, to
ensure that it conforms to our exacting standards.
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Bullers discs are suitable for use on all types
of kilns firing within the temperature ranges quoted. They should be placed
across the kiln setting in such a way as to obtain maximum data on the kiln
performance. They should be placed flat, and not in contact with the product
being fired. On single layer kilns, it is likely that discs will be needed
at each side of the setting and possibly at the middle. Multi-layer kilns
will require discs in several positions across the kiln to determine any
variations. Once a kiln has been mapped, experience with the kiln will determine
how many discs should be placed during routine firing in order to monitor
the quality of the firing.
After firing, the contraction of the disc is measured using the special
dial gauge. Readings to the nearest quarter of a division should be taken
across three diameters and a mean value obtained. Gauge readings should
not be converted to temperature in order to describe firing conditions as
other factors such as time, kiln loading, atmosphere etc can affect firing
contraction. The temperatures shown in the table are for guidance only,
and have been determined at a single set of firing conditions.
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