M970-07/04
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4. Product Description
The 970 furnace series provides a 3zone electrically heated, horizontal tube furnace as
illustrated in Appendix 1. The heated length is divided into 3 zones each with its own
temperature controller, power regulator to the heater winding and a thermocouple
temperature sensor. The power supplied to the end zones is automatically adjusted to
compensate for the heat loss occurring at either ends of the central tube, using a “Master-
Slave” configuration with the centre zone control in order to maintain all 3 zones at a
uniform temperature. This system provides a much longer uniform horizontal temperature
zone than other systems, e.g. single zone vertical or horizontal furnaces. Also adjustment
of the end zone controllers at some points over the wide furnace operating range (100 to
1200 deg.C.) enables the variable losses at the furnace ends to be corrected (extra power
to correct for insulation losses and different operating loads such as number and size of
sensors being calibrated). This provides a similar “flat” uniform zone at any set point
control temperature over 100 to 1200 deg.C. to within better than 0.2 deg.C over a 200mm
central furnace length using a suitable metal equalizing block in the central work tube.
The “radial” uniformity is obtained by the concentric even furnace winding around the
outer diameter of the main furnace tube. This enables uniform radial temperatures for a
series of holes on a common P.C.D. (pitch circle diameter) using a centrally placed
equalizing block in the centre tube.
This 970 furnace is provided in 2 main versions:-
(1) Model 970-2 with a centre main work tube of 3.15ins. (80mm.) internal diameter
When using the main 80mm. i.d. furnace tube a large (up to 3in. diameter) metal
equalizing block can be used allowing for a large number and variety of probe sizes to
be accommodated. This also allows probes with larger “head” diameters / shapes to
be fitted in addition to short probe length designs.
When very short probes are calibrated, the metal equalizing block can be moved
closer to one end of the furnace. With the use of the 80mm. main centre work tube
only, the insulated separately spaced outer furnace end plates can be removed, to
allow larger and shorter access to the inside work tube and larger equalizing block to
3ins. Diameter.
(2) Model 970-1. This uses an additional centre work tube within the main tube. This
tube is supported by the insulated separate outer end furnace plates shown. This
provides improved horizontal heat uniformity along the furnace length. Also complete
electrical isolation is provided at higher temperatures (600 deg.C. and above) for the
metal equalizing block and probes tested from the heater windings / outer main
furnace tubes. This prevents small spurious voltage input when using very sensitive
digital voltmeters for more precise primary standard thermocouple calibrations by
comparison calibration techniques.
