M970-07/04
PAGE 11
7. Theory of Operation
Furnace Control
As illustrated in Appendix 1 the 970-1 and 970-2 horizontal tube furnaces are divided into
3 heated zones, each with its own temperature controller and thermocouple for measuring
and control. The centre zone operates independently as the “master” control to maintain
the furnace centre at its desired set point control value. The two end zones are operated
in a “master – slave” configuration with the temperature input to the end zone controllers
being the difference between the centre zone TC and the end zone TC. As the centre
zone temperature rises the differential input voltage / temperature raises the temperature
of the end zone. The end zones automatically track the centre zone temperature in order
to maintain a constant temperature and horizontal profile.
The power to the outer end zones is automatically adjusted to compensate for heat loss
changes at the ends of the furnace. In order to compensate for variable heat losses
occurring at the furnace ends over its wide operating temperature range and with the
variable “load” conditions (small / large thermometer diameters and the quantity and type
of probes inserted for calibration) the differential “Set-Point” input controls on the end zone
controllers are used to increase or decrease an “offset” control voltage. This will raise or
lower the end zone temperature to obtain a “flat” uniform temperature over the furnace
length.
Equalization Blocks
To provide a constant temperature zone for probes being calibrated with improved
uniformity and short term stability, metal equalization blocks can be fitted to the furnace
tube with suitable close fitting holes (0.015 / 0.02 ins. – 0.4 / 0.5 mm. clearance ). Refer to
examples in Appendix 3 and 4 for the 970-2 furnace. This will also improve immersion
errors as detailed in Appendix 8 – thermometry uncertainty report TU 2.
Calibration Capacity
The advantage of this type of furnace is that probes can be loaded from each end of the
furnace into holes drilled into each end of any equalization block. This doubles the probe
calibration capacity for each calibration program of multiple temperature points. Refer to
the furnace operation and calibration instructions and calibration performance optimization
Sections 13 and 14 for detailed use and operating positions of the equalization block.
Electrical
As illustrated in the electrical diagram under Appendix 2, the supply input is connected to
an independent circuit breaker (combined switch and overload current trip) This provides
both over-current and short circuit protection with complete electrical isolation of the
furnace and electrical controls / instrumentation.
