Carbolite GVC Series - Manual (Page 7)

G range
MF48 -1.18
7
3.0 O PERATION
The instructions for operating the temperature controller are given in a separate manual. If the
furnace is fitted with a time switch, see also the supplementary manual MS0. If cascade control is
fitted, see the supplementary manual MS07.
3.1 Operating Cycle
The furnace is fitted with an instrument switch. The switch cuts off power to the control circuit.
There is also a Heater switch that can be used to disconnect power to the elements.
Connect the furnace to the electrical supply.
Operate the instrument switch to activate the temperature controller. The controller becomes
illuminated and goes through a short test cycle.
Single zone models, GHA and GVA: Set the temperature controller to the desired setpoint or
program - see the controller manual.
3-zone models, GHC and GVC, with control methods A & B: Set the slave (end zone) controllers
to the desired offset temperature, usually zero. Set the main (central) temperature controller to the
desired setpoint or program. See section 3.2 and/or the separate manual(s) for the controller(s).
3-zone models, GHC and GVC, with control method C: Set the three controllers to the desired
temperatures. See the separate manual(s) for the controller(s).
Overtemperature option only. If the overtemperature controller has not yet been set as required, set
it and activate it according to the instructions in the controller manual.
Switch on the Heater switch, located on the instrument panel. Unless a time switch is fitted and is
off, the furnace starts to heat up. The Heat light(s) glow steadily at first and then flash as the
furnace approaches the desired temperature or a program setpoint.
Overtemperature option only. If the overtemperature trip operates then an indicator in the over-
temperature controller flashes, and the heating elements are isolated. Find and correct the cause
before resetting the overtemperature controller according the instructions supplied.
To switch off power to the heating elements, use the Heater Switch. To switch the furnace off, use
both the Heater switch and the Instrument switch. If the furnace is to be left off unattended, isolate
it from the electrical supply.
3.2 3-zone Control Methods – models GHC and GVC
These models are typically designed to achieve an extended uniform temperature zone by the use
of three control zones. The control zones are typically linked so that the end controllers follow the
central controller in a master-slave approach, in one of two ways. Alternatively independent
control zones may be ordered. There are thus three control methods (A, B & C).
Zone lengths: The normal end-zone length for all the 3-zone models is 1 element (150mm). The
“equal zone” models (with suffix E after the model name) have longer end-zones of 2 elements
(300mm); only the models 12/450 and 12/900E have truly equal zone lengths. Any of the control
methods can apply to equal zone models, just as they can to the standard models.
Achieving temperature differences across the zones: a small temperature difference may be
achievable, but this is limited by the effect of radiated heat inside the furnace body, and in the
work tube, and depends also upon the furnace contents.
A. Back-to-Back Thermocouples
This is the most commonly supplied option.
The central zone of the work tube is controlled directly by the central temperature controller. Each
end zone thermocouple is wired in opposition to a central reference thermocouple, and the small
voltage resulting when the zones are at different temperatures is used by the end zone controller.
There are thus four thermocouples. The circuit diagram in section 7.6 (A) shows the thermocouple
arrangement.