Carbolite VST Series - Manual (Page 6)

HST, VST
6
MF13 – 3.40
3.4 Tube Life
A ceramic work tube may be cracked if workpieces are inserted
too quickly or at temperatures below 900°C (when the tube is
more brittle). Large pieces should also be heated slowly to
ensure that large temperature differences do not arise.
Poor thermal contact should be encouraged between the
workpiece and the tube; crucibles or boats should be of low
thermal mass and should have feet to reduce the contact with
the tube (fig. 4).
Do not set too high a heating rate. Large diameter tubes are more susceptible to thermal shock than
smaller. Tubes which extend beyond the heated part of the furnace are more at risk. A general rule
for maximum heating rate is 400/internal diameter (°C/min); for 75mm i/d tubes this comes to 5°C
per minute. The controller can be set to limit the heating rate.
3.5 Pressure
Work tubes are not able to accept high internal pressure. When gas seals or similar fittings are in
use, the gas pressure should be restricted to a maximum of 0.2 bar (3 psi). A pressure of about half
of that should normally be sufficient to achieve the desired flow rate. The customer must ensure
that the exhaust path from the tube is not blocked, so that excess pressure does not occur.
3.6 Power Adjustment
The furnace control system incorporates electronic power limiting. The power limit parameter
OP.Hi is accessible to the operator, and can be used to adjust the furnace to the actual supply
voltage.
The models covered by this manual (March 2001 onwards) are designed for use over the range of
voltages 200V-250V (or, if ordered, 100V-125V), and the power limit parameter is set
accordingly. These models may be relocated to a different voltage within the range: the power
limit should be reset to match the voltage. See section 8.2.
To alter the power limit
Set the Heater Switch to Off to prevent heating while adjusting the power limit.
Press Page  until oP (output list) is displayed. Press Scroll  until OP.Hi (Output High) is
displayed. Press Down  or Up  once to display the value of OP.Hi (it is good practice to write
down the original value). To alter the parameter to the desired value use Down  or Up .
A power setting of zero prevents the furnace from heating: useful for demonstrating the controls
without taking power.
Do not increase the power limit simply “to get more power”. The elements could burn out, or a
fuse could blow.
3.7 Running at Low Temperatures
The power limit may be adjusted to a low level to achieve better control when running the furnace
at a low temperature. No hard and fast rules can be given, but, as an example, to run at
temperatures only up to 600°C try a power limit of 50%. Control stability may fall off again if a
setting below about 40% is used.
Before changing the power limit, record its factory setting for possible future use.