MF12 3.12
7
3.5
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.
3.6
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.
3.7
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.
Note that any power limit applies to all three controllers.
Some models covered by this manual are are fitted with 208V elements and are designed for use
over the range of voltages 200V-250; 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 check whether the furnace is a 208V model:
-
look at the
OP.Hi
parameter – make a note of the value and compare with section 8.2;
-
observe the rating label; if a range of voltages is stated, it is not a 208V multivoltage model;
-
if in doubt, check with Carbolite – each product has its own record card.
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.
3.8
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.
