 2002 by ELTRA GmbH Germany – January 2002 – Service Manual OH 900 / ON 900 / ONH 2000
1.20-1
1.20 The upper electrode gets dark:
It is normal when the surface of the upper electrode insert that gets in contact with the
crucible, darkens. This dark surface builds a ring ( 1 ) of a few milimeters width around the
hole of the electrode. All the rest of the electrode surface keeps the original shiny surface
of the nickel coating, so that after cleaning from the graphite dust it looks like a new
electrode, except the a.m. ring that touches the crucible.
But if the complete surface of the electrode gets dark, this means that the whole electrode
has been overheated.
If the power settings are still the same as they used to be before when the electrodes
didn`t overheat, or if the settings are not higher than advised in the applications of the
operation manual, the reason of overheating must be a bad thermal ( and bad electrical )
contact to the upper furnace block, caused by:
Dark ring ( 1 )
a) not properly fixed screws ( 2 ),
b) grains or any other parts between
the electrode ( 3 ) and the upper
furnace block ( 4 ), or
c) the spring lock washers ( 5 ) are missing or
d) there is too much graphite powder ( 6 ) in the
thread of the electrode underneath the screws.
The graphite powder is used to fill the volume
underneath the lower end of the screws ( 2 )
Shiny
eliminating any air enclosures.
surface
12-0
Sample
drop
Electrode screws
fix properly (2)
Spring lock
washers (5)
Graphite
powder (6)
Make sure that
there are not
any grains or any
other material
between the
electrod and the
upper furnace block.
Upper electrode
insert (3)
Upper furnace
block (4)
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