Parr Instrument 1108 - Manual (Page 5)

1108 Oxygen Combustion Vessel
w w w . p a r r i n s t . c o m 5
Attaching the Fuse
Set the bomb head on an A38A support stand and
fasten a 10 cm length of fuse wire between the two
electrodes. Parr 45C10 nickel alloy wire is used for
most tests, with platinum wire offered as an alter-
nate for certain special procedures. The 45C10 wire
is furnished on cards from which uniform 10 cm
lengths can be cut without further measurement.
Quick-grip electrodes now installed in all new 1108
Oxygen Combustion Vessels eliminate most of the
threading and twisting formerly required when bind-
ing the wire to plain electrodes. To attach the fuse to
quick-grip electrodes, insert the ends of the wire into
the eyelet at the end of each stem and push the cap
downward to pinch the wire into place. No further
threading or twisting is required. The procedure for
binding the fuse to the 4A and 5A plain electrodes
in older Parr vessels is illustrated in the instruc-
tion manual furnished with the original equipment.
For convenience, it is recommended that the user
purchase and install new 4A10 and 5A10 quick-grip
electrodes as replacements for the 4A and 5A styles
in older equipment.
Place the fuel capsule with its weighed sample in
the electrode loop and bend the wire downward to-
ward the surface of the charge as shown above. It is
not necessary to submerge the wire in a powdered
sample. In fact, better combustions will usually be
obtained if the loop of the fuse is set slightly above
the surface. When using pelleted samples, bend the
wire so that the loop bears against the top of the
pellet firmly enough to keep it from sliding against
the side of the capsule. It is also good practice to
tilt the capsule slightly to one side so that the flame
emerging from it will not impinge directly on the tip
of the straight electrode.
Liquids in the Bomb
Most combustion procedures call for a small amount
of liquid to be placed in the bottom of the bomb as
a sequestering agent and absorbent. If the amount
and type of liquid are not otherwise specified, add
1.0 mL of distilled or deionized water from a pipet.
Closing the Bomb
Care must be taken not to disturb the sample when
moving the bomb head from the support stand to
the bomb cylinder. Check the sealing ring to be sure
that it is in good condition and moisten it with a bit
of water so that it will slide freely into the cylinder;
then slide the head into the cylinder and push it
down as far as it will go. For easy insertion, push the
head straight down without twisting and leave the
gas release valve open during this operation. When
working with older bombs which have a removable
compression ring, be sure that the 104A2 ring is in
place above the gasket before attaching the screw
cap. Current model A416A3 Head Assemblies do not
require a separate compression ring. Set the screw
cap on the cylinder and turn it down firmly by hand
to a solid stop. When properly closed, no threads
on the cylinder should be exposed. If the screw cap
tends to bind to the cylinder at this point, indicating
that it might be difficult to open the bomb after it
has been fired, turn the screw cap back slightly – but
only a few degrees – enough to release the binding,
since the bottom thread must remain fully engaged.
It is not necessary to use a wrench or spanner on
the screw cap. Hand tightening should be sufficient
to secure a tight seal.
45C10 Fuse Wire
Set the bomb head on the A38A
support stand when attaching the
fuse and arranging the sample.
To attach the fuse: raise the cap,
insert the wire through the eye-
let, then pull the cap downward
to complete the assembly.
A38A Head Support & Stand Fuse Wire Fastened Between Two Electrodes