Parr Instrument 6400 - Manual (Page 24)

3
Operation
P a r r I n s t r u m e n t C o m p a n y 22
Combustion Capsules
Non-volatile samples to be tested in Parr oxygen
bombs are weighed and burned in shallow capsules
measuring approximately 1” diameter and 7/16”
deep. These are available in stainless steel, fused
silica, fused quartz, and platinum alloyed with
3-1/2% rhodium.
Stainless steel capsules (43AS) are furnished with
each calorimeter. The stainless steel capsules will
acquire a dull gray finish after repeated use in an
oxygen bomb due to the formation of a hard, protec-
tive oxide film. This dull finish not only protects the
capsule, but it also promotes combustion and makes
it easier to burn the last traces of the sample. New
capsules are heated in a muffle furnace at 500 ºC for
24 hours to develop this protective coating uniform-
ly on all surfaces. This treatment should be repeated
after a capsule has been polished with an abrasive to
remove any ash or other surface deposits. Heating in
a muffle is also a good way to destroy any traces of
carbon or combustible matter which might remain
in the capsule from a previous test. Capsules should
be monitored for wear. Do not use the capsule if the
wall or base thickness is less than 0.025”.
Note: After heating, place the capsules in
a clean container and handle them only
with forceps when they are removed to be
weighed on an analytical balance.
When combusting samples that contain metal
particles such as aluminum or magnesium, the non-
metallic 43A3 Fused Silica or 43A3KQ Fused Quartz
Capsule is required.
When superior corrosion resistance is needed, the
43A5 Platinum Rhodium Capsule or 43A3KQ Fused
Quartz is required.
Test Process
Loading the sample
Prepare and weigh the sample to 0.0001g. Gently
tap capsules that contain powdered samples to
compact the material. (Pellets are easier to handle
than loose samples and they burn slower in the
bomb, thereby reducing the chances for incomplete
combustion).
Carefully place the
capsule into the capsule
holder. A cotton thread
(845DD2) is used as an
auxiliary fuse to ignite
the sample. Remove any
moisture from the heat-
ing wire prior to attach-
ing the cotton thread.
Four inches (10 cm) of
thread is recommended
for this auxiliary thread
which is looped over the
heating wire, doubled
on itself, twisted to form
a single strand and fed
into the sample cup to
lay on the sample. When
contact is made through
the heating wire, the
thread will ignite, drop into the sample cup and
ignite the sample. One spool of thread, part number
845DD, is 563 yards. Part number 845DD2 contains
approximately 1000 pieces of thread pre-cut to 4
inches (10 cm).
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 in the calorimeter. 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.
Notice that the bomb head grounding lug extends
beyond the outside diameter of the bomb head. A
slot for this lug is cut into the top of the calorimeter
bucket which holds the bomb cylinder. Position
this lug approximately 20 degrees to the operators
right and slide the head into the cylinder and push it
down as far as it will go. Now rotate the bomb head
20 degrees to the left until the lug contacts the left
edge of the cut out and is pointed to the front of the
calorimeter.
Figure 3-2
Cotton Thread Assembly