Parr Instrument 6300 - Manual (Page 35)

O
PERATING
I
NSTRUCTIONS
6300
5
w w w . p a r r i n s t . c o m 35
S
AMPLES
Particle Size and Moisture Content
Solid samples burn best in an oxygen bomb when
reduced to 60 mesh, or smaller, and compressed into a
pellet with a 2811 Parr Pellet Press.
Large particles may not burn completely and small
particles are easily swept out of the capsule by turbulent
gases during rapid combustion.
Note:
Particle size is important because it influences the reaction
rate. Compression into a pellet is recommended because
the pressure developed during combustion can be reduced
as much as 40% when compared to the combustion of
the material in the powder form. In addition to giving
controlled burn rates, the pelletizing of samples keeps the
sample in the fuel capsule during combustion.
Materials, such as coal, burn well in the as-received
or air-dry condition, but do not burn completely dry
samples. A certain amount of moisture is desirable in
order to control the burning rate. Moisture content up
to 20% can be tolerated in many cases, but the optimum
moisture is best determined by trial combustions.
If moisture is to be added to retard the combustion rate,
drop water directly into a loose sample or onto a pellet
after the sample has been weighed. Then let the sample
stand for awhile to obtain uniform distribution.
C
OMBUSTION
A
IDS
Some samples may be difficult to ignite or they may
burn so slowly that the particles become chilled below
the ignition point before complete combustion is ob-
tained. In such cases powdered benzoic acid, white oil
or any other combustible material of known purity can
be mixed with the sample. Ethylene glycol, butyl alco-
hol or decalin may also be used for this purpose.
Note:
It must be remembered, however, that a combustion aid
adds to the total energy released in the bomb and the
amount of sample may have to be reduced to compensate
for the added charge.
Also, when benzoic acid is combusted for standardiza-
tion runs or for combustion aid purposes, it should be
in the form of a pellet to avoid possible damage to the
bomb which might result from rapid combustion of the
loose powder
O
XYGEN
C
HARGING
P
RESSURE
The 6300 Calorimeter has been designed to operate
with an oxygen filling pressure of 30 atm. Significant
changes from this value are not recommended.
C
OMBUSTION
C
APSULES
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 and platinum al-
loyed with 3-1/2% rhodium.
Stainless steel capsules (43AS) are furnished with each
calorimeter. When combusting samples that contain
metal particles such as aluminum or magnesium, the
non-metallic (fused silica) 43A3 Capsule or 43A3KQ
Fused Quartz is required. When superior corrosion
resistance is needed, the Platinum Rhodium 43A5 Cap-
sule or 43A3KQ Fused Quartz is required.
The stainless steel capsules will acquire a dull gray
finish after repeated use in an oxygen bomb due to the
formation of a hard, protective 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.
Capsules should be monitored for wear. Do not use the
capsule if the wall or base thickness is less than 0.025”.
New capsules are heated in a muffle furnace at 500ºC
for 24 hours to develop this protective coating uniformly
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.
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.