Parr Instrument 6200 - Manual (Page 25)

Operating Instructions
6200 5
w w w . p a r r i n s t . c o m 23
Samples and Sample Holders
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 in-
fluences the reaction rate. Compression into
a pellet is recommended because the pres-
sure developed during combustion can be
reduced as much as 40% when compared to
the combustion of the material in the powder
form. In addition in giving controlled burn
rates, the pelletizing of samples keeps the
sample in the fuel capsule during combus-
tion.
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 con-
tent 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.
Combustion Aids
Some samples may be difficult to ignite or they may
burn so slowly that the particles become chilled
below the ignition point before complete combus-
tion is obtained. In such cases benzoic acid, white oil
or any other combustible material of known purity
can be mixed with the sample. Ethylene glycol,
butyl alcohol 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 compen-
sate for the added charge.
Also, when benzoic acid is combusted for stan-
dardization 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.
Oxygen Charging Pressure
The 6200 Calorimeter has been designed to operate
with an oxygen filling pressure of 30 atm. Signifi-
cant changes from this value are not recommended.
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. When combusting samples that
contain metal particles such as aluminum or mag-
nesium, the non-metallic fused silica 43A3 capsule
or fused quartz 43A3KQ capsule is required. When
superior corrosion resistance is needed, the plati-
num rhodium 43A5 capsule 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
500 ºC for 24 hours by Parr to develop this protec-
tive coating uniformly on all surfaces. This treatment
should be repeated after a capsule has been pol-
ished 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.