Parr Instrument 6200 - Manual (Page 30)

31
6200
Calorimeter Operating Instruction Manual
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 in 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.
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 obtained. 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 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
compensate for the added charge.
Also, when benzoic acid is combusted
for standardization 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.
The 6200 Calorimeter has been
designed to operate with an oxygen
filling pressure of 30 atm. Significant
changes from this value are not
recommended.
Samples
and Sample
Holders
Combustion
Aids
Oxygen
Charging
Pressure
5-5
5