Parr Instrument 6400 - Manual (Page 22)

3
Operation
P a r r I n s t r u m e n t C o m p a n y 20
combustions. If moisture is to be added to retard
the combustion rate, drop the water directly onto
the loose sample or onto a pellet after the sample
has been weighed. Then let the sample stand to
obtain uniform distribution. Low volatile samples
with high water content, such as urine or blood,
can be burned in an open capsule by absorbing the
liquid on filter paper pulp or by adding a combus-
tion aid, such as ethylene glycol.
Sample Types
Because of the difference in combustion char-
acteristics of the many different materials which
may be burned in an oxygen bomb, it is difficult to
give specific directions which will assure complete
combustions for all samples.
The following fundamental conditions should be
considered when burning samples:
• Some part of the sample must be heated to its
ignition temperature to start the combustion
and, in burning, it must liberate sufficient heat
to support its own combustion regardless of the
chilling effect of the adjacent metal parts.
• The combustion must produce sufficient tur-
bulence within the bomb to bring oxygen into
the fuel cup for burning the last traces of the
sample.
• A loose or powdery condition of the sample
which will permit unburned particles to be
ejected during a violent combustion.
• The use of a sample which contains coarse par-
ticles will not burn readily. Coal particles which
are too large to pass a 60 mesh screen may not
burn completely.
• The use of a sample pellet which has been made
too hard or too soft can cause spalling and the
ejection of unburned fragments.
• The bottom of the cup should always be at least
one-half inch above the bottom of the bomb or
above the liquid level in the bomb to prevent
thermal quenching.
• If the moisture, ash and other non combustible
material in the sample totals approximately 20%
or more of the charge, it may be difficult to obtain
complete combustion. This condition can be rem-
edied by adding a small amount of benzoic acid or
other combustion aid.
Foodstuffs and Cellulosic Materials
Fibrous and fluffy materials generally require one of
three modes for controlling the burn rate. Fibrous
materials do not pelletize readily and generally
require either moisture content or a combustion aid
such as mineral oil to retard the burn rate and avoid
development of high pressures. Partial drying may
be necessary if the moisture content is too high to
obtain ignition, but if the sample is heat sensitive
and cannot be dried, a water soluble combustion aid
such as ethylene glycol can be added to promote
ignition. Material such as Napthalene should not be
burned in loose powder form but should be formed
into a pellet.
Coarse Samples
In most cases it may be necessary to burn coarse
samples without size reduction since grinding or
drying may introduce unwanted changes. There
is no objection to this if the coarse sample will
ignite and burn completely. Whole wheat grains
and coarse charcoal chunks are typical of materials
which will burn satisfactorily without grinding and
without additives or a special procedure.
Corrosive Samples
The 1138 bomb is made from alloy 20; a special
niobium stabilized stainless steel selected for its
resistance to the mixed nitric and sulfuric acids pro-
duced during the combustion process. The 1138CL
is made from the halogen resistant Hastelloy G30™.
Hastelloy 30™ is an alloy rich in cobalt and molyb-
denum and is able to resist the corrosive effects
of free chlorine and halogen acids produced when
burning samples with significant chlorine content.
While no alloy will completely resist the corrosive
atmospheres produced when burning samples
containing halogen compounds; users who intend
to test these materials are urged to select the 1138CL
Bomb. These bombs are 250 mL in volume and are
rated to a maximum working pressure of 2000 psi.
The bombs are hydrostatically tested to 3000 psi
and the sample range is ~1g or 5000 – 8000 calories.
Explosives and High Energy Fuels
Materials which release large volumes of gas
which detonate with explosive force or burn with
unusually high energy levels, should not be tested
in this calorimeter. Rather, they should be tested
in a model 6100 or 6200 Calorimeter which can be
equipped with an 1104 High Strength Oxygen Bomb
designed specifically for these types of samples.