Parr Instrument 6400 - Manual (Page 23)

6400
Calorimeter Instruction Manual
4-4
Sample Types
Because of the difference in combustion characteristics 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 turbulence 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 particles 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 remedied 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 produced during the
combustion process. The 1138CL is made from the halogen resistant Hastelloy G30™.
Hastelloy 30™ is an alloy rich in cobalt and molybdenum 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