Parr Instrument 6200 - Manual (Page 27)

Operating Instructions
6200 5
w w w . p a r r i n s t . c o m 25
Figure 5-7
Combustion Capsule with Adhesive Tape Seal
Use the following procedure when filling and han-
dling any of these tape-sealed sample holders:
• Weigh the empty cup or capsule; then cover the
top with tape, trim with a knife and press the
trimmed edge firmly against the metal rim. Also
cut and attach a small flag to the disc (see Figure
5-7).
• Puncture the tape at a point below the flag, then
re-weigh the empty cup with its tape cover.
• Add the sample with a hypodermic syringe;
close the opening with the flag and re-weigh the
filled cup.
• Set the cup in the capsule holder and arrange
the auxiliary fuse so that it touches the center of
the tape disc.
• Just before starting the test, prick the disc with
a sharp needle to make a small opening which
is needed to prevent collapse of the disc when
pressure is applied.
• Fill the bomb with the usual oxygen charging
pressure.
• The calorimeter will fire the bomb and complete
the test in the usual manner.
Volatile samples are defined as one with an initial
boiling point below 180 °C per ASTM D-2.
Low volatile samples with a 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 combustion aid, such as ethylene glycol.
Poor Combustion
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.
• Loose or powdery condition of the sample which
will permit unburned particles to be ejected dur-
ing a violent combustion.
• The use of a sample containing coarse particles
which 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. Either condition can cause
spalling and the ejection of unburned fragments.
• Insufficient space between the combustion cup
and the bottom of the bomb. 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.
• Excessive moisture or non-combustible material
in the sample. If the moisture, ash and other non
combustible material in the sample amounts to
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.