Parr Instrument 1108 - Manual (Page 10)

1108 Oxygen Combustion Vessel
P a r r I n s t r u m e n t C o m p a n y 10
calorimetry, and for the sulfur content of the gelatin
(approximately 0.35%) if used for sulfur determina-
tions. Blank tests must be run to determine the exact
amounts. Gelatin capsules should always be stored
in sealed bottles and handled with due regard for
their hygroscopic nature.
The blank tests should be repeated at frequent inter-
vals since values determined on a weight basis will
change if there are variations in the moisture con-
tent of the gelatin.
Tape-Sealed Sample Holders
Volatile samples can be handled in a standard 43AS
combustion capsule with a flat top rim, or in a 43A6
platinum capsule with a spun rim by covering the top
of the capsule with a disc of adhesive plastic tape.
To seal a capsule; stretch a piece of tape across the
top and press it firmly against the rim with a flat
blade, then trim the excess with a sharp knife. The
seal obtained in this manner will be adequate to re-
tain most volatile samples. The tape used for this pur-
pose should be free of chlorine and as low in sulfur
as possible. Borden “Mystic Tape” No. M-169-C, or
3M Transparent Tape No. 610 are recommended for
this purpose. Equivalent tape can be obtained from
Parr under Part No. 517A. The weight of the tape disc
must be determined separately and a correction ap-
plied for any elements in the tape which might in-
terfere with the determination. This can be done by
running a blank test with the tape alone using a sam-
ple weighing about 1.0 gram. Tape should always be
stored in a sealed container to minimize changes in
its moisture content.
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 at-
tach a small flag to the disc; as illustrated at the top
of page 9. Puncture the tape at a point below the
flag, then reweigh the empty cup with its tape cover.
Add the sample with a hypodermic syringe; close the
opening with the flag and reweigh the filled cup. Set
the cup in the loop electrode and arrange the fuse
wire so that it touches the center of the tape disc.
Just before closing the bomb, prick the disc with
a sharp needle to make a small opening which is
needed to prevent collapse of the disc when pres-
sure is applied. Fill the bomb with oxygen to the
usual charging pressure, but add oxygen slowly so
that the tape will not collapse into the cup. Fire the
bomb and complete the test in the usual manner.
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 or by freeze drying the sample.
Heavy Oils
Oils and other liquids which are not volatile at room
temperature can be weighed directly into open
combustion capsules. The loop of the fuse should
be positioned just slightly above the surface of the
sample. Some operators place one end of a short
piece of fine cotton thread over the fuse loop, with
the other end touching the liquid. In any case, the
fuse wire itself should not be submerged in the
liquid.
Several precautions must be observed when testing
heavy oils because of the intense heat which they
develop. If the wall of the metal combustion capsule
is thin, or if some of the sample happens to have
been spread on the thin rim of the capsule, it is pos-
sible that the metal may become heated to the point
where it will ignite. This condition is serious because
of the excessive heat liberated when metal burns in
oxygen. Also, the molten metal oxides may damage
the interior of the bomb. In extreme cases the bomb
electrodes may also ignite and burn with similar
results. For these reasons, be sure that any capsule
holding a heavy oil is in good condition and not
worn thin from prior usage. Also, bend the straight
electrode so that it does not project over the cup
where it will receive the full flame from the sample.
It is always desirable to tilt the capsule slightly in the
loop holder so as to direct the flame away from both
electrodes. Some operators prefer to use a 10 to 13
mL platinum crucible for holding heavy oil samples
because the added depth in a crucible promotes
slower combustion and a milder flame.
Explosives and High Energy Fuels
Special precautions must be observed when testing
materials which release large volumes of gas upon
ignition, or which detonate with explosive force. Al-
though most slow-burning gun powders and rocket
propellants can be tested in the conventional 1108
Oxygen Combustion Vessel, the user must under-
stand that this bomb is not designed to withstand
the shock pressures produced by primers and high
explosives. It is much safer to test these materials in
a Parr 1104 High Pressure Oxygen Vessel.