Parr Instrument 1108R - Manual (Page 5)

w w w . p a r r i n s t . c o m 5
1108R Oxygen Combustion Vessel
Liquids in the Bomb
Most bomb combustion procedures call for a small
amount of liquid to be placed in the bottom of the
bomb as a sequestering agent and absorbent. If the
amount and type of liquid are not otherwise speci-
fied, add 1.0 mL of distilled or deionized water from
a pipet.
Closing the Bomb
Care must be taken not to disturb the sample when
moving the bomb head from the support stand to
the bomb cylinder. Check the sealing ring to be sure
that it is in good condition and moisten it with a bit
of water so that it will slide freely into the cylinder;
then slide the head into the cylinder and push it
down as far as it will go. For easy insertion, push
the head straight down without twisting and leave
the gas release valve open during this operation.
Make sure that the contact ring is positioned flat
on top of the sealing ring. Set the screw cap on the
cylinder and turn it down firmly by hand to a solid
stop. When properly closed, no threads on the
cylinder should be exposed. If the screw cap tends
to bind to the cylinder at this point, indicating that it
might be difficult to open the bomb after it has been
fired, turn the screw cap back slightly – but only a
few degrees – enough to release the binding, since
the bottom thread must remain fully engaged. It
is not necessary to use a wrench or spanner on the
screw cap. Hand tightening should be sufficient to
secure a tight seal.
Allowable Sample Size
To stay within safe limits, the bomb should never be
charged with a sample which will release more than
8000 calories when burned in oxygen, and the initial
oxygen pressure should never exceed 40 atmo-
spheres (590 psig). This generally limits the mass of
the combustible charge (sample plus benzoic acid,
gelatin, firing oil or any combustion aid) to not more
than 1.1 grams. When starting tests with new or
unfamiliar materials it is always best to use samples
of less than 0.7 of a gram, with the possibility of
increasing the amount if preliminary tests indicate
no abnormal behavior. To avoid damage to the
bomb and possible injury to the operator, it should
be a standing rule in each laboratory that the bomb
must never be charged with more than 1.5 grams of
combustible material.
Attaching the Cotton Thread
A cotton thread (845DD2) is used as an auxiliary
fuse to ignite the sample. Ten centimeters of thread
is recommended for this auxiliary thread which is
looped over the heating wire, doubled on itself,
twisted to form a single strand and fed into the
sample cup to lay on the sample.
When contact is made through the heating wire,
the thread will ignite, drop into the sample cup and
ignite the sample.
A38A Head Support Stand
Set the bomb head on the A38A support stand when
attaching the fuse and arranging the sample.
845DD2 Cotton Ignition Thread
To attach the fuse either twist or tie the cotton thread
to the A40DD2 semi-permanent fuse wire.