Parr Instrument 6200 - Manual (Page 61)

62
6200
Calorimeter Operating Instruction Manual
Thermochemical
Corrections Nitric Acid Correction.
In the high pressure oxygen
environment within the oxygen
bomb, nitrogen that was present as part of the
air trapped in the bomb is burned to nitric
oxide which combines with water vapor to
form nitric acid. All of this heat is artificial
since it is not a result of the sample burning.
The nitric acid correction removes this excess
heat from the calculation.
Sulfur Correction.
In the oxygen rich atmosphere within the
bomb, sulfur in the sample is oxidized to
sulfur trioxide which combines with water
vapor to form sulfuric acid. This liberates
additional heat over the normal combustion
process which converts sulfur to sulfur
dioxide. The sulfur correction removes this
excess heat from the calculation.
Fuse Correction.
The fuse correction applied by the
calorimeter is calculated as:
e
3
= (fuse value)(fuse multiplier
from
calculation factors page)
= (entered value)(fuse multiplier
from thermochemicals page)
“Fuse Value” is the number entered by
the user and the value which appears in the
test report.
Note:
Calculation Factors, - Fuse Multiplier is
normally set to 1.0 so the entered value
is in calories
Users may find it convenient to enter a
fixed value for the fuse correction and avoid
the need to determine this correction for
each test. Fixed fuse corrections can be
entered when Thermochemical Corrections,
is set to ON. By default a fixed fuse
correction of 15 calories is applied to all
tests. Total errors of more than 5 calories
will seldom occur when using a fixed fuse
correction and the fuse wire supplied by
Parr.
When using the 1108P bomb, there are two
components to the fuse correction:
• The heat introduced by heating the wire
used to ignite the cotton thread.
• The heat of combustion of the cotton
thread used to ignite the sample.
The semi-permanent heating wire is
heated by dissipating an electrical charge
from a capacitor. Since this charge is
controlled by the size of the capacitor and
the charging voltage, and because the
capacitor is fully discharged for each test,
the energy released can be calculated. In
the 6200 Calorimeter this is a fixed
correction of 10 calories per test.
Cotton has a heat of combustion of
4000 calories per gram. The actual thread
being used should be weighed to see how
much is being burned. Ten centimeters of a
fine thread will weigh approximately 0.003
grams which would release 12 calories as it
burns. Heavier threads weigh up to 0.010
grams per 10 centimeters and increase this
correction to 40 calories per test. The finer
the thread, the smaller errors will be if the
thread is not exactly ten centimeters in
length. Polyester thread is not
recommended for use in the bomb because
it has a tendency to melt and fall away from
the heating wire before it ignites.
Using the fine thread mentioned above,
the fuse correction for the calorimeter would
be the 10 calories from electrical heating
plus 12 calories from the burning thread for
a total of 22 calories per test. The thread
supplied by Parr has a mass of
approximately 1 milligram per centimeter.
This results in a total fuse correction of 50
calories. This value should be changed
from the default value of 15 calories in the
Thermochemical Corrections menu.
B-2
APPENDIX B