Calculations
50
7
chaPter 7
Calculations
Calculating the Heat of Combustion
The 6400 Calorimeter will automatically make all
of the calculations necessary to produce a gross
heat of combustion for the sample. However, it is
important that the user understand these calcula-
tions to ensure the instrument is set up so the
calculations match the procedures and the units
are consistent throughout the process.
General Calculations
The calculation for the gross heat of combustion is
done by:
H
c
=
WT - e
1
- e
2
- e
3
m
Where:
H
c
= Gross heat of combustion.
T = Observed temperature rise.
W = Energy equivalent of the
calorimeter being used.
e
1
= Heat produced by burning
the nitrogen portion of the air
trapped in the vessel to form
nitric acid.
e
2
= The heat produced by the
formation of sulfuric acid from
the reaction of sulfur dioxide,
water and oxygen.
e
3
= Heat produced by the heating
wire and cotton thread.
m = Mass of the sample.
These calculations are made in cal/g and degrees
Celsius and then converted to other units if required.
Temperature Rise
The 6400 Calorimeter produces a corrected tem-
perature rise reading automatically. Corrections for
heat leaks during the test are applied. For a com-
plete discussion of this process see Introduction to
Bomb Calorimetry, Manual No. 483M.
Energy Equivalent
The energy equivalent (represented by W in the
formula, or abbreviated as EE) is determined by
standardizing the calorimeter as described in
Chapter 6 - Standardization. It is an expression
of the amount of energy required to raise the
temperature of the calorimeter one degree. It is
commonly expressed in cal/°C. Since it is directly
related to the mass of the calorimeter, it will
change whenever any of the components of the
calorimeter (i.e. the combustion vessel, bucket or
amount of water) is changed.
Thermochemical Corrections
Nitric Acid Correction
In the high pressure oxygen environment within
the oxygen combustion vessel, nitrogen that was
present as part of the air trapped in the vessel 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 vessel,
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)
“Fuse Value” is the number entered by the user
and the value which appears in the test report.
Note: Calculation Factors - Fuse Multiplier is nor-
mally 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.
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