Parr Instrument 6400, Operating Instructions Manual

Page: 54 / 110

7
Calculations
P a r r I n s t r u m e n t C o m p a n y
52
c
haPter
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 impor-
tant that the user understand these calculations 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.
e1 = Heat produced by burning
the nitrogen portion of the air
trapped in the bomb to form
nitric acid.
e2 = The heat produced by the
formation of sulfuric acid from
the reaction of sulfur dioxide,
water and oxygen.
e3 = 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 tempera-
ture rise reading automatically. Corrections for heat
leaks during the test are applied. For a complete
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 Chap-
ter 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 calories per degree Celsius. 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 bomb, bucket or amount of
water) is changed.
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:
e3 = (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
normally set to 1.0 so the entered value is in
calories.