Parr Instrument 6400 - Manual (Page 55)

Calculations
6400 7
w w w . p a r r i n s t . c o m 53
Users may find it convenient to enter a fixed value
for the fuse correction and avoid the need to de-
termine 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 50 calories
is applied to all tests. Total errors of more than 3
calories will seldom occur when using a fixed fuse
correction and the thread supplied by Parr.
When using the 6400, 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 dis-
sipating 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 6400 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 cen-
timeters 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 cal-
ories 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 approxi-
mately 1 milligram per centimeter. This results in a
total fuse correction of 50 calories.
ASTM and ISO Methods Differ
Current ASTM, ISO, and British Standard Methods
differ on their treatment of the nitric and sulfuric
acid thermochemical corrections. ASTM Methods
call for titrating the bomb washings to determine
the total acid present. This is assumed to be all nitric
acid with a heat of combustion of -14.1 Kcal per
mole. The amount of sulfur is then determined and
converted to equivalents of sulfuric acid. The differ-
ence between the heat of formation of sulfuric acid
(-72.2 Kcal per mole or -36.1 calories per milliequiva-
lent) and nitric acid is then subtracted as the sulfur
correction.
Most other test methods treat nitric and sulfuric acid
corrections as entirely separate values instead of
combined values. This eliminates the requirement
for a total acid determination and permits the nitric
acid correction to be handled in a variety of ways,
including the assumption of a fixed nitric acid cor-
rection.
The 6400 Calorimeter can be set up to apply the acid
correction by either the ASTM or ISO convention, as
the user prefers. Care must be used to ensure the
proper corrections are applied, and the calculations
made are consistent with the procedure used.
Note: Please review the following section on
Acid and Sulfur Corrections. Different stan-
dard test methods use different values for
the heat of formation of sulfuric acid. These
differences are generally insignificant. The
6400 Calorimeter uses the most recent, pub-
lished values for all thermochemical data.