Parr Instrument 6725 - Manual (Page 26)

6725
Semi-micro Calorimeter Instruction Manual
4-6
should begin to significantly rise at this point, indicating sample combustion. The
calorimetric post-period should last for an additional 6-7 minutes. At the conclusion of
the post-period, turn the stirrer off by pressing the stirrer key once again. The motor
switch may be left in the “on” position for subsequent tests.
If the data log destination is a log file, the log file is located at /flash/datalog.csv and may
be retrieved via FTP. The log file is easily imported into a spreadsheet program where
the calorimeter temperature can be plotted in order to realize a thermal curve.
Instructions for working with or analyzing thermal curves are found in the calculations
section.
At the conclusion of the test, open the cover, remove the bomb and Dewar. Remove the
bomb from the flask and open the valve to release the residual gas pressure before
attempting to remove the cap. This release should proceed slowly over a period of not
less than one minute to avoid entrainment losses.
After all pressure has been released, unscrew the cap; lift the head out of the cylinder
and place it on the support stand. Examine the interior of the bomb and the capsules for
soot or other evidence of incomplete combustion. If such evidence is found, the test will
have to be discarded. Wash all interior surfaces of the bomb with a jet of distilled water
and collect the washings in a beaker. Remove all unburned pieces of fuse wire from the
bomb electrodes; straighten them and measure their combined length in centimeters.
Subtract this length from the initial length of 10 centimeters and multiply this burned
length by 2.3 calories per cm (for Parr 45C10 Fuse Wire) to obtain the fuse correction.
The scale on the fuse wire card can be used to obtain this value directly.
Titrate the bomb washings with a standard sodium carbonate solution using methyl
orange, red or purple indicator. A 0.0709N sodium carbonate solution is recommended
for this titration to simplify the calculation. This is prepared by dissolving 3.76 grams of
Na
2
CO
3
in the water and diluting to one liter. NaOH or KOH solutions of the same
normality may be used.
Analyze the bomb washings to determine the sulfur content of the sample if it
exceeds 0.1%. Methods for determining sulfur are discussed in Operating
Instructions No. 207M.
Automatic Test Sequencing
First, select the appropriate jacket temperature source. For most applications, the
calculated jacket approach works well. Next, select the desired test mode
(standardization or determination) by toggling the Operating Mode key on the
Calorimeter Operation screen. After the calorimeter is fully assembled with a charged
bomb, press the START key located on the right hand side of the screen. This will
activate the stirrer that gently circulates the water that surrounds the combustion bomb.
The thermometer will prompt for sample ID number, a bomb ID number (1-4) and the
mass of the sample in grams. This begins the calorimetric pre-period. After the
thermometer determines that adequate temperature equilibrium is realized, the sample
is ignited. This starts the calorimetric post-period.