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Parr Instrument 6772 User Manual

Page 25

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6772

Calorimetric Thermometer Instruction Manual

4-5

and combustion capsule) 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, turn on the motor on the 1341
calorimeter and 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.

The calorimeter temperature should begin to significantly rise at this point, indicating sample
combustion. The calorimetric post-period will last for an additional 6-7 minutes until the
calorimeter temperature drift rate sufficiently stabilizes. At the conclusion of the post-period
the thermometer will turn off the stirrer and prepare a calorimetric test report indicating the
calorimeter temperature rise, fully corrected for all systemic heat leaks, and whether a heat
of combustion value or an energy equivalent (instrument calibration) factor. Pressing the
REPORT key, on the right hand side of the screen, and following the indicated prompts,
allows the user to view the test report.

At the conclusion of the test, open the cover, remove the bomb and bucket. Remove the
bomb from the bucket and open the knurled valve knob on the bomb head 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 (including the head assembly
and combustion capsule) 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.