Parr Instrument 6772 - Manual (Page 48)

6772
Calorimetric Thermometer Instruction Manual
6-1
CALCULATIONS
Corrections
Fuse Correction
The fuse correction applied by the Calorimetric Thermometer is calculated as:
e
3
= (fuse value) (Fuse Multiplier)
Fuse corrections can be entered when Fixed Fuse in the Thermochemical
Corrections menu, is set to OFF. To enter in the fuse correction in cm while using
the Parr 45C10 fuse wire, enter a Fuse Multiplier of 2.3 cal/cm. The calorimeter will
perform the calculation and provide the correction in calories. To enter in the fuse
correction in calories, enter a Fuse Multiplier of 1. 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. By default a fixed fuse correction of 15 calories is applied to
all tests. Total errors of more than 5 calories will seldom occur when using a fixed
fuse correction and the fuse wire supplied by Parr.
Spiking Correction
It is sometimes necessary to add a spiking material to samples which are very small, have a
low heat of combustion, or have a high moisture content to add sufficient heat to drive the
combustion to completion. Benzoic acid is an excellent material for spiking for all of the
same reasons it is a good standard material. White oil is also an excellent material,
particularly for liquid samples. The 6772 Calorimetric Thermometer can automatically
compensate for the addition of spiking materials to these samples. The calculations are
modified in these cases as follows:
m
m H e e e WT
H s cs
c
) )( (
3 2 1
− − − −
=
Where:
H
cs
is equal to the Heat of combustion of the spiking material in calories per gram
m
s
is equal to the mass of the spiking material
This factor is added to the calculations when Spike Controls, Use Spiking is set to ON. The
Heat of Combustion of the spiking material is entered as calories per gram. The controller
will prompt the user to enter the weight of spiking material. Fixed spikes can also be used.
To do this, set Use Fixed Spike to ON and enter the mass of the spike (Weight of Fixed
Spike key).
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. Users may
find it convenient to enter a fixed value for the acid correction and avoid the need to
determine this correction for each test. Use of a fixed value for the acid correction is highly
recommended. Fixed acid corrections can be entered when Fixed HNO
3
- Thermochemical
Corrections, is set to ON. A correction of 10 calories is a good number for the fixed nitric