Parr Instrument 6725 - Manual (Page 49)

6725
Semi-micro Calorimeter 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 Multipler 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.