Metrohm 746 VA Trace Analyzer - Manual (Page 499)

6 Data processing and evaluation
746 VA Trace Analyzer / 747 VA Stand
6-36
6.10 Formula calculation
The last step in the evaluation is the calculation of the calculation formulae entered
on the dialog page "CALCULATIONS" for the output of the final results Final
results . In addition to the actual calculation formulae with value assignments, vari-
ables, constants and operators, text designations for the result quantities and result
units as well as the number of significant figures can be entered (for details, see
section 5.6.4).
The most frequent application of the formula calculation concerns the conversion of
the mass concentrations of the substances Mass conc. = MC:<subst> obtained in
the content calculation (section 6.9), which refer to the added sample volume
V.sample , into the desired final result which should generally refer to the original
sample size S0 . By pressing the softkey <Upd.from method> on the dialog page
"CALCULATIONS", standard formulae can be read in from the method automati-
cally to provide the mass concentration referred to S0 in g/L and g/g.
The type of standard formula entered depends on whether the sample size has
been entered as a volume or weight or whether diluted or undiluted samples have
been used (see also section 5.4.4 and section 6.2.2). The dependence of the in-
putted sample size S0 and the parameters of the SMPL instructions has the fol-
lowing appearance:
S0 S2 V.fraction V.total Standard formula for final result Result unit
L – – – MC:<subst> g/L
g d – – MC:<subst>/(S2*1000) g/g
L – V.fraction V.total MC:<subst>*(V.total/S0) g/L
g – V.fraction V.total MC:<subst>*(V.total/S0) g/g
The standard formulae shown above refer to the basic quantities g and L, but the
results are automatically provided with the appropriate prefix by simultaneous entry
of #g/L or #g/g in the field Res.unit (see section 5.6.4). Standard formulae can
also be modified later in any way to specify the result in another unit (e.g. ppm) or
for further calculation.
If quantities associated with errors are calculated in result formulae, in addition to
the results the corresponding result scatters Res.dev . are also calculated from the
scatters of these quantities. Of the instrument variables usable for formulae, the
following two quantities are affected by this:
Mass concentration MC:<subst> : Scatter = MC.dev.
Evaluation quantity EV:<subst> : Scatter = Std.dev.
For calculation of the result scatter ∆c for the addition, subtraction, multiplication or
division of two quantities a and b associated with errors with the corresponding
scatters ∆a and ∆b, the following error propagation rules are used:
Result c Scatter ∆∆c
a + b ∆a + ∆b
a – b ∆a + ∆b
a × b  a × ∆b  +  b × ∆a 
a / b  ∆a / b  +  (a × ∆b) / b 2 