Vaisala MMT330 - Manual (Page 144)

User's Guide _______________________________________________________________________
142 __________________________________________________________________ M210733EN-G
NOTE If the oil sample is very dry and the temperatures are close to each other,
it may cause inaccuracy to the calculation model. In order to get the best
possible performance it is recommended to use oil conditions that
represent real conditions in application. Recommended values for
sample are aw approx. 0.5 at 20 °C.
3. Define the correlation between aw, T and PPM (w/w) from the
measured values. Calculate A and B according to the following
example.
[ ] [ ]
( ) ( )
1 / 1 - 2 / 1
1 ( - 2 (
T T
T PPM LOG T PPM LOG
A sat sat
) )
=
[ ] ) (
A/T1 - 1 T PPM LOG B
sat
=
Example:
measured water content 213 ppm
T (°C) aw ppm
saturation
24.1 0.478 213/0.478 = 445.6067
57.6 0.188 213/0.188 = 1132.979
A = (LOG(1132.98)-LOG(445.607))/(1/(57.6+273.16)-
1/(24.1+273.16)) = -1189.4581
B= LOG(445.607)-(-1189.4581)/(24.1 + 273.16) = 6.6503583
Assumptions:
The isoterm of water activity versus water concentration is linear and the
solubility curve has the form of the given equation.