Vaisala MMT310 - Manual (Page 77)

Appendix A ______________________________________________________________ Applications
VAISALA ________________________________________________________________________ 77
Calculation Model with Average
Coefficients
The calculation model of MMT318 and MMT317 is based on the average
water solubility behaviour of transformer oils. The ppm output is
calculated as follows:
10 × = aw ppm
(A/(T+273.16)+B) (1)
Where aw= water activity
A,B= coefficients (average/oil specific)
T= temperature (ºC)
Generally, measurements with MMT318 and MMT317 have an accuracy
better than 10 % of the reading. If additional accuracy is needed, see
section Calculation Model with Oil Specific Coefficients.
Calculation Model with Oil Specific
Coefficients
For additional accuracy, the oil specific calculation model can be used
both for mineral and silicon based oils. An oil sample has to be sent to
Vaisala for modelling. As a result, the specific coefficients (A and B: see
formula in section Calculation Model with Average Coefficients) for the
transformer oil are determined by Vaisala. Using these coefficients
increases measurement accuracy.
The determined coefficients of the transformer oil can be programmed to
the MMT318 and MMT317 by Vaisala or by the user according to the
instructions presented on page 78 .
NOTE The calculation model with oil specific coefficients is always needed for
silicone based oils.