Vaisala MMT162 - Manual (Page 45)

Chapter 4 ________________________________________________________________ Operation
VAISALA _______________________________________________________________________ 43
ppm Conversion
MMT162 ppm Conversion for
Transformer Oils
Traditionally, moisture in transformer oil is measured by using ppm
units. The ppm output shows the average mass concentration of water in
oil.
The moisture and temperature transmitter MMT162 has an option for
ppm-output provided that this has been notified when placing the order
for the transmitter. Vaisala has this conversion readily available for
mineral transformer oils.
Conversion Model with Average
Coefficients
The conversion model of the MMT162 is based on the average water
solubility behaviour of transformer oils. The ppm-output is calculated as
follows:
ppm = aw×10 (A/(T+273.16)+B)
where
aw = water activity
A,B = coefficients (average/oil-specific)
T = temperature (ºC)
Generally, measurements with MMT162 give accuracy better than 10 %
of the reading. If additional accuracy is needed, refer to section
Conversion Model with Oil-specific Coefficients below.