Vaisala MMT162 - Manual (Page 14)

User's Guide ______________________________________________________________________
12 __________________________________________________________________ M210934EN-C
Typical Applications
Method Used for Measuring Moisture in
Oil
The MMT162 transmitter measures water in oil in terms of water activity
(aw) which can be determined as follows: water activity indicates the
amount of water on the scale of 0 - 1 aw. In this scale, 0 aw is an
indication of completely water free oil and 1 aw an indication of oil fully
saturated with water. Water is present in free form.
The most important feature which distinguishes the measurement of
water activity (aw) from the traditional measurement of absolute water
content (in ppm) is that the saturation point remains stable regardless of
the oil type or the aging of oil, additives used etc. As water activity
exceeds 0.9 aw in any system, there is a risk for segregation (especially if
the temperature decreases).
The water activity is used for alarming at the point of >0.9 aw that the
risk for free water in the system is obvious. The most important
advantages of this system are the fact that water activity is immune to the
aging of oil and to additives, and that the MMT162 transmitter can be
used for continuous on-line measurements. In addition, the MMT162 can
be calibrated against salt solutions and no reference oils are needed.
NOTE To avoid self-heating of the probe and consequential measurement
errors, make sure there is some flow in the application process.
Lubrication Oil
In many industrial plants, like paper mills, hydro power plants and off-
shore wind turbines, there is a certain amount of free moisture constantly
present. This means that there is a high risk of the free moisture
becoming into contact with the machine bearings. The most common
reasons for the entrance of water are inadequate sealing of the housing or
moisture absorption from ambient air. Accidental leakages from oil
coolers and other equipment can also cause damage.
Free water in lubrication oil prevents oil from forming a uniform layer on
the metal surfaces and thus deteriorating its lubrication properties. This
can cause, for example, spot heating, cavitation, corrosion and micro
pitting. Free water also ruins additives like AW (antiwear) and EP
(extreme pressure). Note that bearings should never be exposed to oils
that have high water content; this is also important during standstill
because the risk for corrosion increases as the oil temperature decreases.