Vaisala MMT330 - Manual (Page 23)

Chapter 2 ___________________________________________________________ Product Overview
VAISALA ________________________________________________________________________ 23
Typical Applications
Method Used for Measuring Moisture in Oil
The MMT330 transmitter measures water in oil in terms of water activity
(aw) which can be determined as follows: water activity indicates the
amount of oil in 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 MMT330
transmitter can be used for continuous on-line measurements. In addition,
the MMT330 can be calibrated against salt solutions and no reference
oils are needed.
Lubrication Oil in Paper Machines
Typically, a paper machine contains two or three separate lubrication
systems. Usually, one is located at the wet end and the other at the dry
end. There is a certain amount of free moisture constantly present which
means that there is a risk of this moisture becoming into contact with the
machine bearings. The most common reasons for the entrance of water
are an inadequate sealing of the housing and cleaning with high pressure.
However, accidental leakages from oil coolers and other equipment may
also cause damage. In paper machines, the oil should absorb water while
lubricating the bearings and then release this water when collected into
the reservoir. It is to be noted that bearings should never be exposed to
oils that have a high water content; this is especially important during
standstill because the risk for corrosion process increases as the oil
temperature decreases. It is essential to monitor the water content and
keep it on a suitable level.
When measuring the water content of oil in paper machines, it would be
useful to measure the water activity before an oil reservoir and from a
pressure line flow. This way, the performance of dehumidifiers can be
kept under control to ensure that no free water reaches the bearings.