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4.2 Interpretation of the results
By measuring the air speed through a press felt it becomes possible to
quantify the felt’s permeability or openness. Generally the measured air
speed should be between roughly 2 and 10 m/s. If the test results are
too low, the Uhle box will hardly remove any water out of the press felt
and values over 10 m/s will only cost vacuum without increasing the
amount of removed water.
Still, only measuring the air speed is not enough to determine the felt’s
condition. The applied vacuum is very important and should be meas-
ured as well (for example with the Feltest RealVac) as manometers on
Uhle boxes are seldom reliable. To compare the permeability from
several measurements over time with different levels of vacuum, one
could calculate a relative “dynamic permeability”. This means to meas-
ure both vacuum and air speed and then calculating the air speed
towards a standard vacuum of 10kPa.
An example:
Measured Measured air speed Dynamic Perm
vacuum @ 10 kPa
20 kPa 4.0 m/s 2.0 m/s
30 kPa 4.0 m/s 1.3 m/s
35 kPa 4.0 m/s 1.1 m/s
This examples makes clear that, although the measured air speed is
always 4.0 m/s, the felt becomes more closed as the applied vacuum
is increasing.
If the air flow through a felt decreases, this can be the result of
contamination or compaction of the felt. Also a high water content in
the felt will reduce the air flow over the Uhle box. Only by also measu-
ring the caliper with a Feltest Caliper Gauge and the water content,
one gets the whole picture and is it possible to accurately determine
the felts condition.
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