Measuring Back Pressure
At any given position in the exhaust gas system,
the total pressure of the ow can be divided into
dynamic pressure (referring to the gas velocity)
and static pressure (referring to the wall pressure,
where the gas velocity is zero).
At a given total pressure of the gas ow, the
combination of dynamic and static pressure may
change, depending on the actual gas velocity. The
measurements, in principle, give an indication of
the wall pressure, i.e., the static pressure of the
gas ow.
It is, therefore, very important that the back pres-
sure measuring points are located on a straight
part of the exhaust gas pipe, and at some dis-
tance from an ‘obstruction‘, i.e. at a point where
the gas ow, and thereby also the static pressure,
is stable. Taking measurements, for example, in a
transition piece, may lead to an unreliable meas-
urement of the static pressure.
In consideration of the above, therefore, the total
back pressure of the system has to be measured
after the turbocharger in the circular pipe and not
in the transition piece. The same considerations
apply to the measuring points before and after the
exhaust gas boiler, etc.
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