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6 Micro Motion 7829 Viscomaster
®
and Viscomaster Dynamic
™
Viscosity Meters
Installation
Factors which may adversely affect the relevance of the measurements could include:
• Measurement used for control purposes being made too far away from the point of control, so
that the system cannot respond properly to changes.
• Measurements made on fluid which is unrepresentative of the main flow.
2.2 Installation effects
Unlike other Micro Motion meters, the vibrating tines of the 7829 Viscomaster
®
/ Viscomaster
Dynamic
™
viscosity meter are not totally enclosed. The walls of the pipe or tank in which the meter
is installed will introduce boundaries to the fluid flow, and this will have an effect on the calibration
of the sensor.
To overcome this, Micro Motion calibrates the meter under a variety of pre-defined conditions
corresponding to the installation and pipe schedule. This condition is selected when ordering the
7829 Viscomaster
®
/ Viscomaster Dynamic
™
viscosity meter, so that by calibrating the meter under
the same boundary conditions as the installation, the need for additional on-site calibration is
eliminated.
2.2.1 Boundary effects
Any insertion device or meter can only measure the properties of the fluid within the region of fluid to
which it is sensitive.
For practical reasons, it is helpful to consider the sensitive, or effective region, for the viscometer as
an ovoid centered on the tips of the tines with its long axis aligned with the direction in which the
tines vibrate, as shown below. The meter is insensitive to the properties of the fluid outside this region
and progressively more sensitive to fluid properties the closer the fluid is to the tines. Density can be
considered a “mass centered” effect and viscosity a “surface centered” effect in this visualization; i.e.
the measurement of density is more uniformly sensitive to the density of fluid throughout the region
while viscosity measurement is much more critically sensitive to fluid on the surface of the tines.
If part of this volume is taken up by the pipework or fittings there is said to be a boundary effect; i.e.,
the intrusion of the pipe walls will alter the calibration. The diagram below illustrates the meter
installed in a pocket on the side of a 4" (100 mm) horizontal pipe line (viewed from above). The
effective region is completely enclosed within the pipe line and thus is completely fluid.
long axis
short
axis