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6 Micro Motion 7827 Digital Viscosity Meter
Installation (Short Stem)
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 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 7827 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 7827 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