99 / 160
Ma
rc
h
20
17
97
HYD
ROC
ARB
O
N
ENGI
NEER
IN
G
VERIFYING
VISCOSITY
V
iscosity is a physical property that
describes the resistance of fluid to
flow under stress. There are many
different types of viscosities,
e.g., dynamic, kinematic, intrinsic, and inherent,
amongst others. Dynamic and kinematic viscosities
are the most common across all industries. They both
relate to the resistance of a fluid to flow under stress.
However, for dynamic viscosity, the stress can be any
force applied to the fluid, whereas kinematic viscosity
focuses on gravity as the force applied on the fluid.
Practically speaking, kinematic viscosity can be
calculated using dynamic viscosity and density, as the ratio
of one to the other:
Where:
v
= kinematic viscosity (mm
2
/sec.).
η
= dynamic viscosity (mPa
.
sec.).
ρ
= density (g/cm
3
).
This article will explain how a technology used
to measure dynamic viscosity has been adapted
to measure kinematic viscosity. It will then
detail how this technology has been adapted
into an analyser to measure kinematic
viscosity at reference temperature. The
final section of this article will
present the results of the tests to
determine repeatability,
accuracy and
reproducibility of
the new viscosity at reference temperature
analyser.
Existing technology for
kinematic viscosity at reference
temperature
Viscosity measurement at reference temperature is a
measurement where the sample is drawn from the
process line at process temperature and brought to an
analyser with an internal temperature control system to
heat/cool the product to the desired temperature before
measuring the viscosity.
The main technique for this measurement uses the
Hallikainen
1
capillary measurement system combined
with an outside densitometer. Its drawbacks are the use
of an oil bath and its heavy maintenance requirements.
The technology used by capillary systems measures the
pressure difference between the inlet and the outlet of
the capillary to make a direct measurement of
viscosity. The capillary is inserted in an oil bath to
bring the sample to the required temperature and
ensure that the diameter and length of the
capillary do not change with temperature. Other
systems based on the same capillary technology
are available but use an oven instead of an oil
bath. This system is not compliant to the
reference method ASTM D445,
2
since it
uses a pump to flow the sample, it
measures the dynamic viscosity
and needs an external density
measurement and/or a
correlation.
Luc K. Bellière
and Philippe Burg, Sofraser,
France,
evaluate a new kinematic viscosity
analyser working at reference temperature.