PCE Instruments PCE-RVI 2, Руководство пользователя

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Manual
7
6 References to viscosity
6.1 Units
This viscometer is a classic rotational viscometer for quick determination of the viscosity according to the
following standards:
BS: 6075, 5350
ISO: 2555, 1652
ASTM: 115, 789 1076, 1084, 1286, 1417, 1439, 1638, 1824, 2196, 2336, 2364, 2393, 2556, 2669, 2849,
2983, 2994, 3232, 3236, 3716
The operating principle of this viscometer is the same as with all rotational viscometers. A spindle (disc or
cylinder) is immersed into the fluid being analysed, the torque required to set the spindle into motion is
then measured. A spring is attached between the spindle and the drive-shaft of the motor. This rotates at
a set speed. The deviation angle between the spindle and the spring is measured electronically and
turned into a torque value. The torque value is dependent on both the spindle geometry and the rotational
speed of the spindle. From this result, the viscosity of the liquid can be directly derived and displayed in
mPa s/cP (dPa s/P).
Depending on the viscosity, the resistance against the movement of a substance changes in proportion to
the rotational speed or size of the spindle. The viscometer has been calibrated to give viscosity readings
in mPa s or cP (dPa s/P), depending on the rotational speed and the type of spindle used. Using a
combination of various spindle types with different rotational speeds enables readings to be taken across
a wide measurement range.
6.2 Important information
Viscosity:
Viscosity is the characteristic property of a liquid. It is a measure of the internal friction within a liquid
when individual layers within the liquid are brought into motion against each other. Viscosity values are
highly influenced by temperature. The standard units for dynamic viscosity measurement are: mPa s (S.I.)
or cP (C.G.S.)
1 mPa s = 1 cP (centi-Poise)
1 dPa s = 1 P (Poise)
Shear stress:
Shear stress is the force per area unit required to move two fluid layers against each other (internal
friction). Standard measuring units for the shear stress are: N/m² (S.I) or dynes/cm² (C.G.S).
Shear rate:
The shear rate is the velocity at which the various fluid layers move against each other. The standard
measuring unit for the shear rate is the
“reciprocal second”, expressed as s
-1
or 1/s.
Laminar flow:
Occurs when a fluid flows in parallel layers with no disruption between the layers. Laminar flow is
fundamental to the determination of dynamic viscosity.
Turbulent flow:
When a certain flow rate is exceeded, disruption between the layers of liquid can occur. This results in an
apparent increase in shear stress and falsely high viscosity readings. The change from laminar to
turbulent flow can be recognised by a sudden marked increase in viscosity values above a particular
speed. Generally, liquids can be classified by the relation between their shear stress and shear rate.
Newtonian fluids:
In Newtonian fluids the shear stress and the shear rate are directly proportional to each other. At a given
temperature, the viscosity of a Newtonian fluid remains constant regardless of the viscometer, spindle or
rotational speed. Examples of fluids with these properties are water or thin engine oil.