- Ubbelohde viscometer
(1877-1964).
ASTM and other test methods are: ISO 3104, ISO 3105, ASTM D 445, ASTM D 446, IP 71, BS 188 [ [http://www.rheotek.com/scat.asp?id=69 ASTM Ubbelohde Viscometer ] ]The Ubbelohde viscometer is closely related to the Ostwald viscometer. Both are u-shaped pieces of glassware with a reservoir on the right and a measuring bulb with a
capillary on the left. A liquid is introduced and a pressure head forces this liquid from the bulb through the capillary to the reservoir. The time it takes for the liquid to pass through two calibrated marks is a measure for viscosity. The Ubbelohde device has a third arm extending from the end of the capillary and open to the atmosphere. In this way the pressure head only depends on a fixed height and no longer on the total volume of liquid.The determination of viscosity is based on
Poiseuille's law ::
where t is the
time it takes for avolume V to elute. The ratio dv/dt depends on R as the capillaryradius , on the average appliedpressure P, on its length L and on the dynamicviscosity η.The average pressure head is given by:
:
with ρ the
density of the liquid, g theStandard gravity and h the average head of the liquid. In this way the viscosity of a fluid can be determined.Usually the viscosity of a liquid is compared to a liquid with an analyte for example a polymer dissolved in it. The
relative viscosity is given by::.
where t0 and ρ0 are the elution time and density of the pure liquid. When the solution is very diluted
:
the so-called specific viscosity becomes:
:.
This specific viscosity is related to the
concentration of the analyte through theIntrinsic viscosity [η] by thepower series ::
or
:
where ηsp/c is called the viscosity number.
The intrinsic viscosity can be determined experimentally by measuring the viscosity number as function of concentration as the Y-axis intercept.
References
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