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| [[File:Ubbelohde lepkosciomierz.PNG|right|thumb|Ubbelohde viscometer]]
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| [[File:Leo Ubbelohde.jpg|right|thumb|Dr. Leo Ubbelohde, the inventor of the Ubbelohde viscometer.]] | |
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| A '''Ubbelohde type viscometer''' or '''suspended-level viscometer''' is a [[measuring instrument]] which uses a capillary based method of measuring [[viscosity]].<ref>''Introduction to Polymers'' R.J. Young ISBN 0-412-22170-5</ref> It is recommended for higher viscosity [[cellulose|cellulosic polymer]] solutions. The advantage of this instrument is that the values obtained are independent of the total volume. The device was invented by the German chemist [[Leo Ubbelohde]] (1877-1964).
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| [[ASTM]] and other test methods are: ISO 3104, ISO 3105, ASTM D 445, ASTM D 446, IP 71, BS 188 <ref>[http://www.rheotek.com/scat.asp?id=69 ASTM Ubbelohde Viscometer<!-- Bot generated title -->]</ref>
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| The Ubbelohde viscometer is closely related to the [[Ostwald viscometer]]. Both are u-shaped pieces of glassware with a reservoir on one side and a measuring bulb with a [[capillary]] on the other. A liquid is introduced into the reservoir then sucked through the capillary and measuring bulb. The liquid is allowed to travel back through the measuring bulb and 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.
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| ==Determination of viscosity==
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| The determination of viscosity is based on [[Poiseuille's law]]:
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| :<math> \frac{dV}{dt} = v \pi R^{2} = \frac{\pi R^{4}}{8 \eta} \left( \frac{- \Delta P}{\Delta x}\right) = \frac{\pi R^{4}}{8 \eta} \frac{ |\Delta P|}{L}, </math>
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| where t is the [[time]] it takes for a [[volume]] V to elute. The ratio <math>\frac{dv}{dt}</math> depends on R as the capillary [[radius]], on the average applied [[pressure]] P, on its length L and on the dynamic [[viscosity]] [[Eta (letter)|η]].
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| The average pressure head is given by:
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| :<math>\Delta P = \rho g \Delta H \,</math>
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| with [[Rho (letter)|ρ]] the [[density]] of the liquid, g the [[Standard gravity]] and H the average head of the liquid. In this way the viscosity of a fluid can be determined.
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| 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:
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| :<math>\eta_r = \frac{\eta}{\eta_0} = \frac{t \rho}{t_0 \rho_0},</math> | |
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| where t<sub>0</sub> and ρ<sub>0</sub> are the elution time and density of the pure liquid. When the solution is very diluted
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| :<math>\rho \simeq \rho_0 \,</math>
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| the so-called '''specific viscosity''' becomes:
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| :<math>\eta_{sp} = \eta_r - 1 = \frac{t - t_0}{t_0}. \,</math>
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| This specific viscosity is related to the [[concentration]] of the analyte through the [[Intrinsic viscosity]] [η] by the [[power series]]:
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| :<math>\eta_{sp} = [\eta] c + k [\eta]^2 c^2 + \cdots\,</math>
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| or
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| :<math>\frac{\eta_{sp}}{c} = [\eta] + k [\eta]^2 c + \cdots,\,</math>
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| where <math>\frac{\eta_{sp}}{c}</math> is called the '''viscosity number'''.
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| The intrinsic viscosity can be determined experimentally by measuring the viscosity number as function of concentration as the Y-axis intercept.
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| == References ==
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| {{Reflist}}
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| [[Category:Measuring instruments]]
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| [[Category:Laboratory glassware]]
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| [[Category:Polymer chemistry]]
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