Every liquid has got certain flow is due to the friction ca In everyday terms (and for water is "thin ", having a lo ut simply, the less viscous !iscosity in a fluid is the attra ction i n the li quid th a applied to a f lui d at rest, th as shown in $ig. %elow. The force F resisting motion #here μis the proportio nal The applied force is therefo fluid an d in ve rse ly proporti This equa ti on can %e e& pr es . Thus the shear stress %et di re ct ion pe rp endi cu lar to t the velocity gradient VISCOSITY re si stance t o f low and is call ed it s viscos sed when one layer passes over the other fluids only), viscosity is " thickness" or "in er vis cosity, whi le honey is "th ick", havi he f lui d is , the greater its ease of moveme resis tance to its chang e of shape, whic t resists an y chan ge due to fl ow or mot molecular layers in the fl uid ten d to sl id in a fluid is given %y ty factor called viscosity. re prop ort ion al to the a rea ( ') an d veloci nal to the distance %etween the plates (y) sed in terms of shear stress een layers is proportional t o the vel he lay ers. Th is gi ves th e rela tion % etwee [email protected]ity. The resistance to . ternal friction". Thus, ng a higher viscosity. t (fluidity). is due to molecular ion. #hen a force is on top of each other ty gradient (u) in the city gradient in the the shear stress and
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#here, v B !elocityA B ressure difference across the tu%e of length l
Integrating in the limits r / r: and corresponding v / v0.
!olume flow rate,
Cence a%solute viscosity is given %y,
The capillary method has %een successfully used in refineries for measuring viscosities of
petroleum products.
The schematic arrangement is shown in figure %elow. This is a continuous method. Theliquid is forced at a constant velocity through the capillary (friction tu%e). The pressure
drops across the ends of the tu%e is measured %y a pneumatic force %alance type
Differential ressure Transmitter (DT). The indicator can %e directly cali%rated in
viscosity. This method can measure viscosity from 01/2 poise to 2111 poise. It is usa%le at
high static pressures and is suita%le for remote indication and control.
ay%olt?s viscometer works upon the same principle of operation of capillary viscometer,%ut this method measures kinematic viscosity. The schematic set up of the method is
shown in figure %elow.
It consists of a graduated tu%e with a capillary at the %ottom. The tu%e placed in a constant
temperature %ath is filled with the liquid and a measured quantity of liquid is drained
trough the capillary. $or varying viscosity, the time required should vary. The time t
required to drain 0 cc of the liquid indicates viscosity and is denoted %y ay%olt num%er.
'n empirical equation relating kinematic viscosity and time is as given %elow.
Cere ' and ; are constants. ' is having a value of 3F and ;, G00 for ay%olt?s viscometer.
The following formulas may %e used to convert centistrokes (ct units) to
appro&imate ay%olt universal seconds (< units).
$or < values %etween @3 and 00
$or < values greater than 00
Rotameter Type Viscometer&
In this type, rotameter %o%s may %e designed to %e viscosity sensitive. Therefore with a
rotameter in the line of flow, viscosity can %e measured if flow rate is constant. $igure
%elow shows a scheme of the method.
' rotameter with two %o%s, one sensitive and another immune to viscosity is mounted in a
%ypass line through which the flow is regulated %y a suita%le regulator. ;y ad*usting the
regulator, the flow rate inde& is set at the inde& mark. The other %o% will indicate viscosity.If the range is required to %e changed, the flow rate can %e changed and the scale readings
can then %e related to the viscosity %y empirical cali%ration.
This method is commonly used up to a viscosity of 211 centipoises(cp), %ut the range is
e&tendi%le. It measures viscosity and also checks consistency at high static pressure.