Syed Moiz bin Asim

157- M

VARIOUS TYPES OF VISCOMETERS

Various types of viscometers

The various viscometers used for determining the viscosity of different systems can be broadly divided into the following types :

1) CAPILLARY 2) FALLING 3) ROTAIONAL

Capillary viscometer Capillary instruments are very accurate for the

measurement of viscosity of newtonian fluids having low viscosity . During mesurement the time for the fluid to flow by gravity from one mark in a capillary column to the second marked is measured .the time of the flow pf liquid under test is compared with the time required for the flow of known viscosity to pass between the two marks .

•If η1 and η2 ae viscosities of unkown and standrad liquid . P1 and p2 are densities . T1 and t2 are time required to flow in sec then :

η1/η2 = p1 x t1 / p2 x t2η1/η2 = realtive viscosity of liquids

•η = пrt∆P/ 8LV R = radius of the inside capillary t = the time of flow ∆p = the pressure head in dyne/cm² L = length of capillary v = volime of fluid this eq can also be writtnen as : η= Kt∆p

Capillary viscometers are divided into two types

The ostwalds U-tube THE UBBELOHDE SUSPENDED LEVEL FOLLWOING ARE THEIRS DETAILS

The ostwald’s U-tube viscometer

The appartaus consists of a U tube the left armof which has a bulging or bulb as its a lower part and a marking A above the bulb the right srm of the tube also has bulb at the upper part and just below this bulb is a capillary tub . There are two maring B and C above and below the bulb of right arm .

Working ov ostwalds viscometer Liquid is introduced into the viscometer through the left arm untill the level reaches the mark .The visocmeter is fixed vertically in a thermosatated bath and allowed to attain the required temperature . The sample volume is adjusted and the liquid is suck or blown into the right arm untill untill its just above the mark B . The sucktion or pressure is released and the time taken from the bottom of the meniscus to fall from B and C .

Equation :- η1/η2 = p1 x t1 /p2 x t2

Ubbelohde suspended level viscometer

Its a modified form of ostwalds viscometer . In this instrument there is a third vertical arm attached to the bulb below the capillary part of the right arm . Working :- liquid is introduced into the viscometer through the left arm in quantity sufficient to fill the bulb in left arm . The viscometer is fixed vertically in thermostated bath and allow to attain the temp . Equation :- η1/η2 = p1 x t1 /p2 x t2

2) FALLING AND RISING BODY APPARATUS

A)Falling sphere B) Rising sphere

a)Falling spher viscometer Princilple The priciple of this instument is based on strokes law which states that when a body falls through a viscous medium , it experience a resistance which opposes the motion of the body . So , viscous drang on body = force responsible for the downward movement thus :

3пηdv = п/6d³g x Ps – P1 where as D = diameter of the sphere g = acceleration due to gravity v = terminal velocity Ps = density of sphere P1= density of liquid by reaaraning formula : η = d²g ( Ps – p1 ) / 18v

Structure And working :- It consists of a tube having to

markings A and B on the outter surface . The tube is fiiled by the liquid whose viscosity is to be determined . The tube is clamped vertically a constant temp and for removal ofa ir bubbles from the liquid .

A ball of suitable material such as steal or glass is then allowed to fall through the guide inside the falling tube . The time taken for the sphere to fall frm point A to point B and the terminal velocity is obtained by divinding distance .

By puting values in abov formaila we can obtain viscosity .

b) Rising sphere viscometer

Rising sohere viscometerisa n important instrument to study the yield value of non –newtonian systems .

It is 1.9 cm in diamter. Satinless steel sphere connected to strain guage via a platinium rhodium wire is lifted through the material under test at a very slow rate by a motor driver arm the sheering stress caused by the strain in the geal stucture is recorded continously .

Stoke’s equation similar to that used for a falling sphere instrument is used to determine the shear or rate of strain .

3) Rotational viscometers These intuemnts works

on the vicous drag exerted on a body when its rotated in fluid whose viscosity is to be determined . The mai advantage of these instruments over rest of viscometers is that a wide range of shearrate can be achieved using these instruments

Types :-

a) concentri cylinder or cup shaped and bob

b) viscometer cone and plate

a) Cup and bob Structure and

working : this type of instrument consist of two coaxialcylinders of different diameters .. The outer cylinder from the cup into which the inner cylinder or bob is fixed centarlly .the sample to be anaylsed is sheared in space between the outter wall ov the bob and the inner wall of cup .

The dfferent types of commercially available instruments differ mainly in wether the torque set up in th e bob due to roatation of the outer cup or due to rotation of bob it self .

Types of cup and bob COUTTE TYPE SEARLE TYPE

In this instrument cup is rotated

In ths intrument bob is rotated

b) The stromer viscometer The stromer viscometers is an

instrument in which the bob rotates and cup is stationary . This instrument can be used to obatin fundamental rheological prpoerties such as yield value , plastic viscosity and the thixotropic index .

Working ;- in operation , the test system is placed ,in space between the cup and the bob and allowed to reach temp equllibrium .a weight is placed on hanger and the time for the bob to rotate a specific number of time is recorded . The data is converted into rpm .

Equation ;- η = Kv ( w/v ) w = weight in grams v = rmp generated due to w Kv = is an instrument constant which can be determined by analysing an oil of knwn viscosity using the instrument .

the plastic viscosity may be caculated by using eq ;- U = Kv ( W-W1 / V ) w1 = yield value intercept in grams U = plastic viscosity