Viscometer

Deepak Sharma, Mankaran Singh, Dinesh Kumar, Gurmeet Singh M.Pharmacy

PHARMACEUTICS CT Institute Institute of Pharmaceutical sciences  Jalandhar,, INDIA   Jalandhar  AICTE , PCI approved approved

Single point viscometer:-Equipment work at a single rate

of shear s hear.. Capillary Falling

viscometer(Ostwald viscometer)

sphere viscometer

 Application:- Newtonian fluids Multipoint viscometer:-Equipment work at a several rate

of shear s hear.. Cup

and bob viscometer

Cone

and plate viscometer

 Application:- Non-Newtonian, Newtonian fluids

CAPILLARY CAPILLAR Y VISCOMETER(Ostwald viscometer)

Principle: When a liquid flows by gravity ,the time required for the liquid to pass p ass b/w two marks through a vertical capillary tube is determined. The time of flow of the liquid under test is compared with the time required for a liquid of known viscosity viscosity..

The viscosity of unknown liquid can be determined using following equation. n1

=

p1t1 n2 p2t2

p1 -density of unknown liquid p2 - density of known liquid t1 -time of flow of unknown liquid t2 -time of flow of known liquid n2  – viscosity viscosity of known liquid

Application:Used for quality control purposes in the formulation and evaluation of  pharmaceutical dispersion systems such as colloids, dilute suspensions, emulsions etc.

FALLING SPHERE VISCOMETER

Principle:•

Apparatus consists of a glass tube positioned vertically vertically.. A

constant temperature jacket with provision for water circulation is arranged around the glass tube.

The test liquid is placed in the glass chamber. A glass or steel ball is

•

dropped into the liquid and allowed to reach equilibrium with the temperature of the outer jacket. The tube with the jacket is then inverted, which places the ball at

•

the top of the inner glass tube. The time taken for the ball to fall b/w two mark is accurately measured This process is repeated several times to obtain concurrent results.

•

The viscosity of a Newtonian liquid is calculated from n1=t(Sb-Sf )B t is the time interval in seconds for the ball to fall b/w the two points, Sb and Sf are the specific gravity of the ball and fluid under examination

CUP AND BOB VISCOM VISCOMETER ETER This a multipoint viscometer and belongs to the category of  rotational viscometers

Principle:The sample is placed in the cup and the bob is placed in cup upto an appr. appr. height •

The sample is accommodated b/w the gap of cup and bob •

Now, neither the cup or bob is made Now, to rotate and the torque resulting from the viscous drag is measured by a spring or sensor device in the derive of the bob •

The number of revolutions and the torque represent the rate of shear and shearing stress, respectively. respectively. •

The following equation is used to calculate the apparent viscosity of  pseudoplastic system. •

n= Kv w v Where w= weight placed on the hanger, shearing stress v= rpm, shear rate n= apparent viscosity of the fluid Kv =constant for the instrument

CONE AND PLATE PLATE VISCOMETER VISCOM ETER

It possesses several advantages:The

rate of shear is constant throughout the entire sample being

sheared. The

sample required is small 0.1 to 0.2 ml.

Cleaning Less

and filling is easy easy..

time is required for temperature equilibrium.

Principle: •

The cone is driven by a variable-speed motor and the sample is

sheared in the narrow gap b/w b /w the stationary plate and the rotating cone. •

The rate of shear in rpm is increased and decreased by a selector

dial and the viscous traction or torque (shearing stress) produced on the cone is read on the indicator scale.

A plot plot of rpm (rate of shear) v/s scale reading (shearing stress) may

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thus be constructed in the usual manner. The viscosity is estimated by equation

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n=C T v Where C is the instrument constant T is the torque v is the speed of the cone

BROOKFIELD VISCOMETER The Brookfield Viscometer is an instrument used for measuring the

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viscosity of plastisols and other liquids of a thixotropic thixotrop ic nature. •

The instrument measures the shearing stress on a spindle rotating at

a definite, constant speed while immersed in the sample. The degree of spindle lag is indicated on a rotating r otating dial.

This reading multiplied by a conversion factor based on spindle size

•

and rotational speed, gives a value for viscosity in centipoise. By taking measurements at different rotational speeds, an indication

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of the degree of thixotropy of the sample is obtained.