Deepak Sharma, Mankaran Singh, Dinesh Kumar, Gurmeet Singh M.
Pharmacy PHARMACEUTICS CT Institute of Pharmaceutical sciences Jalandhar, INDIA AICTE , PCI approved
Single point viscometer:-Equipment work at a single rate
of shear. Capillary viscometer(Ostwald viscometer) Falling sphere viscometer Application:- Newtonian fluids
Multipoint viscometer:-Equipment work at a several rate
of shear. Cup and bob viscometer Cone and plate viscometer Application:- Non-Newtonian, Newtonian fluids
CAPILLARY VISCOMETER(Ostwald viscometer)
Principle: When a liquid flows by gravity
,the time required for the liquid to pass
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.
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 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. 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 VISCOMETER 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. height
The sample is accommodated b/w the
gap of cup and bob
Now, neither the cup or bob is made
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.
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 VISCOMETER
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 and filling is easy. Less 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 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 of rpm (rate of shear) v/s scale reading (shearing stress) may
thus be constructed in the usual manner.
The viscosity is estimated by equation
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
viscosity of plastisols and other liquids of a thixotropic 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 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
of the degree of thixotropy of the sample is obtained.
