FALLING BALL VISCOMETER

ASSEMBLY: The viscometer is supplied with a glass ball and a stainless steel ball (type 316). A tantalum ball may be ordered separately. The recommended ranges of viscosity for each size and ball are as follows: Size No. Glass Range in cp SS Tantalum 1 0.2 - 2 1 - 10 2 - 20 2 2 - 20 10 - 100 20 - 200 3 20 - 200 100 - 1000 200 - 2000 The proper ball is selected and the instrument is scrupulously cleaned and dried. The ball is added to the tube and the parts are assembled as in the diagram. The position of the knob and lock nut are adjusted so that the PTFE screw will just prevent the ball from falling when the knob is turned to meet the adapter. Unscrewing the knob should allow the ball to be released. This adjustment should be made with care so as not to over-compress the end of the PTFE screw that holds the ball.

FILLING WITH LIQUID:

The nut and the adapter are separated and the cap is removed from the screw. The ball is also removed from the tube. Approximately 5 mL of the test liquid is required to fill the tube. The liquid should be clear of particles and filtered if necessary. Particles in the liquid will interfere with the motion of the ball and reduce the accuracy of measurement. The liquid is carefully pipetted into the tube until nearly full (approximately ¼ in from the top of the flange). The ball is now carefully added and allowed to drop into the tube. The adapter and screw assembly in the release position is now carefully inserted into the tube and liquid allowed to enter the capillary vent. The nut and adapter are tightened on the flange of the tube until secure. The cap is replaced on the screw.

TAKING A READING:

The instrument full of liquid is inverted until the ball enters the PTFE screw and the knob turned until the closed position is reached. The instrument is restored to its normal vertical position and is ready for taking a reading. For the most accurate work, the viscometer tube should be immersed in a constant temperature bath with a transparent window to observe the fiduciary lines. At elevated temperatures, the cap should be removed to permit excess liquid to pass through the capillary vent. Air and gas bubbles should also be vented after equilibrium is reached. Then the cap is replaced.

The ball is released by turning the knob to raise the screw. The time of descent between the two sets of fiduciary lines is measured with a stop-watch. Repeat measurements can be made by removing the viscometer from the bath and inverting to return the ball to the screw and turning to the closed position. With good technique measurements, should be reproducible from 0.2 to 1.0% depending upon the time of descent.

CALCULATING THE VISCOSITY:

For a falling ball viscometer, the viscosity is calculated by the simple formula:

                m    =   K  ( r_t - r ) t
where,     m    =   viscosity in centipoises (cp)
                r_t   =   density of ball (gms/mL)
                            2.53 for the glass
                            8.02 for stainless steel
                            16.6 for tantalum
                r    =   density of liquid (gms/mL)
                t     =   time of descent (minutes)
                K    =  viscometer constant

The approximate K for size 1=0.3, size 2=3.3 and size 3=35. The viscometer constant is obtained by measuring the time of descent for a standard liquid (e.g. a viscosity standard solution).

                       m
     K     =  ________

                ( r_t - r ) t

For the most accurate work, the standard liquid should have physical properties as close to that of the unknown liquid as possible.