Brookfield Falling Ball KF20 - Manual (Page 13)

Brookfield Engineering Labs., Inc. Page 3 Manual No. M09-352-B04
III. CALCULATIONS
III. Dynamic Viscosity
With Newtonian liquids absolute values of the dynamic viscosity are calculated, where as,
for non-Newtonian liquids, relative values of the dynamic viscosity (apparent viscosity) are
calculated.
The dynamic viscosity is calculated according to the following equation:
Equation 1:
η = t(ρ
1
- ρ
2
)K
•
F
where: η dynamic viscosity [ mPa•s ]
t travelling time of the ball [s]
ρ
1
density of the ball according to the test certificate [g/cm 3 ]
ρ
2
density of the sample [g/cm 3 ]
K ball constant according to test certificate [mPa·cm 3 /g]
F working angle constant
Angle of inclination a
(applied to the level)
Working angle constant F
80° (DIN) 1.0
70° 0.952
60° 0.879
50° 0.778
The density and ball constant are each stated in the test certificate.
Consideration for buoyancy of the ball in the sample is accounted for by means of (ρ
1
-ρ
2
) in
equation (1).
The density of the sample can be determined by:
• referring to the material specifications from the manufacturer of the fluid
• measuring with a densitometer
Note: Be sure to measure the sample density at the same temperature at which the viscosity
will be measured.
The density of the sample must be determined exactly when the amount (ρ
1
-ρ
2
) becomes small.
The use of the glass ball requires the determination of the density of the sample ρ
2
to the 3rd
decimal position in g/cm 3 . For metal balls, the 2nd decimal position is sufficient. For glass
balls, the density of the measuring substance is determined to 0.001 g/cm 3 , for metal balls to
0.01 g/cm 3 .