Brookfield Falling Ball KF20 - Manual (Page 17)

Brookfield Engineering Labs., Inc. Page Manual No. M09-352-B04
Appendix B - Calibration of the Ball Constants
Re-calibration of the ball constants is required if:
1. changes in the sample tube or water jacket were made
2. one or more balls were replaced
The calibration requires the use of a Newtonian viscosity standard.
Choice of the viscosity standard is according to the ball; the standards are provided in 100 mL
bottles and can be obtained from your Brookfield dealer.
BALL NO. 2 3 4 5
N44 Viscosity Fluid
(Nominal Value of 92 cP @
20ºC) [mPa•s}
4 20 250 1000 3000
The calibration is made according to the method in DIN 53015 at 20°C ± .05 C. A suitably
calibrated thermometer can be obtained from your Brookfield dealer, on request.
The ball constants are determined from (5) running times, in both forward and reverse direc-
tion.
The ball constant is calculated according to the following equation:
Equation 3:
K =
η
(ρ
1
-ρ
2
)
•
t
t Mean value from 5 running times [s]
η Dynamic viscosity of the calibrating fluid [ mPa•s ] at 20°C ± .05 C
ρ
1
Density of ball [g/cm 3 ]
ρ
2
Density of calibrating fluid [g/cm 3 ]
Note: Be sure to measure the sample density at the same temperature at which the viscosity
will be measured.
The expected value of the constant should be similar to the constant stated in the test certifi-
cate.
For Ball No. 6, the ball constant changes significantly as a function of the falling tube diam-
eter and diameter of the ball, so that the ball constant is calculated according to the following
equation:
Equation 4:
K
6
= 1.4057(D-d
6
)
(
0.75042+1.82637
)
d
6
D
D Falling tube diameter (see calibration certificate)
d
6
Diameter of Ball No. 6 (see calibration certificate)
K
6
Ball constant of Ball No. 6