M19-354
Page 18
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2019 AMETEK Brookfield Inc. All rights reserved.
Appendix B - Calibration of the Ball Constants
Brookfield’s certification of the instrument is performed with Ball #2 and Ball #3 as well as
using the Cannon N44 viscosity standard with a nominal value of 92 mPa
•
s at 20°C.
Re-calibration of the ball constants is required by the operator 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 mineral oil viscosity standard. Choice of the
viscosity standard is according to the ball. Use the appropriate Cannon or Brookfield viscosity
standard that falls within the viscosity measurement range.
BALL NO.
1
2
3
4
5
6
Recommended Viscosity
Standards
(cP, mPa•s)
Cannon
Cannon
Cannon
Cannon
Cannon
N/A
S3
N10
N100
N350
N1000
N/A
3.9 cP @
20
°
C
21 cP @
20
°C
283 cP @
20
°C
830 cP @
20
°C
2900 cP @
20
°C
N/A
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 local authorized dealer, on request.
Ball #6 is not recommended for use in performing a calibration check.
The ball constants are determined from (5) running times, in both forward and reverse direction.
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 certificate.
For Ball No. 6, the ball constant changes insignificantly as a function of the falling tube diameter
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 (15.94 mm)
d
6
Diameter of Ball No. 6 (see calibration certificate)
K
6
Ball constant of Ball No. 6
