IEC 61868 Mineral insulating oils - Determination of kinematic viscosity at very low temperatures
8 Expression of results
8.1 Calculation
8.1.1 Calculate the kinematic viscosity ν from the measured flow time t and the instrument constant C by means of the following equation.
ν = C x t
where
ν is the kinematic viscosity, in square millimetres per second (mm2 x s(-1));
C is the calibration constant of the viscometer, in square millimetres per second squared (mm2 x s(-2));
t is the flow time, in seconds (s).
8.1.2 The result is recorded as a pair of viscosities ν′/ν″. The first viscosity ν′ is the product of the viscometer constant and the first passage time t1. It reflects the influence of the interlocking microcrystalline structure. The second viscosity ν″ is the product of the viscometer constant and the average of the second, third and fourth passage times, t2, t3, t4 (which should be within 0,5 % of each other for ν″ to be reliable). ν″ is the viscosity of the oil after breakdown of the interlocking microcrystalline structure.
A typical example of result would be 2650 mm2 x s(-1) / 2300 mm2 x s(-1).
The test temperature and the soaking period, especially if it is longer than 20 h, should be included in the report.
8.2 Precision
8.2.1 Repeatability (r)
The difference between successive test results, obtained by the same operator with the same apparatus under constant operating conditions on identical test material, would in the long run, in the normal and correct operation of the test method, exceed the following value only in one case in 20:
0,5 % of the mean.
8.2.2 Reproducibility (R)
The difference between two single and independent results, obtained by different operators working in different laboratories on identical test material, would in the long run, in the normal and correct operation of the test method, exceed the following value only in one case in 20:
1,0 % of the mean.
