ASTM D446 for glass capillary kinematic viscometers
ASTM D446 standard specifications and operating instructions for glass capillary kinematic viscometers
6. Calibration of Viscometers
6.1 Procedures:
6.1.1 Calibrate the kinematic glass capillary viscometers covered by this standard using the procedures described in Annex A1, Annex A2, and Annex A3.
6.2 Reference Viscometers:
6.2.1 Select a clear petroleum oil, free from solid particles and possessing Newtonian flow characteristics, with a kinematic viscosity within the range of both the reference viscometer and the viscometer to be calibrated. The minimum flow time shall be greater than that specified in the appropriate table of the annex in both the reference viscometer and the viscometer which is to be calibrated in order that the kinetic energy correction (see 7.1 and 7.2) may be less than 0.2 %.
6.2.2 Select a calibrated viscometer of known viscometer constant C1. This viscometer may be a reference viscometer (driving head at least 400 mm) that has been calibrated by the step-up procedure using viscometers of successively larger capillary diameters, starting with distilled water as the basic kinematic viscosity standard or a routine viscometer of the same type that has been calibrated by comparison with a reference viscometer. See Test Method D2162.
6.2.3 Mount the calibrated viscometer together with the viscometer to be calibrated in the same bath and determine the flow times of the oil in accordance with Test Method D445.
6.2.3.1 The calibration of the reference viscometer should only be carried out by a reputable laboratory meeting the requirements of, for example, ISO Guide 25.
6.2.4 Calculate the viscometer constant C1 as follows:
C1 = (t2 x C2)/t1
where:
C1 = the constant of the viscometer being calibrated,
t1 = the flow time to the nearest 0.1 s in the viscometer being calibrated,
C2 = the constant of the calibrated viscometer, and
t2 = the flow time to the nearest 0.1 s in the calibrated viscometer.
6.2.5 Repeat 6.2.1-6.2.3 with a second oil whose flow times are at least 50 % longer than the first oil. If the two values of C1 differ by less than 0.2 % for those viscometers listed in Annex A1 and Annex A2 and less than 0.3 % for those viscometers listed in Annex A3, use the average. If the constants differ by more than this value, repeat the procedure taking care to examine all possible sources of errors.
6.2.5.1 The calibration constant, C, is dependent upon the gravitational acceleration at the place of calibration and this must, therefore, be supplied by the standardization laboratory together with the instrument constant. Where the acceleration of gravity, g, differs by more than 0.1 %, correct the calibration constant as follows:
C2 = (g2/g1) x C1
where subscripts 1 and 2 indicate respectively the standardization laboratory and the testing laboratory.
6.3 Viscosity Oil Standards:
6.3.1 Kinematic viscosity oil standards4 are available having the approximate kinematic viscosity shown in Table 1. Certified kinematic viscosity values are established by cooperative tests and are supplied with each delivery.
6.3.2 Select from Table 1 a viscosity oil standard with a kinematic viscosity at the calibration temperature within the kinematic viscosity range of the viscometer to be calibrated and a minimum flow time greater than that specified in the appropriate table of the annex. Determine the flow time to the nearest 0.1 s in accordance with Test Method D445 and calculate the viscometer constant, C, as follows:
C = v/t
where:
v = the kinematic viscosity, mm2/s, for the standard liquid, and
t = the flow time, s.
6.3.3 Repeat with a second oil standard whose flow times are at least 50 % longer than the first oil standard. If the two values of C differ by less than 0.2 % for those viscometers listed in Annex A1 and Annex A2 and less than 0.3 % for those viscometers listed in Annex A3, use the average as the viscometer constant for the viscometer being calibrated. If the constants differ by more than this value, repeat the procedure taking care to examine all possible sources of errors.
6.4 Expression of Constant:
6.4.1 Report the constant to the nearest 0.1 % of the determined value. This generally means four significant figures from 1 x 10(N) to 6.999 x 10(N) and three significant figures from 7 x 10(N) to 9.99 x 10(N).
