5 Apparatus
5.1 Viscometers
Viscometers of the glass capillary type, calibrated and capable of measuring kinematic viscosity within the limits of precision given in 8.2, are acceptable (see note 1). The viscometers listed in ISO 3105 meet these requirements (see note 2). Use only calibrated viscometers with constants measured and expressed to the nearest 0,1 % of their respective values. It is not intended to restrict this standard either to the use of only those viscometers or to the use of U tube viscometers; however, glass capillary viscometers of the Cannon Fenske Routine type (series 500 and 600), with constants between 8 mm2 x s(-2) and 20 mm2 x s(-2), have been found particularly suitable at very low temperatures.
NOTE 1 - Procedures for the calibration of viscometers are given in ISO Standard 3105.
NOTE 2 - The calibration constant C is dependent upon the gravitational acceleration at the place of calibration and this shall, therefore, be supplied by the calibration laboratory together with the instrument constant. Where the acceleration of gravity g in the two locations differs by more than 0,1 %, correct the calibration constant as follows:
where the subscripts 1 and 2 indicate respectively the calibration laboratory and the testing laboratory.
5.2 Viscometer holders
The holder shall allow the viscometer to be suspended in a position similar to that adopted for calibration. The proper alignment of a vertical datum part may be confirmed by using a plumb line.
5.3 Viscometer bath and accessories
A suitable set-up, placed in a freezer tank, is described in figure 1. Any transparent liquid bath may be used provided that it is of sufficient depth that at no time during the measurement will any portion of the sample in the viscometer be less than 20 mm below the surface of the bath liquid or less than 20 mm above the bottom of the bath. Methanol has been found appropriate as a bath medium. However, any liquid which is clear, does not freeze at operating temperature, and does no harm to the silicone-based caulking material used for the glass bath construction, can be used instead of methanol, for example 2-propanol.
The temperature control shall be such that the temperature of the bath medium does not vary by more than 0,03 °C over the length of the viscometers, or between the position of each viscometer, or at the location of the thermometer.
5.4 Temperature measuring device
Calibrated liquid-in-glass thermometers of a minimum accuracy after correction of 0,02 °C may be used, or any other thermometric or battery-operated device of equal accuracy. All readings and corrections shall be made at least to the nearest 0,02 °C. IP Standard thermometer 68C or ASTM standard thermometer 73C (corresponding to ASTM specification E1), containing a suitable mercury thallium alloy, have been found appropriate. These thermometers have 0,05 °C graduations, but allow interpolations to be made by good operators down to 0,01 °C.
5.5 Timing device
Any timing device may be used provided that the readings can be taken by the operator with a discrimination of 0,2 s or better, and that it has an accuracy of less than 0,01 % when tested over intervals of 15 min.
Electrical timing devices may be used if the current frequency is continuously controlled to an accuracy of 0,01 % or better. Alternating currents, as provided by some public power systems, are intermittently rather than continuously controlled. When used to activate electrical timing devices, such control can cause large errors in viscosity flow measurements.
Synchronous clocks, dependent on power frequency, should meet this accuracy requirement, as should a high quality mechanical stopwatch, although a quartz-based stopwatch, checked against a broadcast time standard, or a synchronous clock that is driven by a large inter-connected power system would be preferred.
6 Calibration
6.1 Viscometers
Use only calibrated viscometers with constants measured and expressed to the nearest 0,1 % of their respective values.
6.2 Thermometers
Calibrated liquid-in-glass thermometers shall be checked to the nearest 0,02 °C by direct comparison with a suitable certified reference thermometer.
6.2.1 Calibrated thermometers shall be checked for accuracy at total immersion, which means immersion to the top of the mercury-thallium alloy column, with the remainder of the stem and the expansion chamber at the top of the thermometer exposed to the freezer's temperature; the expansion bulb should not be immersed in the liquid.
6.2.2 The ice point of calibrated thermometers shall be determined periodically and the certified correction shall be adjusted to conform to any change in the ice point. The possible change in the ice point reading of new thermometers may require checking every week. Checking at the ice point has been found most appropriate and sufficient. However, checking at the operating temperature against a traceable device may also be used, provided it has the required accuracy.
6.2.3 Thermocouples or other electrical temperature measuring devices may also be used, provided they have the required accuracy and are suitably calibrated.
6.3 Timers
Standard time signals available in some countries may be used for checking the accuracy of timing devices.
6.4 Viscosity oil standards
Viscosity oil certified standards, when available, or a suitable reference oil may be used as confirmatory checks on the procedure in the laboratory. If the measured kinematic viscosity does not agree within +/- 0,5 % of the certified or reference value, each step in the procedure shall be rechecked, including thermometer and viscometer calibration, to locate the source of anomalous result.