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ASTM D445 – Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (incl. Calculation of Dynamic Viscosity)

ASTM D445 describes the standard test method to measure the kinematic viscosity ν (ny) of petroleum products according to ASTM International (formerly “American Society for Testing and Materials”, since 2001 "ASTM International").

The measurement method is based on tracking the flow time of transparent or opaque liquids through a calibrated glass capillary.

The dynamic viscosity η is calculated by multiplying the kinematic viscosity by the density ρ of the sample.

Why measure the kinematic and dynamic viscosity of petroleum products?

The kinematic viscosity and dynamic viscosity of petroleum products are used to

  • determine the quality of petroleum products
  • determine the petroleum’s applicability, operability, and handling
  • develop optimal storage conditions

The flow time is determined with double measurements including cleaning steps in between. To calculate the kinematic viscosity, the flow time is multiplied by the capillary constant, which is obtained by the measurement of certified viscosity reference material. This is an indirect viscosity determination, as the kinematic viscosity is calculated based on flow time.

For the calculation of the dynamic viscosity, additional density measurements according to ASTM D1217, D1480, D1481, or D4052 are required.

See also WIKI articles for ASTM D7042, D4052, D5002 and D7777.

What are the requirements for the viscosity measurement according to ASTM D445?

Temperature control

Temperature is one of the most important factors influencing viscosity measurement. To fulfill the standard’s testing requirements, a maximum temperature deviation of ±0.02 °C for a temperature range from 15 °C to 100 °C (outside this range, it’s ±0.05 °C) is allowed. To meet this requirement, a temperature-controlled bath with an additional external thermometer is needed. For sufficient temperature equalization, the charged capillary must be placed in the temperature bath before the measurement. An equilibration time of about 30 minutes is recommended. However, the optimal equilibration time varies for each device and with different temperatures and kinematic viscosities. It needs to be determined more precisely on an experimental basis.

Timing device

Timing devices with an accuracy of ±0.07 % and a recording discrimination of 0.1 s according to ASTM D445 must be used.

Sample filling and preparation

The ASTM D445 procedure can be applied to both transparent and opaque liquids [1]:

  • Transparent liquids need to be homogenized and degassed in an ultrasonic bath. If the sample fluid contains fibers or particles, an additional filtration step is required.
  • Residual fuel oils and opaque liquids are homogenized by various heating and stirring steps, and particles must be removed with the help of a preheated filter. For opaque liquids, two reverse-flow capillaries are required to determine the kinematic viscosity.

Particles are one of the most common error sources. Filtering is required if the sample contains fibers or solid particles. The pore size of the required filtering screen is 75 µm, so particles larger than 75 µm need to be filtered. Apart from particles in the sample, dust particles inside the capillary can be the cause of faulty measurements. Consequently, correct storage and careful cleaning of the capillaries are essential.

By default, petroleum samples can be measured with manual or automated glass capillary viscometers. Defined Oswald-type, suspended-level type, and reverse-flow capillaries which are specified in the ASTM D445 standard can be used for different samples and viscosity ranges. These viscometers include glass capillaries, capillary holders, a temperature control unit, a temperature-controlled bath, a thermometer, a bath liquid suitable for the measuring temperature, and a time-measuring device that meets the standard’s required specifications.

Common units

According to the ASTM standard, kinematic viscosity and dynamic viscosity must be indicated in millimeters squared per second [mm2/s] and millipascal seconds [mPa·s].

Alternative ASTM standards for viscosity measurements

ASTM 7042: Standard test method for dynamic viscosity of liquids with a Stabinger Viscometer (and the calculation of kinematic viscosity)