0 Bewertungen

ASTM D7042 – Standard test method for measuring dynamic viscosity of liquids using a Stabinger Viscometer (incl. the calculation of kinematic viscosity)

The standard method ASTM D7042 was published by ASTM International (formerly “American Society for Testing and Materials”, since 2001 “ASTM International”) and specifies the procedure of dynamic viscosity η and density ρ measurements of Newtonian fluids like liquid petroleum products and crude oils.

Why perform viscosity and density measurements on petroleum products?

The viscosity of a sample provides information about its flow behavior and applicability. Since many petroleum products are used as lubricants, the viscosity value is a suitable parameter to ensure the correct operation of engines and machinery. Viscosity control is essential to fulfill product specification, and to ensure optimal storage, handling, and operating conditions.

Density is an equally important parameter to classify petroleum products. From measured dynamic viscosity and density values, the kinematic viscosity can be calculated.

Furthermore, the Viscosity Index (VI) calculation and a density extrapolation can be performed if the samples are measured at two temperatures with a difference of at least 20 °C. The calculation of the Viscosity Index is defined in ASTM D2270.

Comparability to ASTM D445: precision and bias.

ASTM D445 describes the standard measurement procedure for the kinematic viscosity measurement of petroleum products by using glass capillaries.

The kinematic viscosity values obtained with the test method ASTM D7042 are equivalent to ASTM D445, as long as the appropriate bias correction is applied. The mathematical conversion formula between the two standard methods is described as follows:

$Y=mX+b$      [1]

The result obtained according to ASTM D7042 is designated by the acronym X, and the corrected kinematic viscosity by the acronym Y. A series of interlaboratory studies conducted by ASTM yields the bias corrections for various materials (including formulated oils, diesel, jet, and residual fuels) and temperatures.

Apart from the bias corrections, the results of the interlaboratory studies showed that measurement repeatability and reproducibility of ASTM D7042 are equal or comparable to ASTM D445.

Consequently, ASTM D7042 is being referenced as an accepted alternative to ASTM D445 in many standards and specifications [2].

What are the requirements for viscosity measurement according to ASTM D7042?

Stabinger viscometer SVM kinematic viscometer

The SVM kinematic viscometer operates according to ASTM D7042. It consists of a rotating coaxial cylinder measuring system and measures viscosity according to a modified Couette measuring principle. The density is measured by means of a U-shaped oscillating sample tube, while the kinematic viscosity is calculated by dividing the dynamic viscosity by the density.

A thermostatic heating and cooling system ensures temperature stability within the set temperature deviation of ±0,005 °C.

Sample filling and preparation

Viscosity measurement according to ASTM D7042 requires homogeneous and bubble-free samples. Air bubbles can be eliminated by treatment with an ultrasonic bath. Homogeneity of samples can be achieved by stirring, e.g. with a magnetic stirrer.

The fluid can be injected into the measuring cell by either manual or automated sample injection equipment. Manual filling describes the injection of the sample by using a 5 mL syringe. Autosamplers automatically transfer the sample into the measuring cell. For both injection methods, air bubbles can be avoided in advance by filling the measuring cell slowly and continuously.

High pour point samples, such as waxes and heavy crude oils, require additional heating treatment before filling by the following means:

  • lab oven for preheating in case of manual syringe filling
  • heated syringe
  • hot filling attachment for manual syringe filling
  • sample changing magazine for heating the sample before the injection, and heated hoses to keep the sample fluid throughout the whole process, from the magazines to the measuring cells and finally to the waste container.

Samples with volatile components and low boiling point may require an application of maximum 1 bar counter pressure on the measuring cells:

  • In case of manual filling with a syringe, a pressurized waste bottle provides suitable measuring conditions.
  • Autosamplers can be combined with a pressurized measurement unit.

Samples that may contain particles, such as used oil or crude oil, must be filtered through a screen with a pore size of 75 μm before measurement. Ferromagnetic particles in particular must be removed using a magnetic particle trap.

Measurements of opaque sample do not require additional preparation or special equipment, as optical properties are insignificant for the modified Couette principle.

Common units

According to ASTM, the accepted units are:

  • millipascal seconds [mPa·s] for dynamic viscosity
  • square millimeter per second [mm2/s] for kinematic viscosity
  • grams per cubic centimeter [g/cm3] or kilograms per cubic meter [kg/m3] (SI unit) for density

Other relevant ASTM standards for viscosity and density measurements and calculations

ASTM D445: Standard test method for kinematic viscosity of transparent and opaque liquids (and calculation of dynamic viscosity)

ASTM D4052: Standard test method for density, relative density, and API gravity of liquids by digital density meter

ASTM D2270: Standard practice for calculating viscosity index from kinematic viscosity at 40 °C and 100 °C

ASTM D1217: Standard test method for density and relative density (specific gravity) of liquids by Bingham Pycnometer