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ASTM D5133 - Standard Test Method for Low Temperature, Low Shear Rate, Viscosity/Temperature Dependence of Lubricating Oils

Temperature-Scanning Technique Helps Ensure Proper Operation of Mechanical Devices

The ASTM D5133 test method is used to analyze the flow behavior of engine oils at low temperatures. The sample is cooled from
-5 °C to -40 °C at a cooling rate of 1 °C/h or until a maximum viscosity value of 40,000 mPa·s is reached. During the test, the sample is exposed to a low shear rate of approx. 0.2 s-1. The test permits determination of the viscosity of the oil at low temperatures. Furthermore, the Gelation Index is determined by calculating the increase in viscosity when the temperature is increased by one Kelvin over the temperature range. The following equation is used:

The maximum value is the Gelation Index. The temperature (T2) at which the Gelation Index occurs is called the Gelation Index temperature. By plotting the Gelation Index values (y-axis) against the temperature (x-axis), the gelation of an oil can be detected with a peak. [1]

Figure 1: Gelated vs. non-gelated engine oil

Figure 1: Gelated vs. non-gelated engine oil

Why measure the viscosity of engine oil at low temperatures?

Measuring the viscosity of oils at low temperatures can help guarantee the proper operation of mechanical devices. Moreover, cold start behavior in car motors or generators using an oil can be simulated. An engine requires a sufficient quantity of lubricant to prevent engine damage immediately or eventually after cold temperature starting. Standardized quality control tests ensure the proper flow and pumpability of the test medium and identify flow problems, which, in oil, can be caused by flow-limited and/or air-binding behavior. Flow-limited behavior is associated with the oil’s viscosity and air-binding behavior is associated with gelation. A viscosity value of 30,000 mPa·s and 40,000 mPa·s is critical for pumping, depending on the engine manufacturer. Determining the viscosity and gelation point at low temperatures helps minimize flow problems.

An oil’s pumpability behavior at low temperature is of particular interest since a catastrophic number of air-binding failures occurred in 1980 due to gelation, with numerous car engines damaged in the winter because of bad engine oil. Clearly, a test method indicating gelation during slow cooling of the oil over a wide low-temperature range was required. Engine oils must not show any gelling within the exposed temperature range, if engine failure is to be avoided. And so, the ASTM D5133 test method was developed. Today, the use of highly paraffinic base oils and vegetable oils is increasing sharply. These oils are prone to gelation and may have a higher low-temperature gelation point, which is why their effects in new engine oil formulations must be studied carefully, to avoid flow-limited or air-binding failure. Especially products sold in countries with cold temperatures need to be tested. [2] 

What are the requirements for a viscosity measurement according to ASTM D5133?

  • Rotational viscometer: For the measurement, a rotational viscometer capable of measuring at least 45,000 mPa·s is required. Measuring system: A special measuring spindle and test tube must be used. A spindle with a length of 65.5 mm (±0.1 mm) and a diameter of 18.40 mm (±0.02 mm) is required. The critical diameter of the test tube is 22.05 mm (±0.02 mm).

  • Temperature device: A temperature device which can perform a temperature ramp with a cooling rate of 1 °C/h is required. For the temperature ramp, a temperature range from -5 °C to -40 °C is required. For the sample pre-treatment a temperature of +90 °C is necessary. The same or a separate temperature device can be used for the sample pre-treatment. Direct control of the temperature device via the viscometer significantly simplifies the operation.
    A Peltier temperature device has many advantages compared to traditional liquid temperature devices for this application:
    • Precise sample temperature control ensures the highest viscosity accuracy
    • No additional space in the lab needed for a thermostat or oven
    • Minimum maintenance thanks to air counter-cooling
    • No flammable cooling liquids

Overview of the test method

The test method includes the following main steps:

  1. 16 mL of homogeneous sample must be filled into the measuring system.
  2. The sample is preheated to +90 °C for 1.5 h to 2.0 h. This step should remove the ‘memory’ of the oil. An oil’s thermal history can influence its future behavior including gelation properties.
  3. The temperature is reduced to -5 °C and the sample is held at -5 °C for 15 min to 30 min for temperature equilibration.
  4. A temperature ramp from -5 °C to -40 °C, or until 40,000 mPa·s are reached with a cooling rate of 1 °C/h, is performed. During the temperature ramp, the sample is exposed to a low shear rate of approx. 0.2 s-1.
  5. Determination of Gelation Index, Gelation Index temperature and critical pumpability temperature: Additionally, the temperatures associated with the following viscosities must be reported: 5,000 mPa·s, 10,000 mPa·s, 20,000 mPa·s, 30,000 mPa·s and
    40,000 mPa·s.

Note: Other test methods also address the pumpability problem of engine oils at low temperatures: ASTM D3829 and D4684. However, the temperature cooling procedure and shear rate are different, and this can lead to significantly different test results. 

What is the difference between ASTM D5133 and D7110?[3]

  • The standard ASTM D7110 method is used for viscosity measurement of used and scoot-containing engine oil.
  • The test procedure differs from ASTM D5133 regarding the cooling rate for the temperature ramp. According to ASTM D7110 a cooling rate of 3 °C/h instead of 1 °C/h is required.
  • The device setup must include a source for dry air or nitrogen. The top of the test tube must be flooded with a low air flow / gas atmosphere of approx. 10 mL/min to 20 mL/min during the measurement. This action prevents condensation and freezing of water on the oil surface.
  • The test result must contain the temperature value at 5,000 mPa·s, 15,000 mPa·s, 25,000 mPa·s, 40,000 mPa·s and
    60,000 mPa·s, in contrast to ASTM D5133.

Common units

According to the ASTM standard, dynamic viscosity must be indicated in millipascal seconds [mPa·s].

Other relevant ASTM standards for rotational viscosity testing

ASTM D7110: Standard test method for determining the viscosity-temperature relationship of used and scoot-containing engine oils at low temperatures

ASTM D2983: Standard test method for low-temperature viscosity of automatic transmission fluids, hydraulic fluids, and lubricants using a rotational viscometer

ASTM D8210: Standard test method for automatic determination of low-temperature viscosity of automatic transmission fluids, hydraulic fluids, and lubricants using a rotational viscometer

DIN 51398: Testing of lubricants; procedure for measurement of low-temperature apparent viscosity by means of the Brookfield viscometer (liquid bath method)

References

[1] https://www.astm.org/Standards/D5133.htm
[2] Selby, T. and Miiller, G., 2008. Thermal History of the Engine Oil and Its Effects on Low-Temperature Pumpability and Gelation Formation. SAE Technical Paper Series.
[3] https://www.astm.org/Standards/D7110.htm