14 Analytical Instrumentation
LOW TEMPERATURE VISCOSITY MEASUREMENTS OF LUBRICANTS WITH ROTATIONAL VISCOMETERS, AND WHY YOU NEED THEM
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Lubricants play a pivotal role in minimizing friction and wear between interacting components, thereby facilitating the effi cient operation and extension of the lifespan of machinery. However, their performance can be adversely aff ected by low temperatures. The low-temperature viscosity of lubricants is particularly critical as it determines the lubricant’s ability to fl ow and provide suffi cient protection in environments with temperatures as low as -40 °C. Under these extreme conditions, it is imperative for lubricants to maintain a suffi ciently low viscosity to ensure eff ective fl ow characteristics.
Viscosity assessment at the lowest operational temperature is essential to guarantee adequate lubrication of critical components. Moreover, cold start behavior in car motors or generators using oil can be simulated. Standardized quality control tests ensure proper fl ow and pumpability of the test medium and identify fl ow problems, which, in the case of lubricants, can be caused by fl ow-limited and/or air-binding behavior. Flow-limited behavior is associated with the oil’s viscosity and air-binding behavior is associated with gelation. Determining the viscosity and gelation point at low temperatures helps minimize fl ow problems.
This article will examine two primary standards for measuring low-temperature viscosity of lubricants, specifi cally ASTM D2983 and ASTM D5133.
ASTM D2983 [1] – distinguishing
between procedures ASTM D2983 utilizes a rotational viscometer to determine the suitability of fl uids such as automatic transmission fl uids, gear oils, hydraulic fl uids, and other lubricants for use at low temperatures, covering a viscosity range of 300 mPa·s to 900,000 mPa·s. The standard describes four different procedures, A, B, C, D, each requiring a different confi guration. Procedure D is widely regarded as the automated test method as the heating, cooling and measurement of the sample are performed in one without the need for further operator intervention and with only one instrument confi guration. A,
Figure 1: Anton Paar’s setup (ViscoQC 300 + Peltier Temperature Device PTD 175) for ASTM D2983 procedure D or ASTM D8210
Test tubes: Rotational viscometer
Procedures A, B and C require a rotational viscometer having a torque range between 0.0670 mN·m and 0.0680 mN·m. Procedure D needs a programmable rotational viscometer that has a torque range between 0.0670 mN·m and 0.1800 mN·m.
For viscosity measurements, the viscometer must have at least the following speeds available: 0.6 rpm, 1.5 rpm, 3.0 rpm, 6.0 rpm, 12.0 rpm, 30.0 rpm, 60.0 rpm and 120 rpm (120 rpm is desirable for procedure A to C, and mandatory for procedure D).
• Procedures A and B: standard test tube with approx. 25 mm ID and 115 mm in length, and 30 mL of sample volume
• Procedure C: SimAir Stator with 15 mm ID, and 15 mL of sample volume
• Procedure D: test tube with approx. 25 mm OD and 150 mm in length, and 20 mL of sample volume
B and C require more intervention as the test samples have to be externally pre-heated before they are transferred to the temperature control unit.
The most important part of the test is the cooling of the sample and subsequent measurement. Procedures A, B, and C rely on a mechanical refrigeration cooling technology which requires the use of CFC (Chlorofl uorocarbons) or HFC (Hydrofl uorocarbons) refrigerants for the compressors, and, in the case of B and C, also the use of liquid cooling baths (usually methanol or ethanol). Procedure D, on the other hand, utilizes the air counter-cooled Peltier technology. The instruments of procedure A, B and C feature multiple positions for test cells and require at least one reference fl uid sample to be tested at the beginning of each run (two reference samples with procedure A). Procedure D typically features one position for a test cell without requiring reference oil tests for each test-run. The precision statement for A, B and C is as follows: Repeatability 13.5 % and reproducibility 18.1 %, while for procedure D it is 8.4 % and 9.7 %, respectively.
Temperature control units
The choice of temperature control unit depends on the test procedure. Samples are cooled as follows: • Procedure A: with an air bath to test temperature
• Procedure B: with a mechanical refrigerated programmable liquid bath
• Procedure C: with a mechanical refrigerated constant temperature liquid bath by means of a simulated air cell
• Procedure D: with a thermo-electric temperature-controlled chamber in a range from -45 °C to +90 °C
Measuring system
Viscometer spindle: Procedures A, B, C, and D require a cylindrical viscometer spindle with the same geometry.
• An uninsulated steel spindle should be used only for procedure A
• A composite spindle with lower thermal conductivity must be used for procedure C
• A spindle with insulation on top (Figure 2) is required for procedure D
Figure 2: Insulated steel spindle for ASTM D2983 procedure D
PIN AUGUST / SEPTEMBER 2024
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