Overview

The USV (Ultra High Viscometer), a computer controlled instrument capable of fully automatic viscosity measurements at 10 million reciprocal seconds.

The USV system comprises of:

• Mechanical unit including PC with data logging software
• User manual, Tool set, Variable displacement laboratory pipette

Viscosity measurement background

Accurate measurement of dynamic viscosity at very high shear rates is of particular concern in the automotive industry. Lubricants developed for motor vehicle applications are typically non-Newtonian making it necessary to reproduce the shear rate found in engines when measuring the viscosity. Several international standards (ASTM, CEC) define procedures to measure the High Temperature High Shear (HTHS) viscosity at 150°C and 10⁶ s-1.

Existing instruments

At present most instruments for measuring HTHS viscosity employ the same technique. The shear stress is generated between a tapered steel plug & copper stator. The gap between the plug and stator is set by adjusting the vertical position of the plug.

In order to minimize the effects of thermal expansion, test results have to be compared with those for a calibration oil at the same temperature. Time-consuming calibration procedures have to be carried out each time the instrument is used. This considerably slows down the test turn around and increases uncertainty in the results.

The temperature challenge

Extending the measuring range on existing instruments is not feasible due to the temperature rise caused by shear heating of the lubricant. As the shear rate rises, the heat dissipated in the film increases. This leads to both a distortion in the film shape and a drop in the test oil viscosity. Consequently, the maximum shear rate is limited at approximately 3 x 10⁶ s-1.

New needs

Lubricant formulators have long been aware of the shortcomings of existing high shear viscometers. Whilst the maximum shear rate of commercially available viscometers has remained around the 10⁶ s-1 mark for many years, developments in engine and lubricant technology mean that in-service conditions are closer to 10⁷ s-1. These new conditions generate a great demand to measure engine oil viscosity at more realistic shear rates, i.e. shear rates greater than 10⁶ s-1.

The ultimate solution

The Ultra Shear Viscometer can carry out fully automated viscosity measurements over a shear rate range from 10⁶ s-1 to 10⁷ s-1 and temperatures between 40 and 150°C. All test parameters are controlled by a built-in microprocessor.

These capacities are enhanced by the simple to use software and a rapid turnaround: a complete temperature/shear rate map for a lubricant can be completed in under 1 hour.

Novel measurement technique

The USV can measure at exceptionally high shear rates thanks to its unique measurement technique. The USV is fitted with a DC servo motor capable of speeds of over 20,000 rpm and an electromagnetic clutch which engages the rotor for only a very short period of time (typically 30 ms). This brief shearing interval minimizes the shear heating in the lubricant.

The motor and clutch are controlled by the built-in micro-processor which also logs torque data from a piezo force transducer coupled to the stator. The radial gap between the rotor and stator is one micron, making shear rates as high as 10⁷ s-1 achievable.

Temperature under control

The temperature is constantly monitored by an independent temperature control micro-processor. Two Platinum RTD probes are used in the instrument. The main probe slides into the stator, in close proximity to the test sample.

The probe is supplied with an internal chip programmed with its own calibration data. The second probe cross-checks temperature at the bottom of the test chamber for safety purposes.

No re-calibration

The rotor and stator are both made from the same material which is heated uniformly to the test temperature. The uniform expansion means that the gap change with temperature is negligible. More importantly, because the shear heating is minimal, thermal distortion of the gap is insignificant so the rotor can have a cylindrical form.

As a result, the radial gap is constant and there is no need for a calibration procedure at each change of lubricant. The rotor position is set and the gap value is fixed for a particular rotor and stator pair. This constant gap allows to the USV to run multi-temperature tests over a wide range of shear rates.

The very high shear rate range and automated testing ability of the USV has attracted the interest of many companies and research centres since its release. Typical USV users are lubricant developers, additives designers, research departments of automotive manufacturers and universities.

USV Results

Data accuracy and repeatability

When measuring viscosity, accurate temperature control and measurement is a key parameter. The USV temperature control constantly controls the test sample to within 0.1°C of the target temperature. Each result is the average of 5 measurements to ensure consistency.

As a result, the USV achieves a repeatability of 2% of the viscosity value. For high shear rates at high temperature, the repeatability is as good as 0.03 mPas, see table below.

Testing time

The USV is fully automated: once the test has started, the user can leave the rig unattended and will find the results logged in a data file at the end of the test. It takes 2 minutes to measure the viscosity at one shear rate. Heating and temperature stabilization of the test chamber require on average 10 minutes. Therefore testing a sample at 10 shear rates for 3 different temperatures would take 3 x (10 x 2 min + 10 min) = 1h30.

Research key tool

Rapid turn around testing, reliability and simplicity of use are the key features of the USV. These qualities make it ideal for research and development on new additives and lubricants as well as studying the effect of ageing.