Advantages of Kinematic Viscosity Measurement in Used Oil Analysis - Part II

In the September-October issue of Practicing Oil Analysis, several advantages of using standardized test methodology were discussed, for example, the use of ASTM D445 for the viscosity measurement of used oil. The primary aim of standardization is improved precision and better interlaboratory reproducibility of results. However, improved precision typically comes with higher costs. Today, the economics of owning and operating automated D445 viscometers have improved with the arrival of newer, smaller, quicker and less capital-intensive designs. These attributes lead to a reduced cost per test while providing higher precision (including determinability, repeatability and reproducibility targets) associated with a standardized test.

D445 Methodology
D445 kinematic viscosity measurement principles are simple - measure the time required for a known volume of liquid to transition between two fixed points in a vertically oriented capillary viscometer under the sole influence of gravity. While the concept is uncomplicated, the challenge with used oil samples involves continual accurate measurement in a repeatable manner - often in the demanding environment of a commercial or mobile analysis laboratory.

To obtain high accuracy, the D445 methodology recommends several critical test parameters be controlled by the instrument. The sample being tested must be held within 0.02°C of the target temperature and its flow time must typically be measured within 0.01 second or better. The instrument's ability to accurately detect the sample meniscus as it drops through the viscometer tube is also important.

D445 includes provisions for both automatic and manual measurements.

Figure 1a. Key Element of Houillon Tube

ASTM and Used Oil Analysis
The ASTM D2 Committee has recently focused its attention on used oil analysis. Following the June 2006 ASTM subcommittee D02.07.A meeting in Toronto, Ontario, a new test method was approved for publication for Houillon-style viscometers (Figure 1). This new test method (ASTM D7279 - Test Method for the Determination of Kinematic Viscosity of Transparent and Opaque Liquids by Automated Houillon Viscometer) has specific provisions for rapid measurement of in-service engine oils. It differs from D445 in that only a single determination (drop) is made per sample. The subcommittee continues to work toward updating D445 - the benchmark kinematic viscosity test method. A task group is moving forward with plans to run an updated interlaboratory round-robin precision study. Part of this study will include analysis of used engine oils and biodiesel fuels.

Figure 1b. Houillon Z-tube Design

Manual vs. Automatic Methods
ASTM D445 began as a manually run glass viscometer test procedure in the late 1930s. At that time, a typical manual test configuration consisted of a constant temperature bath, appropriate D446 glass viscometers, stopwatch or timer, and a kinematic viscosity thermometer with 0.02°C gradation marks. Today, this configuration is still the entry point for making high-accuracy kinematic viscosity measurements with capital costs averaging between five and eight thousand dollars. However, manual measurements have always been a labor-intensive process, often requiring a technician to remain present during the test.

In the mid-1970s, the first fully automated D445 instruments became available. They took up a large amount of space and were expensive - but they freed up laboratory technicians to handle other tasks. Today, automated D445 instruments continue to be the workhorses in most medium- to large-scale petroleum laboratories, replacing rows of benches with manual test configurations.

Unfortunately, D445 automation has traditionally required high levels of capital investment from roughly $40,000 for semiautomation to more than $100,000 for full unattended automation with multiple temperature baths and sample changers. Most automated D445 instruments have been configured for high sample throughput - as many as several hundred samples or more per day.

Until recently, industry professionals interested in improving laboratory efficiencies were faced with few alternatives regarding instrumentation choices. They could choose either low-capital and labor-intensive manual setups or high-capital automation. The key justification was often in the sample throughput and cost per test.

Figure 2. miniAV

The MiniAV
CANNON Instrument Company has introduced a new model to its line of fully automated D445 viscometers — the miniAV® (Figure 2). The viscometer was developed to provide an affordable automated kinematic viscometer for users who are unable to automate their manual viscosity testing due to the high entry cost of traditional automatic viscometers.

The cost of a turnkey viscometer system is less than half of previous D445 entry-level automated instruments. A discussion of the new automated viscometer illustrates how it fulfills the ASTM D445 test requirements without compromising the manual test methodology.

• Temperature Control — The viscometer has a built-in constant temperature bath that controls temperature within 0.01°C for test temperatures between 20° and 100°C - a critical requirement of ASTM D445.

• Timing Accuracy — The viscometer employs a thermal detection system to monitor passage of the sample meniscus, replacing the stopwatch method (Figure 3). Resolution of the automated system is .001 second, far exceeding the trained eye of a technician using manual timing methods. This system has another advantage for used oils; it can reliably detect opaque used oil samples with the same precision as transparent samples.

Figure 3. Thermal Level Sensors in Ubbelohde-style Tube, miniAV Compound Viscometer

• Tube Design — Tube designs for manual viscometers are described in ASTM D446. The viscometer features a compound D446 Ubbelohde-style viscometer tube with a 100-fold viscosity range, equivalent to six different manual viscometers. Sample volume required for testing is typically six mL with an optional ten-fold range "fast-run" tube that requires only three mL. With sample cycle times of between six and ten minutes, the operator can concentrate on other tasks in the laboratory. The fast-run tube reduces the cycle time to approximately three minutes.

• Precision — Precision of the viscometer meets D445, which specifies repeatability of less than 0.11 percent on base oils to 0.26 percent on formulated oils and reproducibilities at 40°C and 100°C of 0.65 and 0.76 percent respectively.

Figure 4. miniAV Sample Loading

How it Works
Operation of the miniAV requires a few simple steps. First, the technician fills a clean vial with approximately six mL of sample. Second, the sample ID is entered into the PC program and the sample vial holder is raised into position (Figure 4). No further interaction with the instrument is required until the next sample test.

The instrument automatically pulls the sample up the tube, holds it for a prescribed thermal soak and allows it to drop while timing the efflux. Once the selected number of test determinations has been completed, the instrument automatically washes (with up to two different solvents) and dries the tube and sample vial. At the conclusion of the test cycle, the instrument lowers the vial holder to accept the next sample. These vials can be reused for the next test - a unique feature that can shave as much as $0.50 off the cost for each test.

The supplied VISCPRO® for Windows® software has a user-configurable reporting/analysis package and built-in dynamic data export capability for easy LIMS connectivity. At the conclusion of each sample test, the data is presented on the screen and can simultaneously be sent to a printer, network hard drive, and via RS-232 or USB to a remote LIMS system (Figure 5). The modular design of the viscometer system allows up to four independent baths to be connected to a single PC through VISCPRO®.

Used oil analysis is receiving more attention from ASTM committees. Automated instruments and methodologies are a proven alternative to manual methods of kinematic viscosity measurement. New technologies are also making kinematic methods more attractive for used oil applications. Small and mid-sized companies may realize cost benefits by transitioning from manual to automated methods to reduce labor costs and improve precision.

(Click Image to Enlarge)

Figure 5. Sample Data, VISCPRO Reporting/Analysis

Editor's Note:

C. Patrick Maggi is chairman of ASTM D02 Subcommittee 7 Section A, and president of CANNON Instrument Company. Since 1938, CANNON has provided worldwide service and application support for the organization's viscometry-related products. If you are interested in learning more about the automated viscometer, please contact the author at patmaggi@cannoninstrument.com.

Related Reading

Maggi, C. Patrick. "Advantages of Kinematic Viscosity Measurement in Used Oil Analysis - Part I." Practicing Oil Analysis magazine, September 2006.