Field-Based Kinematic Oil Analysis

The most commonly used physical property for lubricants is kinematic viscosity. This article describes how using the solvent-free SpectroVisc Q3000 Series device in the field offers accurate and immediate kinematic viscosity measurements.

The SpectroVisc Q3000 Series makes use of new solvent-free technology to accurately perform kinematic viscosity measurements that need no temperature measurement, no density verification, and no calibration.

The article explains the design of the SpectroVisc Q3000 Series, the instrument’s performance and finally, a case study compares the measurement results between the SpectroVisc Q3000 Series portable kinematic viscometer and the SpectroVisc capillary (Modified Zeitfuchs) tube viscometer used in many commercial oil analysis labs.

SpectroVisc Q3000 Series Viscometer Theory and Design

The design of the SpectroVisc Q3000 series viscometer includes an upper sample-loading well, microchannel, and temperature control electronics for fluid measurement at a constant temperature of 40°C. The models available are:

• The Q3000 that measures viscosity over the range 10-350 cSt
• The Q3050 viscometer with a range of 1-700 cSt

The device operation is simple. 60µl of oil is loaded into the upper well of the chip, the fluid sample flows down the microchannel due to gravitational force and a combination of detectors and emitters in the IR range detects its rate of progression in the micro channel.

The principle of working of this viscometer is similar to a Hele-Shaw cell, where Stokes flow is present between two parallel plates. Figure 1 shows how the presence of two parallel plates causes the microfluidic device to be unbounded implying the fluid is exposed to air on both sides. The advantages of the devices are the unbounded microchannel and the optical detection method.

Figure 1. Micro Fluidic Kinematic Viscometer - Schematic

Figure 2 shows how surface tension generates a concave meniscus between oil and air. An olephilic material is required to have a positive pressure that forms this concave meniscus.

Figure 2. Concave Meniscus - Top View

At steady state under laminar flow conditions, viscous and gravitational forces are balanced according to Equation 1.

where µ is dynamic viscosity, u is velocity, ρ is fluid density and g is gravitational acceleration.

Using the average velocity, the kinematic viscosity of the fluid can be determined using the average velocity,

where U is the average velocity, g is the gravitational acceleration, and d is the channel depth. Here the differential term is ignored as the micro channel geometry is straight and the fluid is moving due to only gravitational force.

Figure 3. SpectroVisc Q3000 Series Viscometer

These effects are avoided by placement of the optics sufficiently down the microchannel. To successfully operate the device as a Hele-Shaw cell depends on the aspect ratio of the microchannel being large enough.

Figures 4 and 5 shows the two aluminum plates that were created by an ultra precision computer machining system and how they attach to a hinge that allows easy opening and closing.

Figure 4. Aluminum Plates with Hinge

Figure 5. LEDs and Photodiode Positions

The fluid passing between a photodiode an a LED causes a drop in the photodiode voltage.

Case Study - SpectroVisc Q3000 versus Traditional Benchtop Viscometers

For condition monitoring it is very important to know the viscosity of a lubricant. Hence field-based users need portable viscometers to immediately assess critical equipment while working on-site. The process of comparing results from one instrument to another involves several factors such as:

• Wide variability in the performance of in-use oils
• Variability in the performance of the viscometers
• Variability in application requirements

This case study provides a method for determining whether a portable viscometer will "do the job" given particular requirements.

For this comparison, a SpectroVisc Q3000 serves as a portable viscometer and a SpectroVisc Q300 as a laboratory viscometer. The SpectroVisc Q3000 is a portable, solvent-free kinematic viscometer developed for applications where instant results are required. The SpectroVisc Q300 is a capillary (Modified Zeitfuchs) tube viscometer designed for laboratory analysis.

Both the instruments determined the viscosity of several samples and the measurements were compared from instrument to field viscometer. Two series of comparisons were made. The first sample set consisted entirely of NIST-certified standards and the second sample set consisted of used oils.

Each sample was run three times, each on the SpectroVisc Q3000 and the SpectroVisc Q300 with kinematic viscosity readings taken at 40°C. Results from both sets were averaged and compared.

Figure 6 shows the performance of the Q3000 compared to the SpectroVisc Q300 over a range of certified viscosity standards. The Q3000 performs consistently across the calibrated range, with a relative standard deviation less than 2%.

Figure 6. Q3000 vs. Q300 Performance Using a Range of Certified Viscosity Standards

Results

The following data was obtained using a range of certified viscosity standards:

• Table 1 is a comparison of the Q3000 against the NIST references
• Table 2 compares the Q300 against the NIST references
• Table 3 compares results from the two solutions directly
• Table 4 compares used engine oils

Table 1

Table 2

Table 3

Table 4

It is interesting that the Q300 laboratory viscometer yielded results that are in line with referenced ASTM norms (0.44%) for variation. The portable Q3000 reported results were within its expected 3% specification, and the results from the Q3000 and the lab-based viscometer were also within 3% as shown in Table 3. In the used oil comparison, Table 4 shows that results from the portable Q3000 were consistently within 3% of the bench top Q300.

Conclusion

The issue is whether 3% is sufficient in terms of user's specifications. As mentioned previously used oil can present considerable challenges for repeatability measurements, especially with oil, water, fuel and particle contamination.

OEM engine and rotating equipment providers and users consider viscosity variations greater than 10% (from nominal values) to be the first evidence of potential problems. A portable viscometer can detect such issues immediately, allowing improved decision making and more efficient preventive maintenance.

Undoubtedly, the advent of new solvent-free technology for field-based viscosity monitoring offers the performance necessary to detect variations of in-use oil to signal an impending problem at the equipment site. Furthermore, case study results show agreement within 3% of all samples between traditional laboratory viscometers and the portable viscometers.

This indicates that the SpectroVisc Q3000 Series device can deliver accurate results in a portable setting, using a fraction of the sample volumes required by commercial laboratory viscometers.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.