Screening vs. Testing: Know the Difference When Monitoring Contaminants

Noria Corporation

When it comes to contaminant monitoring, the difference between what is referred to as screening and what could be referred to as testing and analysis is really like talking about apples and oranges. This is an important distinction to remember when dealing in used oil analysis.

When referring to contaminant monitoring, screening is often done at the plant level by plant personnel with field equipment. Conversely, testing is often the job of a lab. It is precise testing done to an exact standard by certified laboratory technicians using laboratory equipment. Make sure that you know the difference when monitoring for contaminants in the field and in the lab.

If we examine the traditional offering of screening tests done in a plant environment, the three most popular screening tests tend to be water, particle count and viscosity.

Identifying Water Content

Water always has been one of the most destructive contaminants in a lubrication system. Not only can water have lasting or irreparable effects on the lubricant (specifically the base oil and the additive package), but water in oil also affects the machine itself.

It goes without saying that this would be an important contaminant to measure. However, the method by which we measure water concentration in oil in a field environment can be much different than the method used by a lab technician.

Many commercial used oil analysis laboratories measure water content using a Karl Fischer titrator. Karl Fischer titration is a classic method in analytical chemistry that uses coulometric or volumetric titration to determine trace amounts of water in a sample. This test follows a set of steps developed and published by the American Society for Testing and Materials.

For this reason, the results are repeatable and highly accurate, providing results in parts per million (ppm). In the field, it would be rare to run the same test to determine the amount of water concentration in a used oil sample. Karl Fischer involves the use of chemical reagents and apparatus not suitable for an industrial field environment. Also, such accurate results are not always required in the field when making maintenance decisions.

Simple pass/fail screening tests may provide you with enough information to take action. The crackle test has long been a field favorite for determining if water is present in an oil sample. The crackle test involves placing two drops of used oil on a pre-heated hot plate.

Based on the formation and audible explosion of bubbles when the drops hit the hot plate, a skilled technician can even determine the approximate quantity of free and emulsified water in the sample down to about 500 ppm. However, the crackle method is limited and highly subjective.

As with any field test, you will not want to seriously compare results to a lab result. In the case of crackle vs. Karl Fischer, these methods use two different apparatus and measure different states of water coexistence.

Measuring Particle Count

Particle count can provide a wealth of information that can limit a potential failure to a less-than-catastrophic issue mitigated with some filtration or breather upgrades. In recent years, the quality and accuracy of particle counters available for the industrial environment has grown significantly.

There are several types of on-site particle counters that you can get your hands on for industrial applications, including light blockage, laser scattering and mesh obstruction.

Laser light scattering certainly offers the most popular particle counters for labs and for the field, but you may come across the other technologies. The challenge when performing particle counts in the industrial environment is being able to work around water or air contamination and color (clarity) of the sample.

These contaminants can make it difficult to get accurate results, and the sample may require additional preparation just to run the particle count. The preparation can be difficult and is less likely to be done in an industrial environment. Additionally, prior to testing the sample for particle count, the sample must be mechanically agitated.

Hand shaking or automatic stir plates will not resuspend the particles to an even and representative distribution. The agitation needs to be aggressive and violent. Reciprocating paint shakers are generally considered to be the best choice for this application.

Considering all of the variables and challenges, particle counting can still be done on-site. Mesh obstruction, pore blockage particle counters make easy work of particle counting and are not as affected by moisture and air contamination or color as optical particle counters, which make them a great choice for the industrial environment.

However, these types of counters are heavy on the labor side and are limited in the area of automation. Because of this, they are less likely to show up in your commercial lab for routine samples. Therefore, field-to-lab comparisons won’t be comparing the same thing.

Likely, you will have two different testing methods or two different types of equipment, which may provide two different results. It’s best to use the on-site equipment for regular screening and for directing maintenance decisions.

I believe the on-site lab shouldn’t replace the commercial lab; rather, they should work in concert. Trending, tracking and confirmation may be best suited for your commercial lab.

Determining the Viscosity

Perhaps the most important property of a lubricant is its viscosity. Viscosity can be tested with several different on-site tools, including a Visgage comparator. This piece of equipment compares a known reference viscosity to the used oil sample. Both oils are normalized to the same temperature, and testing can be performed to an accuracy of 95 percent.

This is a great tool for front-line confirmation that viscosity is where it needs to be. However, keep in mind that on-site viscosity testers and commercial laboratory results will not accurately correspond to each other.

Most on-site viscosity testing units measure resistance to internal flow and shear, known as absolute viscosity, while commercial laboratories use equipment that tests the lubricant’s resistance to flow, known as kinematic viscosity.

The two measurements can be calculated off of one another, but they will never be exact due to the difference in how the tests are performed. This is yet another case for not directly comparing field tests to laboratory tests.

A Two-Pronged Approach to Oil Screening

As mentioned earlier, the goal for on-site used oil screening is not to replace your commercial lab. Ideally, you will want to work in parallel with your commercial lab. On the front line, you can catch problems before they occur and take action on them while your commercial lab confirms results and performs exception testing to further hone in on the root of the problem.

Subscribe to Machinery Lubrication