By Jim Fitch
Machinery Lubrication Magazine
The blotter spot test has been discussed numerous times in the pages of Machinery Lubrication magazine. It not only is one of the oldest oil analysis tests (mid-19th century) but endures as one of the most effective at detecting and even quantifying certain lubricant abnormalities.
However, the blotter spot test is not commonly known as a test for detecting and examining particles in oil such as wear debris and dirt.
As a practical matter, its ability to reveal normal and even slightly abnormal amounts of solid particles is limited, especially without the aid of a microscope. This generally is true with other applications of blotter spot testing.
In other words, the lack of a visible structure (rings, starbursts, pasty center, etc.) is an indication of the absence of the target condition. Because of this, the blotter spot test is less likely to produce a false negative compared to other more advanced analytical methods.
While each method has its own unique interferences and lower sensitivity limits, the ability of blotter spot testing to provide a reliable alert to abnormal particle concentrations is undisputable.