You guessed it. They both can forecast catastrophe.
The use of canaries in coal mines dates to 1911 when miners carried them in cages deep into underground shafts. Canaries, being sensitive to presence of toxic gases including colorless, tasteless and odorless carbon monoxide, were sort of a sacrificial prognostic intelligence system. If there were poisonous gases in the shafts, the birds provided a vital early warning by collapsing or even dying, allowing the miners to make a quick escape.
Ferrous density (FD) meters can foretell disastrous events as well, often in time to save a machine from sudden operational failure. Their role is to monitor the density of a single type of wear metal commonly found in oil or grease: large ferromagnetic particles (typically greater than five microns). A ferromagnetic particle is usually composed of cast iron or low-alloy steel. Such common metallurgy is deployed in a wide range of frictional machine surfaces including bearing rolling elements/raceways, gears, pistons/cylinders, cams/followers and many others.
In fact, it would be rare if at least one of the surfaces in frictional pairs was not composed of iron or steel. The opposing surface may be babbitt, brass, aluminum or steel. Typically, it is this ferrous surface that is the most critical to long-term machine reliability while, in contrast, the nonferrous may be regarded as sacrificial. As such, a sudden change in the concentration of large ferromagnetic particles in a lubricating oil or grease could be the telltale precursor that averts a chain reaction failure or other catastrophic event.
Comparing Elemental Iron
In oil analysis, trending the concentration of iron by means of spectrometric elemental analysis, is helpful and an important assessment of machine condition. However, this is not an alternative to ferrous density measurements. It is widely understood that elemental spectroscopy has limited sensitivity to particles larger than five microns. It is also widely understood that these larger particles are often the most reliable indication of an impending failure condition. Many studies have shown that the more advanced the state of failure the larger and more concentrated the wear particles become. It is even possible for elemental iron to trend downward during the crucial period just prior to machine failure (the calm before the storm). As such, when it comes to wear debris, the concentration of small particles alone is not always a reliable indication of the presence large particles.
Ferrous density meters are known by many names including direct reading ferrograph, particle quantifier, wear particle analyzer, ferrous particle counter and so on. There are approximately 10 different commercial instruments targeted for use, either online or in laboratory applications. However, most of these are bench-level instruments intended for on-site oil analysis laboratories. For this reason, they are most commonly used as a screen for analytical ferrography. After all, it does make sense to avoid the time and expense of preparing a ferrogram if there are relatively few ferromagnetic particles available for inspection.
Partnering Elemental Iron with Ferrous Density
Unlike elemental iron, most particles in the size range detected by ferrous density meters are also in the size range that can be filtered. As such, if filters are in use at or around five microns (say Beta (5) greater than 200), then the concentration of ferrous particles should remain fairly stable and low. In the event that the wear rate should sharply increase, the material balance is lost and the equilibrium ferrous density reading rises to a new plateau. This of course is a reportable alarm condition. By contrast, the concentration of elemental iron can increase progressively over the service life of the oil, reaching levels in excess of 100 ppm, without concern in some cases. It is the rate of increase of iron, not its specific concentration that typically triggers alarms.
In general though, any unusual increase in either elemental iron or ferrous density is a cause for concern. In most cases, exception tests should be performed to better characterize the nature and severity of the problem. By partnering elemental iron with FD, more precise diagnostics and prognostics can usually be achieved. Following are general guidelines on these technologies when used in tandem:
Iron and FD Both Trending Upward
Iron Stable but FD Trending Upward
FD Stable but Iron Trending Upward
In my view, machines that have critical ferrous surfaces and high mission criticality are great candidates for ferrous density meters. Sadly, most commercial oil analysis programs tend to detour around the use of this valuable technology. Perhaps you should review the tests included on your oil analysis reports to see what is missing. It could be a life-saving canary.