Possible Interferences
Following reasons may result in unexpected results:
1. Contamination Problems: contaminations on rod and flat electrodes affect results very seriously and obviously. In most case, the contaminations are the sole cause of unexpected results. For instance, the concentration of calcium is about 3000 ppm in engine oil. While running 50 ppm standard, a tiny of engine oil contamination on the electrodes through fingers, would seriously affect Ca results. Therefore, when unexpected results are reported, think about possible contaminations first. But what enjoyable is, a piece of paper and hygiene habits can always resolve the contamination problems.
While sample is burned in the sparking chamber, ashes and combustion residuals can be adsorbed or deposited everywhere in the chamber. Use dry paper tissues to clean the dirty stuff in order to maintain the instrument to a good and operable condition. Particularly, the groove of the rod electrode holder, the lens, and the fixture of flat electrode shall be cleaned on a daily base. Most importantly, the fixture of graphite flat electrode shall be cleaned on per sample base.
Major sources of Contaminations:
a) Operator’s fingers
b) Flat electrode plastic holders
c) Rod electrode rest screw
d) Lens of light collector
Contaminated Standardization: if a standardization procedure is affected by the above contaminations, followed test results might be erroneous. To correct this problem, carefully re-standardizing the analyzer will always resolve the problem.
2. Particle Suspension—large particles can only remain in suspension for a short time after vibration or ultrasonic bathing, depending on the viscosity of an oil sample. Wear metal and contamination elements would be significantly under-reported if the sample is not shaken immediately before poured down to a flat electrode.
3. Spectral Inferences: When the concentrations of iron, copper, and aluminum are extremely high, their spectral lines might cause the elevation of results of other elements.
Concentration of Non-Suspendable Particles (CNSP)-Virtual RFS
Because of the gravity and shapes of wear metal and dust particles, large and very large particles cannot remain in suspension in a standing sample bottle for a certain time. These particles can be defined as Non-Suspendable Particles. But in a mechanical system, the oil in-service is in constant disturbance, that is, the Non-Suspendable Particles are indeed in suspension in the modules of equipment. According to ASTM Practice D4057 or so, non-suspendable particles can be always collected into an oil sample. But whether they can be reported is another story.
Because SFE/AES method posts no limitation to the size and gravity of non-suspendable particles, COA analyzer is very sensitive to the disturbance of oil samples, that is, for the same sample, if it is tested immediately after shaken, the wear metal and silicon results may be much higher than those from the sample that stands for 10 to 30 minutes, in which the large particles settles down to bottom and cannot be sparked. The settling time of the particles depends on the viscosity of the oil in question.
Virtual Filter: exploring this advantage, the concentration of non-suspendable particles (CNSP) can be easily obtained without tedious filtering procedure. The only cost is to run a sample twice, one is tested without being shaken (Settled Run), another with shaken (Shaken Run), and a waiting time about 30 minutes:
Shaken Run (total) – Settled Run (partial) = concentration of non-suspendable particles
Just as Rotrode Filter Spectroscopic (RFS) results, CNSP represents the level of large particles in used oil, separates severe machine wear from normal one, and thus is considered directly associated with machine potential problems. If CNSP value is ignorable, ferrography and particle counting tests may not be necessary. Also, trend analysis of CNSP can be developed for close monitoring the conditions of machinery, through which mechanical failure may be found much relevant to CNSP. For on-site oil analysis where other instruments lack, CNSP results may be advisable for whether to send sample out for further tests.
COA software provides user interfaces to report CNSP (see Product page).
Standard Test Method for Determination of Wear Metals and Contamination Elements in Used Industrial Oils by Sweeping Flat Electrode Atomic Emission Spectrometry.