Detecting metal particles in oil- the testing misconception
Q: “ I am sick of this nonsense about the limitation of ICP (inductively Coupled plasma) and RDE (rotating disk electrode) spectrometers to large wear particles. Has anyone ever heard of acid/microwave digestion? Every commercial lab who is capable of offering this technique, eliminates the large particle concern, and they just don’t bother to offer it to their clients”.
- Forwarded to us by an Internet chat board by a confused reliability engineer.
A: Whoa! There are several issues brought up here, and each needs a discrete response. Let’s take each of these statements in turn.
- “Nonsense about limitation of ICP and RDE to large wear particles….”
ICP, RDE, and almost all Optical emission spectrometers have only a finite amount of energy available to energize a sample. The bigger the particle size in the sample, the more energy needed to convert to plasma. If you do not have the energy available, the sample will not be completely energized, resulting in “blindness” to the large particle. So the statement is strictly true. - “Anyone heard of Acid digestion…”
Yes!!!!!! To anyone who has performed chemical analysis with the instruments we refer to here, all would agree to the limitation to large particles. However in the same breadth, a preparation technique to get around the large particle problem would be suggested as it was here. Acid/microwave digestion is a preparation technique widely used, especially those engaged in environmental testing, wastewater analysis and biological work. The sample material is reduced to sub micron slurry, easily detectable by ICP or RDE spectrometers. - “Every lab can do it, just don’t bother to offer it to their clients…”
Here’s where things get interesting. Strictly speaking about labs that offer oil analysis, No, not every lab offers this preparation technique. Almost all labs involved in this business use the “dilute and shoot” approach, that is, there is no preparation technique other than dilution before the sample is energized. There are a number of thoughts as to why this is the norm for oil analysis, but the most common reasons are based on the fact that: - Oil analysis is encouraged to be periodic and timely in order to make maintenance decisions about equipment. The preparation time using acid digestion for a large quantity of samples is quite time consuming, to a point that by the time the analysis is complete, it no longer has much value in making maintenance decisions.
- Cost of the preparation was considered to be too high relative to the goal of the analysis is, i.e. develop a wear metal trend- this preparation adds a 4 to 10 times increase in the cost of the analysis
- Early users of oil analysis were engine owners, who typically have a more predictable wear trend composed primarily of small (less than 5 micron diameter) wear particles. Extra preparation techniques did not influence the decision taken from the basic dilute and shoot approach.
It is worth noting that these points were formulated at a time when the oil analysis industry centered around engine oil monitoring, which had to be fast and cheap, with the tradeoff being you would detect most but not all the wear debris of interest.
We argue that “dilute and shoot” is acceptable for metals analysis for oil, however an extra analysis is always necessary for critical samples, particularly those from industrial equipment. Currently most oil analysis users who are getting their analysis from lubricant suppliers and several commercial labs are not getting digestion prepped metals, but straight dilution of the oil into the spectrometer, and NO extra tests (such as ferrography, Rotrode filter spectroscopy (RFS), X ray fluorescence (XRF) etc.) for large debris. That’s a problem, because missing that large debris can mean missing the early warning of a serious failure. These tests should be done even if acid digestion is performed before spectroscopy, as morphology and size of large particles is critical information.
The oil analysis industry is shooting itself in the foot here, because cases will and do occur where customers are holding up their “Satisfactory Condition” oil reports for a machine that just failed without warning!
-DPW
