Rotrode
type Atomic Emission Spectrometer for lube Oil Analysis:
Introduction
For the past forty
years or so, spectrometric analysis of used oil samples has been applied as a
machine health monitoring technique (the terms condition monitoring and
predictive maintenance are more commonly used today). Spectrometric analysis
determines the elemental concentration of various wear metals, contaminants,
and additives present in a used oil sample. Results are usually reported in ppm
(parts per million). Commercial oil analysis laboratories report on as many as
32 different elements.Atomic emission spectroscopy was relied on to provide an
insight into abnormal wear rates, even though the technique was known to have
decreasing sensitivity as particle size increased.
Particle Size Limitation of Spectrometers
What is not widely
appreciated is that spectroscopy is more-or-less blind to the larger particles
in an oil sample, precisely those particles which are more indicative of an
abnormal wear mode. Most severe wear modes such as spalling, severe sliding
wear, and cutting wear generate large particles which go undetected by
spectroscopy. Large contaminant particles are also missed by spectroscopy. The
particle size at which spectrometers begin to lose their detection ability
depends on a number of factors including spectrometer make and type, but it is
generally agreed that spectrometers lose their ability to detect particles in
the 1 to 10 micrometer range. For purposes of this note, particles larger than
10 micrometers will be called large.
SPECTROMETERS MEASURE
ONLY VERY SMALL PARTICLES AND DISSOLVED MATERIAL IN OIL
Consequently,
spectrometer readings increase steadily between oil changes because, unlike
large particles, small particles and dissolved material are not captured by
filters, nor do they settle out easily in piping or tanks.
Traditional
Methods of Determining Large Particles
Acid Digestion Method
Acid digestion
methods have been developed whereby the particles in an oil sample are filtered
and then dissolved using such acids as hydrochloric, nitric, and hydrofluoric.
The resultant sample is then processed very carefully in a specially prepared
spectrometer. This expensive and hazardous method has proven too costly for
regular use in predictive maintenance.
Microwave Digestion Method
Microwave digestion
methods are in use developed whereby the particles in an oil sample are
collected and then dissolved with microwaves in a specially designed autoclave.
The resultant sample is then processed very carefully in a specially prepared
spectrometer. This method does provide a total concentration of elements in the
sample, but it is expensive and time consuming. Like the acid digestion method,
you do not know the ratio of large and small particles in the original sample.
Rotrode Filter
Spectroscopy
RFS (Rotrode Filter
Spectroscopy) was developed to provide an improved spectroscopic method for
analysis of used oils for condition monitoring/predictive maintenance without
the particle size or metal-type limitations of previous combined
spectrochemical and DR ferrographic techniques.
This patented
method uses a rotating disc electrode spectometer, known as an RDE spectometer,
already in use in many military and commercial laboratories which perform spectrographic
oil analysis
Method
·
In the RDE
spectrometer, a graphite carbon disc is pressed onto the end of a shaft which
rotates causing the disc to rotate.
·
In the normal use
of this spectrometer, a quantity of oil is poured from the sample bottle into
the sample bottle cap and positioned so that the bottom of the rotating disc
passes through the oil.
·
A spark gap is
formed between the top of the rotating carbon disc and the tip of the carbon
rod electrode.
·
An electric
discharge across the gap vaporizes the oil which has adhered to the rotating
disc. The light emitted contains wavelengths characteristic of the elements in
the oil sample.
·
The spectrometer
optics and electronics quantify these wavelengths and report in ppm of up to 20
elements in 30 or 40 seconds. The carbon discs are known as rotrodes.
Preparation
Station
The rotrodes are
porous/Graphite carbon discs and this characteristic makes them ideal filters.
A fixture has been designed to to clamp the discs so that oil can be drawn
through the outer circumference of the discs when a vacuum is applied to the
inside (hub) of the discs. The particles in the oil are captured by the surface
of the rotrode. The oil is then washed away with solvent, the disc is allowed
to dry, and the particles are left adhered to the rim of the rotrode in just
the right position to be vaporized and detected when the rotrode is
"zapped" in the RDE spectrometer. The sensitivity of the method is
excellent. A multi-station fixture is used so a number of samples can be
filtered at once. The procedure is fast and economical to perform. It is an
ideal screening test for analytical ferrography.
Advantages
of Technique
·
The technique has
several advantages which make it a powerful predictor of equipment failure.
Most abnormal wear modes cause a significant increase in concentration and size
of wear particles. Using porous graphite rotrodes as a filtering media, large
particles are captured and subjected to RDE (Rotating Disk Emission)
Spectroscopy to obtain a multi-elemental analysis. These captured coarse
particles are measured essentially independently of fine and dissolved particle
contaminants in the sample. Removal of the used oil from the spectroscopic
analysis reduces the energy required to vaporize the sample. This, in effect
presents a more concentrated particle sample to the plasma produced during the
RDE spectroscopy, and lead to greater sensitivity of the instrument.
·
By combining such
large particle results with conventional RDE analysis of the dissolved and fine
particles in the oil sample, a complete wear analysis picture for a machine of
interest can be obtained. After seven years of applying this new technology in
our lab, we and our customers are convinced of its powerful diagnostic
capability.
·
Rotrode Filter
Spectroscopy is fast and efficient, and is used as a standard screening test
for every oil sample entering the lab.
·
The large particle
data will flag abnormal wear modes at a very early stage, indicating when an
analytical ferrogram should be made, and be of great benefit in defining the
metallurgy of the wear particles.
·
The data is also
excellent for contamination analysis as it will give elemental composition of
large contaminant particles( such as silicon). The nice feature of this is that
you may determine whether the element is sourced from an addtive package (such
as a silicone polymer for defoaming) or a contaminant(sand/dirt particles)
