RDE Atomic Emission Spectrometer for Lube oil Analysis

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)