Overview of Rotating Disc Electrode (RDE) Optical Emission Spectroscopy for In-Service Oil Analysis

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Courtesy of Spectro Scientific

Introduction
The basis of modern oil analysis is the use of optical emission spectroscopy (OES) to measure the ppm (parts per million) levels of wear metals, contaminants and additives in oil samples. Whatever else an oil lab may measure, a multi-elemental analysis is the core of in-service oil analysis. This paper gives an overview of Rotating Disc Electrode Elemental Spectroscopy and its use for in-service oil analysis applications.

Sometime after World War II, the Denver and Rio Grande Railroad, now defunct, began analyzing diesel locomotive engine oil by looking at the spectral lines emitted by an inservice oil sample when excited by a strong electric arc using carbon electrodes. Today, spectrometric oil analysis is widely applicable to any closed loop lubricating system, such as those found in gas turbines, diesel and gasoline engines, transmissions, gearboxes, compressors and hydraulic systems. In practice, an oil sample is periodically taken from a system. The spectrometer analyzes the sample for trace levels of metals worn from moving parts as well as contamination and additive element levels. The resulting data, when compared to previous analyses and allowable limits, may indicate a sound mechanism showing only normal wear, or it may point out a potentially serious problem in its early stages. With this advanced warning, steps may be taken to correct the situation before serious damage or injury occurs.

Spectrometric oil analysis works because fine particles are generated by relative motion of metallic parts in an oil-wetted system. The lubricating oil may be thought of as a diagnostic medium because the oil carries with it the particles generated by the wear contact. Abnormal wear modes such as corrosion, abrasion, severe wear, spalling, etc., cause an increase in the concentration of wear metals in the oil. Contaminants are detected and lubricant mix-ups or badly degraded lubricants are identified by the concentration of additive elements. Multi-element analysis, coupled with knowledge of the materials of construction, often allows identification of a specific component in distress. Table 1 shows typical metal elements can be analyzed by spectroscopy and their sources.

So how does a spectrometer work?

Principles of Spectroscopy
Spectroscopy is a technique for detecting and quantifying the presence of elements in a material. Spectroscopy utilizes the fact that each element has a unique atomic structure. When subjected to the addition of energy, each element emits light of specific wavelengths or colors. Since no two elements have the same pattern of spectral lines, the elements can be differentiated. The intensity of the emitted light is proportional to the quantity of the element present in the sample allowing the concentration of that element to be determined. The light has a specific frequency or wavelength determined by the energy of the electron in transition. Since many transitions of different energy are possible for complicated atoms which have many electrons, light of many different wavelengths is emitted. If this light is dispersed by using a dispersing element such as a prism, a line spectrum will result.

These spectral lines are unique to the atomic structure of only one element. For the hydrogen atom with atomic number 1, the spectrum is fairly simple (Figure 1). On the other hand, the spectrum of iron with atomic number 26 is much more complicated with many emission lines in the visible spectrum corresponding to the many possible electronic transitions that may occur (Figure 2). If more than one element is present in the sample, spectral lines of distinctively different wavelengths will appear for each element. These lines must be separated in order to identify and quantify the elements present in the sample. Usually only one spectral line among many possible choices is chosen to determine the concentration of a certain element. This line will be chosen for its intensity and freedom from spectral line interference of other elements. To accomplish this, an optical system is required.

Figure 1: Emission Spectrum of Hydrogen

Figure 2: Emission Spectrum of Iron

Rotating Disc Electrode Optical Emission Spectroscopy (RDEOES)
Spectrometers that look at the multitude of spectral lines from a heated (or “excited”) sample are called optical emission spectrometers. All optical emission spectrometers consist of three main components, these components are:

  1. Excitation Source - introduces energy to the sample.
  2. Optical System - separates and resolves the resulting emission from that excitation into its component wavelengths.
  3. Readout System - detects and measures the light that has been separated into its component wavelengths by the optical system and presents this information to the operator in a usable fashion.

One typical method used in the excitation source in modern spectrometers is an electric discharge. The source is designed to impart the energy generated in an arc or spark to the sample. For oil analysis spectrometers, a large electric potential is set up between a disc and rod electrode with the oil sample in the gap between them. An electric charge stored by a capacitor is discharged across this gap creating a high temperature electric arc which vaporizes a portion of the sample forming plasma. A plasma is a hot, highly ionized gas which emits intense light. The light given off as a result of this process contains emissions from all the elements present in the sample. These emissions can now be separated into individual wavelengths and measured using a properly designed optical system. Temperatures in the 5000 to 6000 °C range are achieved and hard to excite elements emit enough light to be readily detected.

Since the early days of spectroscopic oil analysis, oil has been sparked (or “burned”) between a rotating carbon disc electrode and a carbon rod electrode. The sample is placed in a sample cap, the disc is partially immersed in the oil sample and the disc rotates as the burn proceeds (Figure 3). This requires about 2 or 3 ml of sample depending on the exact cap being used. A fresh disc and a newly sharpened rod are required for each sample to eliminate sample carryover. This method is called rotating disc electrode (RDE) optical emission spectroscopy (OES), or combining the two, RDEOES. Alternatively, one often sees it referred to as RDE-AES, for rotating disc electrode atomic emission spectroscopy.

The light coming from the plasma is separated by the optical system, in a spectrometer, into the discrete wavelengths of which it is comprised. An optical device called a diffraction grating is used to separate the discreet wavelengths. The diffraction grating is a concave mirror with very fine lines on its surface that causes incident polychromatic light to be separated into component wavelengths.

Figure 3: RDE Spectrometer Sample Stand Showing Oil Sample Being “Burned”

Figure 4 shows the major components of an oil analysis spectrometer using a polychromator optic based on the Rowland Circle concept. Light from the excitation process (burn) exits the fiber optic cable and passes through the entrance slit and is concentrated on the diffraction grating by a lens. The entrance slit introduces light made up of all the elements present in the oil sample and defines the shape of the spectral lines at the focal curve after it is diffracted by the grating. The purpose of the grating is to separate (diffract) this light into its component wavelengths. The spectral lines can be photographed or electronically quantified by photomultiplier tubes (PMTs) or charge coupled devices (CCDs).

Figure 4: Schematic of a Rotating Disc Electrode Optical Emission Spectrometer for Oil Analysis

An important consideration when designing a spectrometer is the region of the spectrum where the wavelengths of interest occur. Many elements emit light in the visible region of the spectrum. However, there are elements that emit mainly in the Far Ultra Violet (FUV) region of the spectrum. This is significant because FUV radiation does not transmit well through air; rather, it is mostly absorbed. For the optic to see FUV spectral lines, it is necessary for the optical system to be mounted in a vacuum chamber or otherwise filled with gas transparent to FUV light, so the emitted light can reach the grating, be diffracted, and then be detected at the focal curve. Thus, a sealed chamber and a vacuum pump or gas supply system become part of the system.