After the end of world war ii, the United States department of railway in Denver, Colorado was carried out with atomic emission spectrum analysis of diesel engine oil in the related work, they through the high pressure generated between two graphite electrode arc implements the various elements in the sample to be measured in the excitation of the atom, and then to inspire study analyzed the characteristics of each element of lines. At present, oil element spectral analysis technology (oil spectrometer or oil spectrometer) is widely used in oil monitoring of various closed-loop lubrication systems, including gas turbine, diesel/gasoline engine, transmission system, gear box, compressor and hydraulic system, etc. The analysis of the typical process for: drawn from the controlled equipment lubrication circuit part of the sample, using spectrum analyzer analysis sample to be tested in the characterization of the wear, contamination and the various elements of additives composition and concentration, by referring to the alarm limit of historical data and has been set, the equipment lubrication, pollution and wear conditions are measured and evaluated. Based on the analysis results, the equipment maintenance management personnel can take corresponding measures as early as possible to effectively reduce the operation risk of the equipment.
In the lubrication system, the relatively moving two workpiece surfaces will produce tiny wear particles. The essence of wear analysis on the equipment by spectrometer is to analyze the element composition and concentration of these tiny wear particles. As the testing medium, the lubricating oil being used by the equipment contains the tiny particles produced by equipment wear, and the concentration of these particles corresponds to the wear degree of the equipment. In general, the occurrence of abnormal conditions such as corrosion, severe wear, and ablation will directly cause the concentration of some elements to increase significantly. Oil contamination, mixed use and aging state are identified and determined by monitoring the change of contaminating elements or additive elements in the measured oil sample. Combining the results of element analysis with the design and selection of key parts of the equipment, the wear position of the equipment can also be directly judged. Typical measured elements and their sources in the spectral analysis results of oil elements are listed in Table 1.
The principle of spectroscopic analysis
Spectral analysis technology refers to the detection method of identification and quantitative analysis of various elements in the measured material. Each element has a unique atomic structure that emits light of a specific wavelength (or color) (called a signature line) when excited by external energy. There is no case in nature where the characteristic lines of two different elements coincide exactly. Therefore, different primitives (elements) can be recognized by identifying characteristic spectral lines. At the same time, the intensity of the characteristic spectral line is proportional to the concentration of the corresponding element in the material to be measured. Therefore, the element concentration can be measured by measuring the intensity of the spectral line. Since it is impossible for the material to be measured to be composed of a single element, light of various wavelengths will be emitted in the excitation state. It is also necessary to disperse the light of various wavelengths through a prism and other spectroscopic devices so as to analyze the spectral line by line.
These characteristic lines correspond to specific atomic structures (elements). In the case of hydrogen, which has an atomic number of 1, the emission spectrum is very simple (see Figure 1). However, for iron with an atomic number of 26, the emission spectrum becomes a little more complicated because of the various transitions in its outer electrons (see Figure 2). When there is more than one element in the measured substance, a series of spectral lines corresponding to each element at various wavelengths will appear in the spectrum. These spectral lines must be separated in order to identify and quantify the target element. In the process of spectral analysis, only one of its multiple characteristic spectral lines is usually selected to realize the determination of an element composition and its concentration. The selection of the spectral line requires a comprehensive consideration of the intensity (also known as optical density) of the spectral line and its interference with the characteristic spectral lines of other elements. Therefore, each atomic emission spectrometer contains a whole set of complex optical system, and the accuracy and reliability of the optical system directly determine the accuracy and reliability of the spectrometer.
Figure 1. Emission spectra of hydrogen
Atomic emission spectrometry based on rotating disc electrode technique (RDE OES)
A typical excitation source of atomic emission spectrometer is based on the principle of discharge, that is: arc excitation (the design of the excitation source is the electric arc or spark generated by the discharge phenomenon directly on the sample to achieve the excitation process). During the operation of the oil spectrometer, the large capacitor in the excitation source charges the electrode. There is a huge potential difference between the graphite disk electrode and the rod electrode. When the potential difference between the electrodes reaches the discharge state, the high voltage discharge phenomenon (generating instantaneous high temperature) will be generated at the gap between the disk electrode and the rod electrode. The measured oil sample existing in the discharge gap will be gasified and isoionized under the action of high temperature arc, and the isoionized measured oil sample (containing various elements) will be excited out of the corresponding characteristic spectrum. The instantaneous temperature of the discharge gap can reach 5000-6000 degrees Celsius, which can fully excite some elements that are difficult to excite and produce stable emission spectrum. The optical system of the spectrometer then collects, distinguishes and quantifies the emitted spectra.
During the operation of the oil spectrometer, the measured oil sample is excited at the discharge gap between the rotating graphite disk electrode and the rod electrode. The measured oil sample is placed in a small oil cup, and the graphite disk electrode is partially immersed in the small oil cup (oil sample). The rotating disk electrode continuously transfers the measured oil sample to the discharge gap, thus realizing a continuous "sample burning" process (as shown in Fig. 3). A sample burning process requires 1-2ml of the measured oil sample. At the same time, in order to effectively avoid the cross-contamination between different oil samples during the firing process, a new graphite disk electrode and a well-dressed rod electrode are needed for each sample firing process. This method is known as RDE (Rotating Disk Electrode) Atomic Emission Spectroscopy (OES), or RDE-OES, and is also often referred to as RDE-AES(Atomic Emission Spectroscopy).
Figure 3. Analysis process of oil spectrometer based on RDE technology (called burning sample)
Emission spectra are produced after isoionization of the measured oil sample. The diffraction grating in the optical system of the spectrometer separates the emission spectrum into several discontinuous spectral lines. Diffraction gratings in the spectrometer are specially designed concave spherical gratings, which contain a series of precise lines on the surface. Due to the different diffraction angles of different wavelengths, the emission spectra of multi-color light containing various elements will be divided into discontinuous and independent characteristic spectral lines after passing through the diffraction grating. The optical system design of the NFL oil spectrometer is shown in Fig. 4.
Fig. 4. The optical system used in the SCNFL oil spectrometer
The composition of an oil spectrometer based on RDE technology is shown in Figure 5. The spectrometer (spherical grating) is placed on the Rowland Circle specially designed by the installation platform. The excited light passes through the optical fiber, enters the incident slit, and is focused to the diffraction grating under the action of a lens. The incident slit transmits multicolor light containing the emission spectrum of various elements to the diffraction grating and determines the shape of the characteristic spectrum lines that pass through the grating. The function of grating is to separate multicolor light into independent, discontinuous, monochromatic light of a single wavelength. The monochromatic light is then quantitatively analyzed using a photomultiplier tube (PMT) or a charge-coupled device (CCD)