Compressed air is widely used in various industries, but do you know how to check whether compressed air contains oil? How to test an oil-free air compressor system?

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Oil-free compressed air is widely used in the food industry. The main factors affecting the quality of compressed air are oil, water, and solid particles; however, oil content is the hardest to detect. For this reason, various measurement techniques and methods have been developed in the industry, but their accuracy is not high. The common methods for determining oil content currently include gravimetric analysis, ultraviolet spectrophotometry, infrared spectrophotometry, chemical indicator tube method, particle size spectrometry, photoionization detector method, and gas chromatography-mass spectrometry combined technology. The gravimetric method is a technique in which a certain amount of sample gas is placed in a container or medium with a constant weight (such as filter paper or filter membrane), and after evaporation, drying, and reaching a constant weight, the analysis results are determined. The gravimetric method was the first technique used to determine oil content, and it is still employed by many people to measure the oil content in water bodies with high oil levels. When using the gravimetric method, it is necessary to strictly control the moisture level of the sampling tube, absorbent cotton, and quantitative filter paper, as well as the impact of dust contamination, and to prevent contamination by other oils. During sampling and analysis, it is absolutely not allowed for any additional debris to adhere to the container or medium being weighed. Weighing with a balance must be done quickly and accurately, as mechanical and dust particles in the air can affect the accuracy of the oil content measurement. However, the low sensitivity of the gravimetric method makes it unsuitable for accurately determining the oil content in gases, as the oil content in gases is quite low; using the gravimetric method is clearly inappropriate in such cases. Infrared spectrophotometry and ultraviolet spectrophotometry have become one of the methods widely used in China, thanks to their advantages such as short testing time, simple sampling procedures, and high measurement accuracy. The ultraviolet spectrophotometry method is based on the fact that aromatic compounds containing conjugated bonds and benzene rings in oil have characteristic absorptions in the ultraviolet region; therefore, this method can be used to determine the concentration of substances with conjugated double bond structures, thereby assessing the oil content in gas samples. The theoretical basis of this method is the Lambert-Beer law of light absorption. That is: A = cbc, where A is the absorbance; e is the absorptivity coefficient; c is the concentration; and b is the path length. Oil substances all have characteristic absorption bands in the ultraviolet region. Aromatic compounds containing benzene rings absorb light primarily at wavelengths of 250–260 nm. The two absorption peaks of crude oil are generally at 225 and 256 nm respectively. Compounds with conjugated double bonds absorb light mainly at wavelengths of 215–230 nm. The absorption wavelengths of other oils such as fuel oil and lubricating oils are also similar to those of crude oil. Different samples have different absorption values. To achieve high sensitivity and accuracy in the measurement results, it is necessary to carefully select the incident wavelength. The key is to make the detection wavelength equal to the absorption wavelength of the substance being measured; the selected wavelength should be both the substance’s maximum absorption wavelength and its optimal absorption wavelength. Only when all the above conditions are met can it be considered as the measurement wavelength for that substance. . The chemical indicator tube method: The main manufacturers of instruments based on the principle of this method are Johnson Controls in the United States and Dräger in Germany. Johnson Controls’ oil level indicator contains a gas-detecting agent that reacts with the oil to form a colored layer; quantitative analysis is carried out based on the intensity of the color of this layer or the length of the colored area. Delge’s oil detection method involves capturing oil mist in the indicator layer of the oil detection tube; under the action of a catalyst, this mist reacts with concentrated sulfuric acid present in the tube to produce a black substance. The intensity of this color is correlated with the oil content, and colorimetry is used to determine the oil content in the air. This method generally only indicates whether the oil content is above a certain value, without allowing for the determination of an exact numerical value. This method is simple to use for sampling, but it takes a long time; it can only indicate the approximate range of oil content without providing specific values. . Particle size spectrometer method: A particle size spectrometer is an instrument that counts suspended particles by utilizing the principle of light scattering; therefore, it is unable to distinguish between suspended oil and solid particles. However, this method can be used to measure the oil content in specific gases (such as those containing only oil particles), and its greatest advantage is its fast measurement speed, allowing for real-time testing. Therefore, this method is widely used on test benches; for example, on test benches for filter performance, solid particles in compressed air are removed in advance, followed by the intentional addition of oil. A particle size spectrometer is then used to measure the concentration of suspended oil before and after the filter, thereby determining the filter’s efficiency in a convenient and reliable manner. The photoionization detector method: In recent years, German companies Beckum + Weil and Hilscher have introduced online instruments for measuring oil content, utilizing the photoionization detector method. This is a technique for measuring the concentration of hydrocarbon vapors through a PID (photoionization detector). In principle, this method mainly measures oil vapor, so it is generally used for online monitoring of changes in the oil content in compressed air; however, it is difficult to accurately measure the total amount of oil present. Gas chromatography-mass spectrometry technology is primarily used for measuring oil vapors. Its principle involves using activated carbon to absorb the oil vapors, followed by analysis and measurement using gas chromatography-mass spectrometry technology. As the requirements for the oil content level in compressed air become increasingly stringent, the proportion of oil vapor also increases, and this method will thus become an important approach for measuring oil content.