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With the widespread use of compressed air, different industries have varying requirements regarding its quality. The main factors affecting compressed air quality 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 at present include gravimetric analysis, ultraviolet spectrophotometry, infrared spectrophotometry, chemical indicator tube method, particle size spectrometer method, photoionization detector method, and gas chromatography-mass spectrometry combined technique, among others. 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, its weight is measured to determine the analytical results. 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 levels of oil. 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 pollution 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 impurities 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; therefore, using this method is clearly inappropriate. 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 structure, and high testing 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 represents 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 the above conditions are met simultaneously can it be used as a determination wavelength for that substance. The chemical indicator tube method – the instruments manufactured using this principle are mainly produced by 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 determines 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 determine the approximate range of oil content without providing specific values. The 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. In recent years, German companies Beckum + Heun and Hilsch have each introduced online testing instruments for 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 primarily 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 increases, and this method will also become an important approach for measuring oil content. The most common oil content detectors for compressed air available on the market currently include the following: 1. Dräger Aerotest. Dräger’s oil tubes and oil containers are used by Dräger’s compressed air quality detectors to measure the oil content in compressed gases in applications such as pipelines, air compressors, and hyperbaric chambers; they enable qualitative detection of oil concentration and comply with national standards. Application range of Delge Oil Detection Kit 8103560: Used to detect oil mist and oil vapor in compressed air; no other information regarding the compressed air is provided. Standard measurement ranges: 0.1mg/m3, 0.5mg/m3, or 1.0mg/m3. Detection limit: 0.05mg/m3. Environmental conditions for oil mist: Temperature: 10°C to 30°C; Humidity: Up to 60% relative humidity; Pressure: Suitable only for compressed air that is not under pressure. Purpose of this product: To determine the concentration of oil in compressed gases, especially in compressed air. Measurement ranges: 0.1mg/m3, 0.3mg/m3, 0.5mg/m3, 1.0mg/m3. Detection limit: As indicated by the color changes in the oil type chart. Color changes: White --> Light brown or yellow. Environmental conditions: Temperature: 10–30°C; Pressure: Suitable only for compressed air that has been depressurized. Advantages: Different detection limits for various concentrations (0.1/0.5/1.0mg/m3); signals are generated for all synthetic oils; lower detection limit of 0.1mg/m3; easier to read; independent of oil type or viscosity. Disadvantages: This type of detector can only determine the oil content in air through color comparison, and this method generally only indicates whether the oil content is above a certain value, without providing an exact figure. This method is simple to use for sampling, but it takes a long time; it can only determine the approximate range of oil content without providing specific values. 2. Hillside SUTO S120 is an oil residue detector used to measure the oil content in compressed air and other gases within the limits specified by the technical parameters. It is mainly used in compressed air systems in industrial environments. But it cannot be used in explosive environments. Measure the oil vapor content in compressed air or other gases. Advantages: • Connected via a sampling tube and a quick connector. • It can be used for fixed or portable applications. • The measured minimum oil content is as low as 0.003 mg/m3. • PID sensor, high precision. • LED indicators indicate services and alerts. • It can be connected to a CS-iTEC display and data recorder, or third-party display and control devices. • IP65 enclosure provides excellent protection even in harsh industrial environments. • The built-in display of the residual oil detector (optional) can directly show the actual values. It is a relatively accurate detector for measuring the oil content in compressed air, capable of providing precise numerical values and displaying them visually in the form of curves. However, it should be noted that it should not be used to detect excessively high levels of oil, as this can damage the device due to high oil contents. 3. German CS Instruments gas measurement device – OILCHECK 400, a compressor air oil content detector. Advantages of the OILCHECK 400, a fixed-type device for detecting oil content in compressed air from German CS Instruments: · Equipped with a PID sensor that ensures high-precision measurement of residual oil as well as oil vapor (using a photoionization detector). · An ideal choice for mobile measurements: the PID sensor can provide accurate results within about 30 minutes. · Automatic zero-point calibration ensures long-term stable measurement results. The integrated mini-catalyst is reliable and capable of generating reference gas for zero-point calibration. The accuracy of measurements using the reference gas provided by the activated carbon filter is affected by the aging and saturation of this filter. The catalyst that generates \"zero air\" does not age or wear out, eliminating the need to replace the activated carbon filter; sampling can be done easily through PTFE hoses or stainless steel tubes