The oil film thickness measuring instrument achieves non-contact measurement through optical or electrical principles. The core principles and methods are compared as follows:
Optical measurement methods
Optical interference method
Calculate the thickness by utilizing the interference fringes generated by the reflection of light on the upper and lower surfaces of the oil film. When the incident light penetrates a transparent substrate (such as a glass disk), the two beams of light reflected from the oil film and metal surface interfere, and the spacing or movement of the stripes is inversely proportional to the thickness of the oil film. For example, PCS' EHD2 measuring instrument in the UK is based on the principle of ultra-thin film optical interference, which can achieve precision measurements of 1-1000nm and synchronously obtain the traction coefficient. This method has high resolution and strong visualization, but requires a transparent substrate and is sensitive to environmental vibrations. It is commonly used in laboratory research.
Infrared absorption method
Based on the absorption characteristics of oil substances towards specific wavelengths of infrared radiation. After the infrared light source penetrates the oil film, the remaining light intensity changes are detected, and the thickness is calculated using the Beer Lambert law. For example, the infrared oil film thickness gauge can quickly measure the oil film on metal surfaces with an accuracy of micrometers, and is less affected by environmental light interference, making it suitable for industrial sites.
Electrical measurement methods
Capacitance method
Calculate the thickness by measuring the capacitance between the upper and lower electrodes of the oil film. The capacitance is inversely proportional to the film thickness and is suitable for full film elastohydrodynamic lubrication scenarios. But when the film thickness is less than 0.5 μ m, electrical breakdown is prone to occur, and the unstable dielectric constant of the lubricant can lead to errors, which is mostly used for contact measurement.
Discharge voltage method
Applying voltage between metal friction pairs, the discharge voltage is linearly related to the film thickness (for every 1 μ m increase in film thickness, the voltage rises by approximately 0.18V). This method has been used for measuring gear oil film, but its accuracy is greatly affected by the cleanliness of the lubricant.
Method comparison and recommendation
Accuracy and Range: Optical methods (such as optical interferometry) can achieve nanometer level resolution, suitable for thin oil films; The electrical method (such as capacitance) has a large range, but its accuracy is limited by the stability of the medium.
Application scenarios: Optical interferometry is commonly used in laboratory tribology research; Infrared absorption method and capacitance method are more suitable for industrial sites; The discharge voltage method has gradually been phased out due to significant errors.
Development trend: Optical methods have become mainstream due to their non-contact and visualization advantages, and the combination of fiber optic sensing or machine vision technology can further enhance dynamic measurement capabilities.