Cleanliness refers to the amount of contaminants remaining on the surface of a part or product after cleaning. The indicators of dirt include type, shape, size, quantity, weight, etc. The specific indicators used depends on the degree of influence of different contaminants on product quality and the requirements for the accuracy of cleaning control.

The purpose of ensuring the cleanliness is to enable the product to reach the specified service life, and not to shorten the service life due to contaminants in the process of manufacture, use and maintenance. By measuring the cleanliness and limiting its numerical value that the damage of contaminants to the product can be avoided or reduced. Commonly used detection methods are as follows:

Visual inspection method: that is, the outer surface or inner cavity surface that can be seen by the part is inspected directly with the eyes under the microscope. Larger and obvious particles, spots, rust spots and other contamination remaining on the surface of the part can be checked. Contact angle method: The angle held by a liquid when it is in thermodynamic equilibrium on a solid surface. A large contact angle indicates that the surface is contaminated with hydrophobic contaminants (oil/grease, etc.), whereas a small contact angle indicates that the surface is clean when the droplet is broken or thinned. Fluorescence method: Under the irradiation of ultraviolet light, the contaminants particles on the surface will emit fluorescence. Gravimetric method is the most commonly used method to determine cleanliness in industrial production and test. The measuring principle is to wash a certain number of samples under certain conditions, and then fully filter the cleaning liquid through the filter membrane. The contaminants is collected on the surface of the dried filter membrane, and the filter membrane is fully dried again. According to the analytical balance, the mass of the dried filter membrane before and after the filter cleaning is weighed, and the added numerical value is calculated as the mass of the solid particle pollutants on the sample.

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