The air compressor Blotter Test is performed before sandblasting process i.e. SSPC-SP 5 (white metal cleaning), SSPC-SP 10 (Near-White Metal Cleaning), etc. The test can be performed with white absorbent paper. Place the absorbent paper in the air flow of the compressor for few minutes and then look for any visible oil and water. The ASTM D 4285 requires the absorbent paper mounted on the rigid backing, and the test to be conducted on the discharge point close to the use point, also the test should be done after inline oil and water separators. The testing intervals is specified on the coating specification.
As per ASTM D 4285, any oil indication (discoloration of white absorbent paper) is a cause for rejection of air compressor. Similarly, water indication of absorbent paper is not acceptable for sandblasting work, and air compressor should be rejected and subjected to the repair.
To distinguish between oil and water contamination, use an ultraviolet light and a dark room, stay for 5 minutes at the dark room to get your eyes adjusted. Then flash ultraviolet radiation on the absorbent paper surface, if it was shiny or glossy, it means your contamination is an oil contamination otherwise, it is a water contamination.
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