Entrapped air and its effects on dye penetration of voids

This study was conducted to determine the effect of entrapped air on the ability of methylene blue dye to penetrate known and measurable voids. Thirty six capillary tubes were sealed at one end and had standardized voids created within the lumen. Three equal groups of 12 test specimens and 4 controls were subjected to dye penetration by the methods of passive immersion for 7 days, centrifuging at 3,000 rpm for 5 minutes and immersion in a vacuum of 26 mm of mercury for 5 minutes. Measurement of linear dye penetration was made by removing shavings from the sealed end until dye could be observed. Negative controls showed no dye penetration and positive controls showed complete penetration. The vacuum method showed 100% penetration in all cases, while centrifuging had a mean of 91.7 /+ 8.7% penetration and passive immersion had a mean of 20.7 +/- 5.4% penetration. This study demonstrates that passive dye leakage studies are unreliable and very variable. The vacuum method whereby all entrapped air is removed is the most reliable method for dye penetration studies.