T.O. 33B-1-12-60Figure 2-21. The Effects of Optimum, Insufficient, and Excessive Hydrophilic Removal.2.5.5.5.1Another advantage of hydrophilic remover is its relative insensitivity to removal of penetrant entrapped in adiscontinuity. This permits complete removal of fluorescent background in most cases. In contrast, when usinglipophilic emulsifier on slightly rough surfaces, it is desirable to leave a faint residual background when maximumsensitivity is required. The reduction of background fluorescence with the hydrophilic technique improves the contrast,making faint indications easier to see. The hydrophilic method also allows spot or touch-up removal on local areasduring the final clear water rinse. This cannot be done with the lipophilic method, since the oil base emulsifier will nottolerate water. The use of hydrophilic emulsifiers also provides better control, handling and recycling of the penetranteffluent. This can significantly decrease waste water treatment costs and minimize water pollution.Table 2-4. Comparison of Hydrophilic versus Lipophilic Methods.Hydrophilic Lipophilic1. Supplied as a concentrate 1. Supplied as a ready to use fluid2. Water base when mixed 2. Oil base3. Low viscosity 9 to 12 cs 3. High viscosity 35 to 120 cs4. Limited penetrant tolerance 4. Miscible with penetrant in all concentrations5. Miscible with water in all concentrations 5. Limited water tolerance6. Applied as dip or spray 6. Applied as a dip7. Action: Dip-detergent with scrubbing wash 7. Action: Diffusion activated by scrubbing8. Reduced drag-out 8. Critical emulsion time
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