Process Control of Temperature
The temperature of the penetrant materials and the part being inspected can have an effect on the results. Temperatures from 27 to 49oC (80 to 120oF) are reported in the literature to produce optimal results. Many specifications allow testing in the range of 4 to 52oC (40 to 125oF). A tip to remember is that surfaces that can be touched for an extended period of time without burning the skin are generally below 52oC (125oF).
Since the surface tension of most materials decrease as the temperature increases, raising the temperature of the penetrant will increase the wetting of the surface and the capillary forces. Of course, the converse is also true, so lowering the temperature will have a negative effect on the flow characteristics. Raising the temperature will also raise the speed of evaporation of penetrants, which can have a positive or negative effect on sensitivity. The impact will be positive if the evaporation serves to increase the dye concentration of the penetrant trapped in a flaw up to the concentration quenching point and not beyond. Higher temperatures and more rapid evaporation will have a negative effect if the dye concentration exceeds the concentration quenching point, or the flow characteristics are changed to the point where the penetrant does not readily flow.
The method of processing a hot part was once commonly employed. Parts were either heated or processed hot off the production line. In its day, this served to increase inspection sensitivity by increasing the viscosity of the penetrant. However, the penetrant materials used today have 1/2 to 1/3 the viscosity of the penetrants on the market in the 1960's and 1970's. Heating the part prior to inspection is no longer necessary and no longer recommended.