It is easy for some of us to speak about “old times” in the penetrant business. In the early days, which implies something like 30 or 40 or 50 years ago, one of the requirements of the QPL was to list the penetrant viscosity. Certificates ofcompliance with the QPL then had the viscosity of the batch of penetrant listed on them, and it was a practice to test the viscosity of inuse penetrants to assure that they were satisfactory for continued use. Changes in the viscosity can result from several sources. These include changes in the water content of water washable penetrants due to inadvertent water additions resulting from the pre cleaning process, or from the minor evaporation of some of the constituents of the penetrant. However, there is no credible evidence that any small changes in the viscosity of in-use penetrants affects their ability to perform properly. However the viscosity might change, the bottom line of a satisfactory penetrant line is the system performance test specified in ASTM E-1417 and other specifications. If this test shows that the line is performing properly, there is no reason to test the viscosity, especially since there are no known specifications to use that require the viscosity to be between certain limits, and no known documentation that would support those limits.