The Kester PS-22 is a kit used to determine the acid number of the flux. The test is a simple drop test with an indicator for the endpoint (similar to a swimming pool test). You count the drops and then go to a chart. From the chart you determine how much thinner to add to bring the acid number back down to specifications. - The accuracy problems encountered with automatic specific gravity controllers in conjunction with low-solids "no-clean" fluxes make the flux kit a better alternative for process control. Control of the flux concentration in the flux becomes more critical when using a low solids flux.