The drying rate of organic coatings varies with changes in temperature, air flow, and relative humidity. In particular, the drying rate of waterborne paints depends on the evaporation of water and thus is much more dependent on relative humidity than are solvent based paints or paints that are 100 % solids. Measurement of the rate of drying of waterborne paints under ambient conditions in laboratories cannot be adequately replicated without some control of the drying conditions. A test chamber will be described that provides a means of controlling relative humidity above ambient humidity and minimizing the effects of air flow variability at ambient room temperatures. If desired, the test chamber without water in it and with vents wide open can be placed in a temperature and humidity controlled room to test dry speed at various temperatures as well as humidity while using the chamber to minimize the effect of air flow.

This practice is particularly useful for testing the drying rate of waterborne pavement marking (traffic) paints where fast dry at elevated ambient humidity is an important feature. For waterborne traffic paints, the test chamber can be used to evaluate dry to no-pick-up (Test Method D711) and water wash-off resistance (Practices D7377 and D7538) at elevated ambient humidity.