Bulk Water versus Surface Conditions
Definition
The distinction between what a water test measures in the bulk fluid and the very different chemistry occurring at metal and deposit surfaces.
Most water tests describe the bulk water, but most failures happen at the surface. Bulk pH may look acceptable while tube skin temperature drives local calcium carbonate precipitation, and bulk biocide residual may be present while the inside of a biofilm stays chemically protected. Every free body diagram should ask what is likely happening at the surface that the bulk water does not reveal.
Related concepts
Scale · Biofilm · Under-Deposit Corrosion
Part of The Blueprint of Water · 📏 measurement