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Pool Journal · Word of the Week
Meniscus: The Little Moon at the Water’s Edge
by Larry Szulc, Spectrum Pool Care
Look closely at water in a glass and its edge curls gently upward, like the thin curve of a crescent moon. That curve has a name, and it shows up every time you test your pool.
A meniscus is the curved surface a liquid makes where it meets the side of its container. The word comes from the Greek mēniskos, “little moon” or “crescent,” from mēnē, “moon,” because the curve looks like the thin sliver of a new moon. The same root reaches back to an old way of keeping time: month and moon are close relatives in many languages.
Water in glass curves upward at the edges, because water is more attracted to the glass than to itself. Mercury in a thermometer does the opposite and bulges into a dome. Both are the work of surface tension, the pull of water molecules on one another that lets a water strider skate across a pond and lets a glass be filled just a little past the brim.
Every edge of every pool holds a small crescent of water, curving to meet the tile.
The Pool
Where the Little Moon Shows Up
Reading a test kit
When filling a test vial, set it at eye level and fill to the line using the bottom of the curve. Reading from the top adds extra water and can shift the result.
The water line
Where water meets tile, it rises in a tiny curve. It’s why the tile line is where oils and minerals tend to collect first.
The skimmer
The skimmer draws water from the very top layer, where leaves, pollen and sunscreen float. A healthy surface flow keeps that layer moving.
Surface tension
The same pull that shapes the meniscus lets leaves and dust ride on the surface, right where the skimmer can catch them.
From our routes: a careful eye at the bottom of the curve is a small habit that makes every reading that follows more reliable.
Written by Larry Szulc, owner of Spectrum Pool Care, a family-owned company serving the San Fernando Valley, Westside, Santa Monica and Malibu since 1999.
