Sand Filter
A filter using sand, or sand and gravel as the filter medium.

Sanitize
To kill all microorganisms, including bacteria and algae, and to remove unwanted contaminants.

Scale
The deposit that forms on surfaces in contact with water when the calcium hardness, pH or total alkalinity levels are too high. Scale may appear as grey, white or dark streaks on the plaster, fiberglass or vinyl. It may also appear as a hard crust at the waterline.

Scum
The foreign matter floating on the surface of the water, forming a layer or a film. It can also refer to a residue deposited on the tiles or walls of the pool.

Sequestering Agent (Also Chelating Agent)
A chemical or compound that combines with dissolved metals or minerals in the water to prevent them from coming out of solution, coloring the water or causing stains.

Shock Treatment
Adding an oxidizing compound to the pool water to chemically break up (oxidize) contaminants such as suntan oils, cosmetics, perspiration and chloramines.

Skimmer
A device installed in the wall of a pool that is connected to the suction line of the pump. The suction pulls in water and floating contaminants.

Skimmer Face Plate
A cover, stainless steel or plastic, used to stop the flow of water out of the pool when the filter system is removed for the winter months.

Soda Ash
Chemically, Sodium Carbonate. A base that is used to raise the pH of acidic (below pH 7.0) water.

Stabilized, Chlorinating Products
Form of chlorine used to sanitize pool water. Favored for its economy and ability to remain active in strong sunlight.

Stabilizer
Cyanuric acid; a compound that prevents the dissipation of chlorine residuals by sunlight.

Stain Inhibitor (Also a Sequestering or Chelating Agent)
A chemical that will combine with dissolved metals in the water to prevent the metals from coming out of solution and so avoiding discoloration of the water or stains.

Submersible Pumps
Any of a number of pumps used to pump water off the winter cover of a pool. These pumps may be below water level and work efficiently.

Super chlorination
Adding 7 - 10 times the normal dose of chlorine to the water to destroy ammonia, nitrogen, chloramines and other contaminants.

Suspended Solids
Insoluble solid particles that either float on the surface or are in suspension in the water causing cloudiness. They may be removed by filtration, but if the particles are too small a flocculant or coagulant is necessary to enable the filter to trap them.

Test Kit
A manual or electrical device used to measure specific chemical residuals, levels or demands in pool water. Kits usually contain reagents, vials, titrates and color comparators for the tests. The most common tests are: pH, free available chlorine, combined chlorine, total alkalinity, calcium hardness, cyanuric acid and metals.

Test Strips
Small plastic strips with pads attached that have been impregnated with reagents to test pool water. The strips are dipped into the water and the resulting colors are compared to a color scale to determine the values.

Total Alkalinity
The total amount of alkaline materials present in the water, usually measured as carbonate alkalinity. It indicates the water's resistance to change in pH. Low total alkalinity causes pH bounce. High total alkalinity causes the pH to constantly rise.

Trichlor
A slow-dissolving, tablet or granular stabilized organic chlorine compound which provides 90% available chlorine. It must be dispensed using a floating feeder or an in-line chlorinator. Trichlor contains cyanuric acid that prevents the chlorine from being destroyed by the ultraviolet rays of the sun.

Undesirable Compounds
Substances introduced in the water by people or the environment that interfere with ideal pool conditions.