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U Frame: A previously used NEMA designation indicating frame size and dimension (prior to 1965 the standard frame sizes per horsepower rating).

U.L. (Underwriter's Laboratory): An independent testing organization, which examines and tests devices, systems and materials with particular reference to life, fire and casualty hazards. It develops standards for motors and controls used in hazardous locations through cooperation with manufacturers. U.L. has standards and tests for explosion- proof and dust ignition-proof motors, which must be met and passed before application of the U.L. label.

Ultra Filters: A membrane based filtration system in which the pore sizes range from 0.002 to 0.1 microns.

Ultra Violet Light Treatment: Using UV wavelength radiation to destroy contaminants in water. UV light is also used to create ozone molecules for the same purpose.

Ultra Violet Light: Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. It is so named because the spectrum starts with wavelengths slightly shorter than the wavelengths humans identify as the color violet (purple).

Ultraviolet (Uv) Light: Radiation (light) having a wavelength shorter than 3900 angstroms, the wavelength of visible light and longer than 100 angstroms, the wavelengths of x-rays. This wavelength puts ultraviolet light at the invisible violet end of the light spectrum. UV light is used as a disinfectant. Water treated by ultraviolet light should be free from particulate materials or turbidity so as to prevent micro-organisms from being shielded from the incident UV radiation.

Unbalanced seal: Not hydraulically balanced to generate low heat at the seal faces. Typical of original equipment designs.

Underdrain: The distribution system at the bottom of the filter which collects the water uniformly during the filter cycle, and which distributes the backwash water uniformly during the cleaning operation. Normally applies to sand filters.

Underdrain-1-: The lower collection system in a filter which directs filtered water back towards the pool. It also distributes water in reverse during backwashing. See laterals.

Underwater lights: A light designed to illuminate a pool from beneath the water surface.

Unfilled carbon: Containing carbon/ graphite and nothing else. Filled carbons contain inorganics that will be sensitive to some chemicals and temperature extremes. Unfilled carbons are the preferred seal faces.

Universal Gravitation Constant: The acceleration of a free falling body in the gravity field of the Earth. g = 32.2 feet/second/second.

Unloader Valve: Similar to relief valve, but not adjustable.

Unsaturated Zone: The area above the ground water level or water table where soil pores are not fully saturated, although some water may be present.

Unsaturated zone-1-: The zone immediately below the land surface where the pores contain both water and air, but are not totally saturated with water. These zones differ from an aquifer, where the pores are saturated with water.

Upper Distribution System: Those devices designed to distribute the water entering a permanent medium filter in a manner so as to prevent movement or migration of the filter medium. This system shall also properly collect water during filter backwashing unless other means are provided.

Upthrust: A submersible turbine pump run with an extremely high flow rate and very low discharge head causes the thrust direction up. This happens when the impellers are not generating enough downward thrust.

Utility Pump: An inexpensive pump similar to a sump pump except that it is not.