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O.D.: Outside diameter.

OC Voltage: The terminal voltage of a cell or battery when open circuited — i.e., not supplying any significant load current.

ODP: Open Drip Proof: a type of motor enclosure.

Off Float: A panel float that turns pump off.

Ohm:  A unit of measurement that indicates the amount of resistance in a circuit.

Ohm’s Law: The law describing the relationship between voltage, amperage, and resistance. Ohm's Law states that volts equals amperes multiplied by ohms.

Ohmmeter: Device used to check the resistance of the flow of electricity.

Ohm's Law: The formula that describes the amount of current flowing through a circuit. Voltage = V = I x R  (Voltage = Current multiplied by Resistance).

ON/OFF Float Switch: A device commonly used with sump or other types of drainage pumps that sense water level and turns the pump motor on if the level exceeds the high level setting and off when the water level drops below the low level setting.

On-Site System: A natural system or mechanical device used to collect, treat, and discharge or reclaim wastewater from an individual dwelling without the use of a community-wide sewer. The system includes a septic tank and leach field.

Open Circuit: Condition of a battery which is neither on charge nor on discharge (i.e., disconnected from a circuit).

Open Drip-proof (ODP): Same Drip Proof. Ventilation openings in bearing housings and some yokes placed so drops of liquid falling within an angle of 15° from vertical will not affect performance. Normally used indoors in fairly clean, dry locations.

Open Impeller: impellers designed with open blades or vanes, both front and back.

Open or Infinity: the flow of electricity is so small that the meter cannot sense it; there is no flow taking place.

Open Water Heating Systems: These heating systems are open to the city water pressure. If the pressure of the system increases due to expanding water, it can be relieved by the backflow of water through the supply line.

Open-Circuit Voltage: The difference in potential between the terminals of a cell when the circuit is open (i.e., a no-load condition).

Operating length: Measured after the seal has been compressed the proper amount. The measurement is usually made from the face of the stuffing box.

Operating point: The point (flow rate and total head) at which the pump operates. It is located at the intersection of the system curve and the performance curve of a pump. It corresponds to the flow and head required for the process.

Operating point-1-:  the point on the system curve corresponding to the flow and head required to meet the process requirements.

Operating Pressure: The manufacturer's specified range of pressure expressed in pounds per square inch (PSI) in which a water processing device or water system is designed to function.

Operating Temperature: The manufacturer's recommended feed water or inlet water temperature for a water treatment system.

Organic Contaminants: Carbon-based chemicals, such as solvents and pesticides, which can get into water through runoff from cropland or discharge from factories. EPA has set legal limits on 56 organic contaminants.

Organic matter: Plant and animal residues, or substances made by living organisms. All are based upon carbon compounds.

Orifice Plate: A disc, placed in a water flow line, with a concentric sharp-edged circular opening in the center, which creates a differential pressure to measure flow and to operate feeders and instruments or other hydraulic equipment.

O-ring groove: The space into which an O-ring is inserted. Dynamic O-ring grooves use a different dimension than static O-ring grooves.

O-Ring: A loop of elastomer with a round (o-shaped) cross-section used as a mechanical seal or gasket. They are designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.

Osmosis: The movement of water molecules through a thin membrane. The osmosis process occurs in our bodies and is also one method of desalinating saline water.

Osmosis-1-: The natural tendency of water to pass through a semi-permeable membrane, so as to equalize concentrations on both sides of the membrane.

Osmotic Pressure: The force (pressure) resulting when two liquids, having different solute concentrations, are separated by a semi-permeable membrane. For every 100 ppm, an osmotic "back pressure" of 1 psi is generated and this "back pressure" must be overcome in the reverse osmosis process.

OTO: Another method of testing for free available chlorine levels in your pool, as in an OTO test kit.

Outboard: Toward or beyond the boat's sides. A detachable engine mounted on a boat's stern.

Outfall: The place where a sewer, drain, or stream discharges; the outlet or structure through which reclaimed water or treated effluent is finally discharged to a receiving water body.

Outlet/Discharge: The opening through which water exits the pump.

Over hung impeller: Not supported with bearings on either side of the impeller.

Overcharging: Attempting to store more charge into a battery than its electrochemical system can safely absorb beyond its capacity. Overcharging can cause overheating and irreversible structural damage, including explosion.

Over-discharging: Withdrawing too much energy from a battery, which can shorten its working life or in extreme cases cause irreparable damage.

Overflow Gutter: The gutter around the top perimeter of the pool which is used to skim the surface of the water and to carry off the waste, or to collect it for return to the filters (sometimes incorrectly referred to as “scum gutter” or “spit trough”).

Overflow System: Refers to removal of pool surface water through the use of overflows, surface skimmers and surface water collection systems of various design and manufacture.

Overload Protector: A device that interrupts the flow of current in an electric circuit if the flow becomes sufficiently high to constitute a danger.

Owner’s Manual: A reference book for individual items of equipment, including instructions for installation, operation and repair. It usually includes a list of available repair parts.

Oxidants (Oxidizing Agents): Chemicals, which provide oxygen and accept an electron in an oxidation-reduction reaction. Free chlorine and chloramines are oxidants, which are widely used for disinfection.

Oxidation: A chemical reaction that results in the release of electrons by an electrode's active material.

Oxidation-1-: The "burning up" of organic waste and compounds in the pool water. It also refers to what you may see on your metal pool surfaces if your water is corrosive. Rust is a form of this kind of oxidation.

Oxidizer: Combines with carbon to form carbon monoxide or carbon dioxide. The oxidizers attack all forms of carbon including the seal face and any black O-rings in the system.

Oxidizing Filters: Filters that use a catalytic media, such as managanous oxides or potassium permangenate, to oxidize iron, manganese and other impurities from water.

Oxygen demand: The need for molecular oxygen to meet the needs of biological and chemical processes in water. Even though very little oxygen will dissolve in water, it is extremely important in biological and chemical processes.

Ozone: An extremely active oxidizing agent and bacteriocide, which consists of three oxygen atoms. It is formed by the action of a high voltage electrical field on oxygen or air (such as occurs during an electrical storm). Some degree of ozone can also be produced by certain types of ultraviolet lamps.

Ozone-1-: Created by oxygen atoms combining with oxygen molecules in a high-energy atmosphere. Will prematurely age Buna rubber. Ozone forms in the shop as a result of the sparking of electric motor brushes.

Ozone-2-: The molecule containing three atoms of oxygen; known to be a very powerful sanitizer. Ozone producing equipment creates this molecule by UV radiation or corona discharge generators.