OCCLUSION - An absorption process by which one solid material adheres strongly to another, sometimes occurring by co precipitation.

OCTANE RATING - A classification of gasoline according to its antiknock qualities. The higher the octane number or rating, the greater are the antiknock qualities of the gasoline.

OCTYL ALCOHOL - ETHYL HEXANOL - Additive in absorption machines to reduce surface tension in the absorber.

OFFSET - Term used to describe the difference between the set point and the actual operating or control point.

OHM - The unit of electrical resistance equal to the resistance through which a current of 1 ampere will flow when there is potential difference of one volt across it.

OHM’S LAW - The relationship between current and voltage in a circuit. It states that current is proportional to voltage and inversely proportional to resistance. Expressed algebraically, in DC circuits I=E/R; in AC circuits I=E/R.

OIL - A liquid of vegetable, animal, mineral, or synthetic origin that feels slippery to the touch.

OIL BINDING - Condition in which an oil layer on top of refrigerant liquid may prevent if from evaporating at its normal pressure temperature.

OIL GROOVE - A channel or channels in a bearing to improve oil flow through the bearing.

OIL RING LUBRICATION - A system of lubrication for horizontal shafts. A ring of larger diameter rotates with the shaft and collects oils from a container below.

OIL SEPARATOR - Device used to remove oil from gaseous refrigerant or steam.

OIL, MINERAL - A refined hydrocarbon oil without animal or vegetable additives.

OIL, MULTI-GRADE - An oil having relative little change in viscosity over a specified temperature range.

OIL, NEUTRAL - A lubricating oil obtained by distillation, not treated with acid or with alkali.

OIL, TURBINE - An oil used to lubricate bearings in a steam or gas turbine.

ONCE-THROUGH BOILER - A steam generating unit usually operated above the critical pressure in which there is no re-circulation of the working fluid in any part of the unit.

ON-OFF CONTROL - A two position action which allows operation at either maximum or minimum condition, or on or off, depending on the position of the controller.

OPEN CIRCUIT - The absence of a direct connection between two points in an electrical network.

OPEN CYCLE - A gas turbine arrangement, in were the exhaust gases from the turbine are exhausted to the atmosphere without any further treatment.

OPEN RE-CIRCULATING WATER SYSTEM - A system, using continuously circulated water as a heat-transfer medium, in which the water is exposed at one point to the atmosphere for either discharge or absorption of heat.

OPERATING POINT - The value of the controlled condition at which the controller actually operates. Also called control point.

OPERATING PRESSURE - Actual pressure at which the system works under normal conditions. This pressure may be positive or negative (vacuum).

ORGANIC GROWTH - A substance resulting from the growth of biological organisms such as fungi, algae, and slime bacteria.

ORGANIC MATERIAL - Contain carbon and usually hydrogen and are derived from living things.

ORGANIC OXYGEN SCAVENGERS - These are organic compounds such as hydroquinone and ascorbate to remove dissolved oxygen from the boiler feedwater and condensate.

ORIFICE - Accurate size opening for controlling fluid flow.

ORSAT ANALYZER - A furnace atmosphere analysis device in which gases are absorbed selectively (volumetric basis) by passing them through a series of pre-selected solvents.

OSHA - Occupational Safety and Health Administration.

OSMOSIS - The passage of water through permeable membrane separating two solutions of different concentration; the water passes into the more concentrated solution.

OUTSIDE AIR OPENING (HVAC) - Any opening used as an entry for air from outdoors.

OVER CURRENT DEVICE - A device such as a fuse or a circuit breaker designed to protect a circuit against excessive current by opening the circuit.

OVERFLOW PIPE - A pipe installed at a top of a tank to enable the liquid within to be discharged to another vessel when the tank is filled to capacity.

OVERLOAD - A condition of excess current; more cur rent flowing than the circuit was designed to carry.

OVERRIDE - A manual or automatic action taken to by pass the normal operation of a device or system

OVERSPEED TRIP - On steam turbines, a mechanism that provides absolute reliable overspeed protection by shutting off the steam supply.

OXIDATION - The reaction of an element or substance with oxygen, e.g., iron is oxidized by reaction with oxygen to form rust (iron oxide).

OXIDIZING AGENT - Any substance such as oxygen, or chlorine, that will readily add (take on) electrons.

OXIDIZING ATMOSPHERE - A furnace atmosphere with an oversupply of oxygen that tends to oxidize materials placed in it.

OXIDIZING BIOCIDE - An agent, such as chlorine, which will kill bacteria by the chemical process of oxidation.

OXYACETYLENE WELDING - An oxyfuel gas welding process in which the fuel gas is acetylene.

OXYGEN CONCENTRATION CELL - (see differential aeration cell).

OXYGEN CONCENTRATION CELL - The surface area in contact with the media of higher oxygen concentration becomes the cathodic area, and the area with the lower oxygen concentration becomes the anode.

OXYGEN PITTING - Damage caused due to the presence of oxygen in the feedwater. Damage results are small pit-like holes in the metal.

OXYGEN SCAVENGER - A substance that will absorb oxygen by chemical reaction.

OZONE - Triatomic oxygen (0₃). Sometimes used in cold storage or air conditioning installations as an odor eliminator. Can be toxic in certain concentrations.