Glossary of MEP

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Absorption Coefficient
The percentage of incoming energy which is absorbed. In measur­ing radiant energy (light or heat) it is a unitless ratio which may vary depending on wavelength. In acoustics, it is called a sabin and may vary depending on frequency.
Absorption Refrigeration
A cooling process resulting from the absorption of vapor by a brine solution that is then heated to remove the mois­ture. The heat may be supplied by solar or other heating sources.
The abbreviation for either air condition­ing or alternating current, depending on the context.
Acoustic Power Level
See Power Level.
Air Gap
An unobstructed vertical path, open to the atmosphere, separating the outlet of a faucet from the overflow rim of the fixture it serves. The purpose is to prevent a momentary vacuum in the supply pipe from siphoning water from the fixture back into the supply pipe.
Alternating Current
An electric current that reverses its direction at regular intervals, gener­ally 60 times per second in the United States. A plot of the voltage over time is a sine wave.
Relating to a general or all surround­ing condition. In thermal processes, it refers to the air temperature, as distinct from that of surfaces or objects. In lighting, it refers to the background light level, and in acoustics, it refers to the background noise level.
The abbreviation for the American Society of Heating, Refrigerating, and Air Conditioning Engineers, the source of most of the standardized information on the subject.
Aspect Ratio
The ratio of the longer to shorter dimension of an air conditioning duct, that affects duct friction, or of a room, that relates to light reflection.
Ball Drip
The automatic drain valve at the base of a dry standpipe.
Blow Down
The drain that removes dirt that builds up in the floor pool of an evaporative cooling tower.
The abbreviation for British Thermal Unit, a unit of heat energy, which is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
The abbreviation for Btu's per hour, an energy flow rate.
Building Automation
The control by auto­matic equipment of many functions in a large building, usually including the HVAC system, the fire detection and alarm system, and building security.
The abbreviation for cubic feet per minute, which is the flow rate of air in a mechan­ical system or duct.
A piece of equipment which cools water for use in an air conditioning system.
Chill Factor
A fictitious temperature assigned to a combination of actual temperature and wind velocity that has the same physiolog­ical effect as still air at the chill factor temperature.
The addition of small amounts of chlorine to a water source to kill bacteria.
Circuit Vent
In sanitary drainage, any vent that serves two or more traps.
A window above eye level that admits daylight, such as the high windows in a cathedral.
An organized body of rules and regula­tions adopted and enforced by a governmental unit, such as a building code.
Coefficient of Performance
The ratio of the amount of heat energy delivered by a heat pump to the amount of energy supplied, or the ratio of the amount of heat energy removed by a refrigeration machine to the amount of energy expended in its removal. It is similar to effi­ciency, but often exceeds 100 percent, and is therefore expressed as a number, i.e., 2.7, which is typical for a heat pump.
Coefficient of Utilization
The ratio of useful light arriving at the work plane to the amount of light emitted by the source. The CU depends on the reflectivity of different surfaces and the aspect ratios of the ceiling, wall, and floor cavities.
Comfort Zone
The combination of thermal and environmental conditions within which a human is comfortable, often shown on a psy-chrometric chart.
Conductance (C)
The rate at which a specific thickness of a given material conducts heat.
The heat transfer process that occurs when a warm fluid rises, displacing cold fluid which then falls.
One complete set of repeating events, typically used with alternating current or sound.
Cycles per Second (CPS)
A measure of frequency in electric current or acoustics, i.e., the number of cycles per second of a wave or oscillation. In acoustics, the term has been largely replaced by Hertz, where 1 cps = 1 Hz.
The use of natural light from out­side to replace or augment electrical light out­doors, which produces energy savings.
A decibel measured in the A scale, that is weighted to account for the special sensitivi­ties of the human ear.
Decibel (dB)
A logarithmic measure of sound intensity level.
Degree Day (DD)
The amount by which the average outdoor temperature at a particular loca­tion is below 65°F for one day. Degree days may also be summed and stated for a month or a year.
Delta Connection
A method of connecting windings on a three phase transformer, end to end, that results in a triangular shape.
Dew Point Temperature
The temperature of air at that the water contained in the air begins to condense and form dew. The dew point for a given air sample is always lower than or equal to its current temperature.
A device through which the air from a duct enters a room, or a device through which the light from a fixture enters a room.
Dry Bulb (DB)
The temperature of air as read on an ordinary glass thermometer.
Dry Pipe Sprinkler
A sprinkler system whose pipes are normally pressurized with only air, thus being invulnerable to freezing temper­atures. Upon actuation, the air is vented and supply pressure forces water through the sys­tem.
Economizer Cycle
An energy-saving strat­egy in which a part of the HVAC system is shut off while the rest is used, such as shutting off the refrigeration when the outside air tempera­ture is low, while the fan continues to operate.
Effective Temperature
A fictitious tempera­ture having the same physiological effect as air of a standardized temperature, humidity, and velocity.
The ratio of the lumens emitted by a lamp to the electrical power consumed by the lamp.
A factor that represents the rate at which a given surface material gives off or emits radiant energy. The emissivity varies from 0 to 1.0, where 1.0 is the theoretical emissivity of a perfect black box at the same temperature.
The total of sensible plus latent heat stored in the air. It is also known as Total Heat.
A continuous and unobstructed means of egress to a public way. Its minimum width is generally 44 inches.
Fire Assembly
A complete fire-resistive assembly consisting of a fire door, fire damper, or fire window and its mounting frame and hard­ware. The entire assembly, not just its compo­nents, must be approved and labeled by a testing agency that inspects the materials and workman­ship during fabrication at the factory. Available ratings are 3/4, 1, 1-1/2 and 3 hours.
Fixture Unit
A unit of liquid flow used in sizing both supply and drainage pipes.
Flame Spread Rating
A numerical classifi­cation indicating the rate at which flame will spread in or on a given material, in which higher numbers flame up more rapidly.
Flushometer Valve
A valve that releases a definite amount of water into a plumbing fixture each time it is actuated.
The basic unit of illumination arriving at a work plane. One footcandle is equal to one lumen per square foot.
Forced Air System
A heating or cooling sys­tem that uses a fan to circulate heated or cooled air through ducts to the occupied spaces.
Forced Convection
The movement of a fluid by a fan or a pump, in order to force heat exchange.
Four Pipe System
A hot and chilled water system having separate return lines for each sup­ply line, and with no mixing of the two streams.
The number of cycles that occur per second, either in alternating current or acoustics. In acoustics, the frequency deter­mines the pitch.
Frost Line
In a given location, the maximum depth in soil that is expected to freeze in cold weather. Water piping must ordinarily be buried below the frost line to protect against freezing.
Fusible Link
A piece of wax or paraffin that melts at a predetermined temperature, setting off a sprinkler head, an alarm system, or otherwise actuating a fire protection device.
Globe Thermometer
A thermometer that measures Mean Radiant Temperature (MRT).
An electrical conductor connected to the earth or to a pipe extending into the ground, used to dissipate hazardous currents into the earth.
A gaseous fire extinguishing medium that smothers fires, often used in automatic sys­tems in computer rooms.
Heat Pump
A refrigeration loop used to bring heat into a space instead of removing heat from it. The term is also used for an entire system of such units attached to a recirculating heat sink.
HEPA Filter
A high efficiency particulate air filter, which removes dust and other tiny parti­cles from a moving air stream.
Hertz (HZ)
The frequency of a sound, or of an alternating current, equal to the number of cycles per second.
High Intensity Discharge (HID)
A family of lamps consisting of a quartz envelope inside a glass envelope. The inner quartz tube can stand higher temperatures, and allows for the current to arc between the two electrodes exciting a plasma of mercury, metal halide, or high pres­sure sodium.
Horsepower or brake horse­power, a unit of power, equal to roughly 746 watts.
The intensity of light falling on a surface, usually expressed in footcandles.
Impact Isolation Class (IIC)
A rating of the degree of isolation of a floor against the trans­mission of impact noises.
The leakage of air through cracks around windows and other building elements.
Intensity Level
The intensity of sound at a given location, measured in watts per square meter, or more commonly in dB where the ref­erence level is 10~12 watts/meter2 or 10~16 watts/cm2.
Inverse Square Law
A physical principle that states that the intensity of a phenomenon is inversely proportional to the square of the dis­tance from the source to the measuring device. It holds true for point sources of light, and for sound in an open field.
The lowest point of the inside of a drain, pipe, channel, or other liquid-carrying conduit.
Ion Exchange
A process of water softening in which calcium and magnesium ions are replaced by sodium ions. This process is also known as the zeolite process.
lonization Detector
A fire detector which detects the products of combustion (POC) even before they are visible to the naked eye.
K Factor
The thermal conductivity of one square foot of a material per inch of thickness, with a surface temperature difference of one degree F.
Kilowatt (KW)
A unit of electric power, equal to 1,000 watts.
A rating for transformers equal to the product of volts and amperes divided by 1,000. The product of the KVA and the power factor gives the power in kilowatts.
Latent Heat
The heat added to or removed from a substance when it changes its state. See Sensible Heat.
Light Shelf
An overhang, either outside or inside or both, that is used with a clerestory to reflect light up onto the ceiling, and reduce direct light adjacent to the window below.
A unit of light, defined as the amount of light passing through one square foot at a distance of one foot from a one candlepower source.
A complete light fixture, includ­ing lamps.
Mass Law
The theoretical law that states that for each doubling of mass in a wall, there is a 6 dB drop in the amount of sound transmitted.In actual practice, it is usually closer to 5 dB.
Mean Radiant Temperature (MRT)
The weighted average of all of the temperatures of all of the surfaces visible from a given position.
NC Curves
A single-number system for spec­ifying a maximum SPL level in a given location, using standardized reference contours. The curves weight the frequencies to which the human ear is sensitive.
The abbreviation for National Electrical Code.
The wire or conductor in an electrical system that is equidistant in voltage from the phase conductors of the system. It is not the same as ground.
Noise Reduction Coefficient (NRC)
A one-number rating system giving the average sound absorption coefficient of a material at frequen­cies of 250, 500, 1,000 and 2,000 Hz.
The purpose for which a building is intended to be used.
Occupancy Group
A designation for a group of several occupancies that have comparable fire safety considerations, and which are therefore grouped together by the code.
The abbreviation for the Occupational Safety and Health Act, which regulates working conditions.
Passive Solar Design
The practice of orient­ing and sizing a building, its windows, and its internal masses in such a way that it responds to the sun and to the climate, without the use of mechanical equipment.
The unit of permeability for a given material, expressing the resistance of the mate­rial to the penetration of water or water vapor through it. One perm is equal to the flow of one grain of water vapor through one square foot of surface area per hour with a pressure difference of one inch of mercury.
The property of permitting pas­sage of water or water vapor through a material without causing rupture or displacement.
Potable Water
Water that is suitable for drinking.
Power Factor (PF)
In an electrical circuit, the ratio of real power in watts to the product of voltage and current.
Power Level (PWL)
The logarithmic expression for the acoustical power at the source of a sound. It is also known as Acoustic Power Level.
The abbreviation for parts per million.
Psychrometric Chart
A graph showing the relationships between temperature, humidity, relative humidity, and enthalpy.
The adding of sensible heat to a sup­ply air stream which has been previously cooled.
Relative Humidity (RH)
The ratio of the moisture content of the air to the maximum pos­sible content at the same temperature.
The persistence of sound in an enclosed space after the source has stopped.
Reverberation Time
The time it takes a 60 dB sound to completely die away in a closed room after the source has stopped.
The unit of sound absorption equivalent to the absorption of one square foot of open window.
Sensible Heat
Heat that changes the temper­ature of a substance, and does not represent the addition of any moisture to the substance
Shading Coefficient (SC)
The ratio of the solar heat gained through a window with adding devices to the solar heat gained by a single pane double strength clear glass window. Shading devices, such as Venetian blinds, lower the SC.
Siamese Fitting
A Y-shaped hose attachment at the base of a building, which allows the fire department to connect a fire hydrant through a pumper truck to provide or augment water flow to a standpipe.
Smoke Developed Rating
A numerical rat­ing derived from a standardized fire test proce­dure. Larger numbers indicate a greater density of smoke.
Smokeproof Enclosure
A continuous enclosed stairway separated from the building at each floor by an open vestibule that allows smoke to vent away without entering the stair.
A sanitary drainage term referring to the waste from urinals, water closets, and fixtures of similar function.
A subjective system of measuring loud-ness, based on the reference point of one sone equal to a sound pressure level of 40 dB.
Sound Level Meter
A meter that measures the sound pressure level and gives a reading in dB.
Sound Pressure Level (SPL)
The logarith­mic expression of the pressure exerted by sound waves on the receiver. The reference pressure is 2 x 1O~5 newtons per square meter.
Sound Transmission Class (STC)
A single-number rating for the evaluation of a particular construction cross-section in terms of its transmission of airborne sound. The higher the STC rating, the more effective the construction is at stopping airborne sound.
Sprinkler System
A system used to extin­guish fires automatically by releasing water or other substances. See Dry Pipe Sprinkler and Wet Pipe Sprinkler.
Stack Vent
The portion of a soil or waste stack that is above the highest branch drain con­nected to the stack. Its sole function is to vent to the outside air.
A vertical supply pipe for firefight-ing. Dry standpipes are empty, and must be con­nected to a fire hydrant via a Siamese connection and a pumper truck. Wet standpipes are pressur­ized and filled with water, to serve attached hoses within the building, on each floor. Wet standpipes also have Siamese connections to allow the water flow to be augmented from fire hydrants.
Star Connection
A method of connecting the windings on a three-phase transformer in which one end of all three windings is connected to a common neutral center point, forming a Y shape. Same as Wye Connection.
A device that starts the arc in a neon or fluorescent lamp, or a contactor and overload relay used in starting some electric motors.
Steam Trap
A valve that permits passage of air or water, but not steam, often used with steam radiators.
The method of soldering copper plumbing, or the condensation of water on cold pipes or building materials.
The method of using a heated surface and the resulting convection to move a fluid out of a space. In solar hot water heaters, the collector is below the storage tank, and the water is circulated automatically by convection when it is heated. Thermosiphoning may also be used to ventilate a building by sending the warmed air out the top, and siphoning in cooler air at the bottom.
Three Pipe System
A hot and chilled water system having a common return pipe for both supply lines. Ton The amount of cooling required to create a ton of ice in a 24 hour period, equal to a steady rate of 12,000 Btuh.
Transmission Loss (TL)
The reduction of sound that occurs when a given wall transmits sound from one room to an adjacent room, expressed in decibels.
Trombe Wall
A form of mass wall that trans­fers heat by causing convection into the room behind it, as well as conduction.
Two Pipe System
A hot or chilled water sys­tem having only a supply and return line. It can supply only heated or chilled water to a zone, but not both simultaneously.
U Factor or U Value
The thermal conductiv­ity of a particular wall section, expressed in Btu's per hour per degree Fahrenheit per square foot.
Vent Stack
A vertical pipe that vents several sanitary drainage lines, in order to break the siphoning suction which would occur when water drops through the system.
The basic unit of electrical power, equal to the product of volts and amperes in direct current systems, equal to 3.41 Btuh.
The length of one complete cycle or waveform, for light or sound waves. In light, the dominant wavelength determines the per­ceived color.
Wet Bulb Temperature (WB)
The tempera­ture attained by a glass thermometer whose bulb is covered with a wet sock and placed in an air stream moving at 1,000 feet per minute.
Wet Pipe Sprinkler
A sprinkler system that is continually pressurized with water. If a fusible sprinkler opens, water is immediately forced through the sprinkler head.
Vacuum Breaker
An automatic valve that admits air into a supply pipe rather than allow­ing the pipe to suction or siphon polluted water back into the supply.
Variable Air Volume (VAV)
An air condi­tioning system that accommodates thermal load changes by varying the flow of supply air into
A substance used in water softening, in which a filtering tank is recharged by passing a salt solution through it. The tank may then be used as a filter for the free ions associated with hard water.
A portion of a building controlled by a single thermostat, because its spaces have sim­ilar heating or cooling needs.
Absolute filter (H)
A high-efficiency mechanical filter built originally for removing radioactive particles and having an efficiency of 99.9% or higher.
Absolute humidity (H)
Weight of water vapor per unit vol­ume of air-steam mixture. Usually expressed in grains per cubic foot.
Absolute pressure (G, H)
The pressure above an absolute vacuum; the sum of the pressure shown by a gage {ox gauge) and that indicated by a barometer.
Absolute temperature (H)
Temperature measured from absolute zero; on the Fahrenheit scale, the absolute tem­perature is approximately °F + 459.6° and on the Celsius scale °C + 273°.
Absorption (AS)
The ability of a material to absorb acoustical energy. Measured in sabins.
Absorption (H)
An assimilation during which the absorbent undergoes a physical or chemical change ior both:. Absorbers such as lithium chloride and calcium chloride are used in dehumidifying. The process is also used in absorption refrigeration.
Absorption coefficient (AS)
The fraction of sound energy impinging on a surface that is absorbed by that surface, usually denoted by a. Frequency sensitive.
Absorption refrigeration (H)
A process whereby a secondary fluid absorbs the refrigerant and, in doing so, gives up heat. Afterward, it releases the refrigerant, during which time it absorbs heat.
Acoustics (AS)
The science of sound, including its generation, transmission, and effects.
Acrylonitn'le-butadiene-styrene, ABS (PS)
A thermoplastic compound from which fittings, pipe, and tubing are made.
Active sludge (PS)
Sewage sediment, rich in destructive bacteria, that can be used to break down fresh sewage more quickly.
Adaptive reuse (G)
Converting existing buildings to new functions to avoid demolishing them; generally considered a sustainable design by avoiding the need to dispose of existing building materials and conserving the use of new materials along with the resources needed to produce and erect them.
Adiabatic (H)
A change at constant total heat; an action or process during which no heat is added or subtracted.
Aeration (PS)
An artificial method in which water and air are brought into direct contact with each other to furnish oxygen to the water and to reduce obnoxious odors.
Aerobic (PS)
Living or active only in the presence of free oxygen (said of certain bacteria).
Airborne sound (AS)
Sound that is transmitted through air by a series of oscillating pressure fluctuations.
Air change (H)
The quantity of infiltration or ventilation air, in cubic feet per hour or minute, divided by the volume of the room.
Airfoil fan (AS, H)
A fan with an airfoil-shaped blade that moves the air in the general direction of the axis about which it rotates.
Air gap (PS)
The unobstructed vertical separation between the lowest opening from a pipe or faucet conveying water or waste to a plumbing fixture receptor.
Alternating current, AC (E)
Flow of electricity that cycles or alternates direction. The number of cycles per second is referred to as the frequency.
Ambient (AS, H, I)
Encompassing on all sides. Thus ambient air is the surrounding air; ambient light is the lighting envi­ronment of a space; ambient noise is the surrounding sound in a space.
Ambient lighting (I)
Lighting throughout an area that pro­duces general illumination.
Ambient noise (AS)
The sound associated with a given envi­ronment, usually a composite of many sources.
American Standard Code for Information Interchange, ASCil (DP)
A system for referring to letters, numbers, and com­mon symbols by code numbers. The numerical value assigned for each binary digit of an eight-digit byte is, from left to right, 128, 64, 32, 16, 8, 4, 2, and 1.
American Wire Gauge, AWG (E)
The standard system for measuring the size of wires in the United States.
Ampacity (E)
The current, in amperes, that a conductor can carry continuously under conditions of use without exceed­ing its temperature rating.
Analog (DP, TC)
Varying continuously (e.g., sound waves). Analog signals have their frequency and bandwidth meas­ured in hertz. See Digital.
Anechoic (AS)
An acoustical environment free of any reflected sound; a free field.
Angstrom (I, TC)
A unit of length, 0.1 nm or 1CT10 m, often used to measure wavelength, but not part of the SI system of units. See Nanometer.
Apparatus dew point (H)
Dew point temperature of the air leaving the conditioning apparatus.
Aspect ratio (H)
In a duct, the depth of elbow along (paral­lel to) the axis of bend divided by the width in the plane of the bend.
Atmospheric pressure (G, H)
Pressure indicated by a barom­eter. The standard atmospheric pressure is 760 mm of mer­cury (29.921 in. of mercury) or 14.7 lb per square inch (in.2) at sea level.
Attenuation (AS)
Lessening or reduction, e.g., from 80 dB to 70 dB.
Aural (AS)
Pertaining to the ear or to the sense of hearing. A-weighting (AS) Prescribed frequency response defined by ANSI Standard SI.4-1971. Used to obtain a single number representing the sound pressure level of a noise in a manner approximating the response of the ear, by deemphasizing the effects of the low and high frequen­cies. See dBA.
Axial flow fan (H)
A fan with a disk- or airfoil-shaped blade that is mounted on a shaft and that moves the air in the general direction of the shaft axis.
Backflow (H, PS)
In plumbing or HVAC systems, water flow into water-distributing pipes from any source other than that for which the system is designed.
Backflow connection (PS)
In plumbing, any connection or arrangement which permits backflow.
Backflow preventer (PS)
A device to prevent backflow into a water supply line from the connection at its outlet.
Background sound (AS)
Noise from all sources in an envi­ronment, exclusive of a specific sound of interest.
Backup (DP)
A copy of a program or document that can be used if the original is destroyed.
A device used with fluorescent and HID lamps to provide the necessary starting voltage and to limit the cur­rent during operation of the lamp.
Bandwidth (DP, TC)
The highest frequency that can be trans­mitted in an analog operation. Also (especially for digital systems), the information-carrying capacity of a system.
Baud (DP, TC)
Strictly speaking, the number of signal-level transitions per second in digital data. For some common coding schemes, baud equals the number of bits per second, but this is not true for more complex coding, where the term is often misused. Telecommunication specialists prefer bits per second, which is less ambiguous.
Baud rate (DP)
The number of bits per second. Baud rates are most commonly used as a measure of how fast data are transmitted by a modem.
Beam angle (I)
The angle between the two directions for which the intensity is 50 percent of the maximum inten­sity, as measured in a plane through the nominal beam centerline.
Bell-and-spigot joint (PF)
The joint commonly used in cast-iron pipe. Each piece is made with an enlarged diameter or bell at one end into which the plain or spigot end of another piece is inserted when the pipe is being laid. The joint is then made tight with cement, oakum, lead, or rubber caulked into the bell around the spigot.
Bellows (H, PS)
An expansible metal device containing a fluid that will volatilize at some desired temperature, expand the device, and open or close an opening or a switch, as in controls and steam traps.
Bimetal (H)
Two metals of different coefficients of expan­sion welded together so that the piece will bend in one direction when heated and in the other when cooled. Thus it can be used in opening and closing electrical circuits, as in thermostats.
Binary numbers (DP)
The base-2 numbering system of almost all computers, as opposed to the base-10 (decimal) numbers in common use.
Binary system (DP)
A numbering system using 2 as base, as opposed to 10 as base in the decimal system. For example, 1 in binary system is 1; 10 is 2; 11 is 3; 100 is 4; 101 is 5; 110 is 6; 111 is 7; 1000 is 8; 1001 is 9; and 1110 is 10. The binary system is used in almost all computers.
Bit (DP)
Abbreviated from "binary digit," the smallest possi­ble unit of information. A bit represents one of two things: yes or no, on or off, or, as expressed in the binary numbers used in computers, 0 or 1.
Blackbody (H, I)
A body that absorbs all the radiant energy falling upon it.
Boiler (H)
A closed vessel in which fuel is burned to generate steam. (A vessel that produces hot water should be called a hot-water heater^)
Boiler horsepower (H)
The power required to evaporate 34.5 lb of water at 212°F per hour, or the equivalent of 33,475 Btu per hour (Btuh).
Booting (DP)
The starting up of a computer by loading an operating system into it. (The name boot comes from the idea that the operating system pulls itself up by its own bootstraps.)
Branch circuit (E)
The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
Branch interval (PS)
In plumbing, a vertical length of waste pipe, usually one story high, to which the horizontal branches from that floor are connected.
Branch vent (PS)
A vent connecting one or more individual vents with a vent stack.
British thermal unit, Btu (G, H)
The amount of heat required to raise the temperature of 1 lb (0.45 kg) of water 1 degree Fahrenheit (0.565°C).
Broadband (TC)
In general, covering a wide range of fre­quencies. The broadband label is sometimes used for a network that carries many different services or for video transmission.
Buffer (DP)
An area of memory or a separate memory cache that holds information until it is needed. Buffers are used to speed up printing, redraw the screen, etc.
Bug (DP)
A mistake or unexpected occurrence in a piece of software (or, less commonly, in a piece of hardware).
Building drain (PS)
The section of the horizontal drainpipe connecting the drainage pipes inside the building with the building sewer outside the inner face of the building wall.
Building sewer (PS)
The pipe extending from the outer end of the building drain to the public sewer.
Building trap (PS)
A trap in the building drain to prevent air circulation between the building drainage system and the building sewer.
Bulletin board (bulletin board system), BBS (DP)
A com­puter dedicated to maintaining messages and software and making them available over phone lines at no charge. People upload (contribute) and download (gather) messages by calling the bulletin board from their own computers. BX (E) An electrical cable consisting of a flexible metallic cov­ering inside of which are two or more insulated wires for carrying electricity.
Byte (DP)
A group of eight bits that the computer reads as a single letter, number, or symbol. For example, 01000101 is a typical byte of the binary system.
Calorie (H)
In engineering, the large calorie is usually used; it is defined as one one-hundredth of the energy or heat required to raise the temperature of 1 kg of water from 0°C to 100°C. The small calorie is the heat required to raise the temperature of 1 g of water.
Candela, cd (I)
The unit of measurement of luminous inten­sity of a light source in a given direction.
Candlepower, Cp (I)
Luminous intensity of a light source in a specific direction.
Candlepower distribution curve (I)
A curve, generally polar, representing the variation in luminous intensity of a lamp or luminaire in a plane through the light center.
Cathode ray tube, CRT (DP, TC)
The display technology used on virtually all computer monitors and television sets.
Cathodic protection (PS)
The control of the electrolytic cor­rosion of an underground or underwater metallic structure by the application of an electric current.
Central processing unit, CPU (DP)
The central part of a computer. The CPU includes the circuitry built around the CPU chip and mounted on the motherboard that actually performs the computer's calculations.
Check valve (FP, H, PS)
A valve designed to allow a fluid to pass through in one direction only. A common type has a plate suspended so that the reverse flow aids gravity in forc­ing the plate against a seat, shutting off reverse flow.
Circuit breaker (E)
A device designed to open and close a circuit manually or automatically on a predetermined overcurrent.
Circuit vent (PS)
A branch vent from two or more traps extending from in front of the last fixture connection of a horizontal branch to the plumbing vent stack.
Clean room (H)
A room in which the air is highly purified, particularly with regard to dust and other particulate mat­ter. Clean-room techniques are used in the processing of delicate materials and products.
Clock rate (or clock speed) (DP)
The operations of a com­puter are synchronized with a quartz crystal that pulses millions of times each second. These pulses determine, for example, how often the screen redraws an image and how often the CPU accesses RAM or a hard disk. The frequency of the pulses—how often they occur—is measured in megahertz (millions of cycles per second) and is called the clock rate or clock speed.
Close nipple (H, FP, PS)
A nipple with twice the length of a standard pipe thread.
Closed cycle (H)
A system in which the fluid is used over and over without introduction of new fluid, as in a hot-water heating or mechanical refrigeration system.
Coefficient of performance, COP (H)
In a heat pump, the ratio of the effect produced to the electrical input in consis­tent units.
Coefficient of utilization, CU (I)
The ratio of the luminous flux (lumens) from a luminaire, calculated as received on the work plane, to the luminous flux emitted by the luminaire's lamps alone.
Color-rendering index, CRI (I)
Measure of the degree of color shift objects undergo when illuminated by a light source, as compared with a reference source, normally incandescent.
Color temperature (I)
The absolute temperature of a black-body radiator having a chromaticity equal to that of the light source.
Community antenna television, CATV (TC)
Cable television, a broadband transmission system generally using 75-ohm coaxial cable that simultaneously carries many frequency-divided TV channels.
Compiler (DP)
Software that implements a program by trans­lating it all at once.
Condensate (G)
Water that has liquefied from steam or pre­cipitated out from air.
Condensate (H, PS)
Liquid formed by condensation.
Condenser (H)
Apparatus used to liquefy a gas.
Condensing unit (H)
An assembly attached to one base and including a refrigerating compressor, motor, condenser, and receiver, and the necessary accessories.
Conductance (H)
The quantity of heat (usually BTU) transmitted per unit of time (usually 1 hour) from a unit surface of material under a unit temperature (usually 1°F) differential between the surfaces. Unit abbreviation is Btu/hr-ft2oE
Conduction (H)
The transmission of heat from one part of a body to another part of the same body, or from one body to another in contact with it, without appreciable displace­ment of the particles of the body.
Conductivity (H)
The quantity of heat (usually Btu) trans­mitted per unit of time (usually 1 hour) from a unit surface (usually 1 sq ft) to an opposite unit of surface of one mate­rial per unit of thickness (usually 1 in., but occasionally 1 ft) under a unit temperature differential (usually 1°F) between the surfaces.
Continuous load (E)
A load such that the maximum current is expected to continue for 3 hours or more.
Contrast (I)
Difference in brightness between an object and its background.
Convection (H)
The transfer of heat from one point to another within a fluid (such as air or water) by the mixing of one portion of the fluid with another. If the motion is due to differences in density, from temperature differences, the convection is natural; if the motion is imparted mechan­ically, it is forced convection.
Convector (H)
A unit containing heating elements that allow air to be heated through natural convection without using external power.
Cooling tower (H)
A device for cooling water by evaporation in the outside air. A natural-draft cooling tower is a cooling tower in which the airflow through it is due to the natural chimney effect; a mechanical-draft tower uses fans.
Coprocessor (DP)
A chip that specializes in mathematics, graphics, or some other specific kind of computation. When the CPU is handed the kind of job the coprocessor special­izes in, it hands the job off to the coprocessor.
Counterflow (H)
In a heat exchanger, a situation in which the fluid absorbing heat and the fluid losing heat are directed so that the lower and higher temperature of the one are adja­cent to the lower and higher temperature of the other, respectively. Ordinarily, one fluid is flowing in the opposite direction from the other, hence the term.
Critical angle (I, TC)
The angle at which light undergoes total internal reflection.
Critical velocity (H)
The point above which streamline flow becomes turbulent.
dBA (AS) (TC)
Overall A-weighted sound pressure level expressed in decibels referenced to 20 micropascals. An A-weighting is a single number approximating the response of human hearing.
Dead room (AS)
A room whose boundaries and furnishings absorb a great amount of sound.
Debug (DP)
To search out bugs or defects in a piece of soft­ware and eliminate them.
Decay rate (AS)
The rate at which the sound pressure level (in dB) decreases when the source of the sound is eliminated.
Decibel, dB (AS, TC)
Ten times the logarithm to the base 10 of a quantity divided by a reference quantity; dB = 10 X logl0(X/Xref).
Degree day, DD (H)
The number of Fahrenheit degrees that the average outdoor temperature over a 24-hour period is less than 65°F.
Demand charge (E)
That part of a utility service charged for on the basis of the possible demand as distinguished from the energy actually consumed.
Demand factor, DF (E)
The ratio of the maximum demand to the total connected load of a system.
Demand load, DL (E, H, PF)
The load, in appropriate units, such as kW, Btuh, or gpm, that an electrical or mechanical system encounters.
Density (G)
The weight of a unit of volume—usually, pounds per cubic foot.
Developed length (H, PF)
The length along the centerline of a pipe and its fittings.
Dew point, DP (G, H)
The temperature of a gas or liquid at which condensation or evaporation occurs.
Dielectric fitting (H, PS)
A fitting having insulating parts or material that prohibits the flow of electric current.
Diffraction (AS)
Alteration of the direction of propagation of a sound wave in the vicinity of a boundary discontinuity on the edge of a reflecting or absorbing surface. Frequency sensitive.
Diffuse sound field (AS)
A sound field in which the intensity of the sound is independent of its direction; an area over which the average rate of sound energy flow is equal in all directions.

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