A treatment that resists moisture with no hydrostatic pressure, typically a thin coatingIn: Below-grade waterproofing
Dark fiber vs leased wave
Dark fiber is unlit glass you light and run yourself; a leased wave is a lit channel a carrier runs for you under an SLAIn: Data center interconnect (DCI)
DAS
Distributed antenna system, the antennas and cabling that spread a signal through a buildingIn: DAS and public-safety radio
The day-2 discipline of running a commissioned facility 24/7: monitoring, rounds, procedures, alarm response, and incident handling that deliver the designed uptime.In: Data center operations and the NOC
Data date
The line in a schedule update dividing actual progress from remaining forecastIn: Project scheduling
Double check valve assembly; two checks, no relief, for low hazard and both mechanisms (ASSE 1015)In: Backflow assembly types
DC arc flash
An arc on a DC system; with no zero crossing it does not self-extinguish, so it is harder to interrupt and carries higher continuous energyIn: Rack DC power distribution (HVDC)
DC fast charging / Level 3
High-power DC delivered straight to the battery; the station, not the car, converts AC to DCIn: DC fast charging station design
DCDA / RPDA
Double check and reduced pressure detector assemblies; fire-line versions with a metered bypass (ASSE 1048 / 1047)In: Backflow assembly types
DCI
Data center interconnect, the transport that carries traffic between data centers across campus, metro, or long-haul distanceIn: Data center interconnect (DCI)
Digital diagnostic monitoring in an optical module, reporting Tx and Rx power, laser bias, temperature, and voltage for failure predictionIn: Network observability for the AI fabric
Treating ice or pack that has already formed to melt it and break its bond to the pavementIn: Snow and ice management
Dead band
The temperature range between heating and cooling where the box holds minimum airflow and the reheat valve is shutIn: VAV box commissioning field guide
Dead bus
A de-energized bus the first set closes to without synchronizing, under first-start arbitrationIn: Generator paralleling switchgear
The percentage the lender keeps from what it funds to you in exchange for the promotional planIn: HVAC customer financing guide
Debarment
Removal from eligibility to bid covered public contracts for a period, a top consequence of serious violationsIn: Certified payroll and prevailing wage
Reducing the chlorine in the flush water to a permitted level before discharge, per AWWA C655In: Potable water disinfection guide
Decisions log
The record of what was decided, by whom, when, and on what basis, separate from open actionsIn: Progress meetings field guide
Deck (substrate)
The structural surface that spans the framing and that the roof system attaches toIn: Roof deck substrate types
Deck ledger
The board that fastens the deck to the house band joist; must be bolted or lag-screwed and flashed, never nailed aloneIn: Deck construction field guide
Deck overlay (LMC / polyester)
A bonded wearing course added to extend deck life: latex-modified, polyester polymer, silica-fume, or membrane and asphaltIn: Bridge deck construction
Deck-level (source-capture) exhaust
Exhaust drawn from low at the water and deck, where the heavy chloramine layer sits, instead of from the ceilingIn: Natatorium HVAC field guide
Datacom equipment cooling system, the OCP term for the fluid, heat exchangers, circulation, and controls within the tank or rackIn: Immersion cooling acceptance
Dedicated equipment space
The reserved volume above the gear footprint, kept free of foreign piping and duct, per 110.26(E)In: Working clearance (110.26)
Dedicated exhaust
A corrosion-resistant, exhaust-only duct system that serves the hoods and never mixes with or recirculates to the building comfort airIn: Lab fume hood exhaust guide
Injecting a nutrient solution under pressure into the root zone below the surface, a delivery method still governed by the soil testIn: Plant health care (PHC)
Deep-seal trap
A trap with a seal deeper than the standard range, used to resist siphonage on hard-to-vent fixturesIn: Plumbing traps and trap seal guide
Deer-resistant
A plant deer tend to avoid by smell, texture, or chemistry; relative and local, not deer-proof, and it fails under high pressureIn: Deer and wildlife protection
DEF
Delayed ettringite formation, internal expansion and cracking from the core running too hot when young, commonly capped at 158F (70C)In: Mass concrete thermal control
The fire-resistant zone around a structure; the first 5 ft should be noncombustibleIn: Mulch beds and weed control
Defensive driving
Operating to avoid crashes others may cause, through following distance, scanning, speed control, and leaving an outIn: Fleet and driver safety program
Deferred interest / same-as-cash
No interest if the balance is paid in full inside the window; retroactive interest if it is notIn: HVAC customer financing guide
Deferred maintenance
Scheduled maintenance not completed on time; a latent risk that grows until correctedIn: Data center PM program
Deferred revenue
Prepaid fees for service not yet delivered, recorded as a liability and recognized as revenue as the visits are performedIn: Service agreements and recurring revenue
Deficiency / issues log
The tracked list of every finding from identify through verify and close, the program's most important operational artifactIn: Data center commissioning process
Cycle that reverses to cooling to melt frost off the outdoor coil, with aux heat covering the roomIn: Heat pump fundamentals field guide
Defrost (off-cycle / electric / hot-gas)
Periodic melting of coil frost: off-cycle for medium temp, electric heaters or hot discharge gas for low temp, ended on coil temperature with a time fail-safeIn: Commercial refrigeration field guide
Degauss
A magnetic-media method that erases by demagnetizing; it does nothing to solid-state drivesIn: Data center decommissioning
Controlling outdoor air to measured CO2 or occupancy, adding fresh air when CO2 rises and cutting it when a space emptiesIn: IAQ monitoring and sensors field guide
Demarcation point, the specific port where one party's ownership ends and the next party's beginsIn: Cross-connect patch records
Demolition robot
A remote-controlled tracked machine with a breaker, shear, or bucket that lets the operator work a dangerous or unstable structure from a safe distanceIn: Construction robotics
Dense-graded aggregate (DGA)
A well-graded crushed blend from top size down to fines that compacts tight; also crusher run, 21A, 411, GABIn: Aggregate base and gravel roads
Dense-graded aggregate base
Crushed stone blended down to fines that locks tight when compacted; the layer that carries the loadIn: Paver hardscape installation
The supply-to-return difference the plant was sized for, e.g. 44 F supply and 56 F return is a 12 F riseIn: Low delta-T syndrome field guide
Design density and area
The water rate in gpm per square foot applied over the most remote design area; the basis the calculation must meetIn: Fire sprinkler system design guide
Design ESALs
Total equivalent single axle loads expected over the design life, the traffic number the section is sized toIn: Asphalt pavement thickness design
Mixing the warm ceiling air back down to the floor with high fans to cut the vertical temperature spreadIn: Unit heater field guide
Desuperheater
An auxiliary heat exchanger that captures compressor discharge heat to warm domestic hot waterIn: Geothermal heat pump field guide
DETC / LTC
De-energized tap changer, moved only dead, versus a load tap changer designed to move energizedIn: Padmount transformer energization
Detect / recognize / identify
The three goal levels: detect a presence (about 20 to 25 ppf), recognize a known person (about 40 to 50), identify a stranger (80 or more)In: CCTV system design field guide
Detect vs isolate
Detection finds and locates the leak; isolation stops it by closing a valve. Both are required, not just the firstIn: Liquid-cooling leak detection
Ion-exchange polishing that trades dissolved ions for hydrogen and hydroxide on resinIn: Pure water RO/DI field guide
DI water
Deionized water, the low-ion makeup and dilution water the fluid is formulated for, far cleaner than tap or softened waterIn: Liquid cooling loop chemistry
Diagnostic fee
The charge to come out and find the fault, commonly credited to the repair if the customer proceedsIn: Flat-rate pricing price book
Diaphragm
The roof or floor that ties the panels together as the permanent lateral system, letting the braces come offIn: Tilt-up bracing and erection
Progressive death of branch tips and limbs from the top down, a sign of a tree losing the establishment fightIn: Tree establishment and aftercare
Dielectric
The electrical property of the concrete that sets the pulse velocity; roughly 6 to 8 in dry concrete, higher when wetIn: Concrete scanning before you drill
Zone protection that compares current in against current out and trips fast for an internal fault without a coordination delayIn: Coordination study field guide
Differential (thermal gradient)
The temperature difference between the core and the surface, commonly limited to 35F (19.4C)In: Mass concrete thermal control
The test instrument that reads pressure across one part at a time through high, low, and bleed needle valvesIn: Backflow test procedure guide
Differential pressure
The static pressure drop across the coil; the rise over a clean baseline at the same airflow is the cleaning triggerIn: HVAC coil cleaning and maintenance guide
Differential pressure (dP)
The pressure difference the CDU holds across the manifold headers to keep each rack branch fedIn: CDU commissioning
Differential relaying
Protection that trips on the difference between current entering and leaving a zone; fast and selective on its own zoneIn: Arc energy reduction methods
Differential settlement
One part of a foundation settling more than another, the movement that cracks and racks a structureIn: Foundation underpinning and repair
The top and bottom manifolds that spread flow to a low velocity to preserve stratificationIn: Thermal energy storage tank
Diffusion vs air leakage
Diffusion is slow vapor movement through solid material; air leakage is bulk air through gaps, carrying far more waterIn: Roof vapor retarder and air barrier
Combing ridges in one direction and sliding the tile across them to collapse voids; back-buttering adds a skim of mortar to the tile back for large or warped tile.In: Ceramic and porcelain tile installation
Discharge means
Bleed resistor or device that drains the capacitor's residual voltage after disconnectionIn: Power factor correction
Chlorinating a line to kill the bacteria the install brought in, then proving it clean before serviceIn: Potable water disinfection guide
Disinfection CT
Disinfectant residual concentration (mg/L) multiplied by contact time (minutes); the measure of how much disinfection the water receivedIn: Treatment plant systems field guide
Operating while attention is on something other than driving, most dangerously a phone, by hand or by mindIn: Fleet and driver safety program
Distress
A defect recorded in the survey, named from the standard's catalog: alligator cracking, rutting, raveling, and so onIn: Pavement condition assessment (PCI)
Distribution box (D-box)
Buried box that splits effluent evenly among the drainfield trenches; must be and stay levelIn: Septic system field guide
Distribution unit
A manifold that splits one primer's metered water evenly among several floor-drain traps, commonly up to fourIn: Trap primer field guide
District heating
A network distributing heat to many buildings through insulated pipe, the largest-scale outlet for recovered heatIn: Data center waste heat reuse
A physically separate route, conduit, and building entrance from the working path, so a single fiber cut cannot isolate the siteIn: Data center interconnect (DCI)
A zinc, brass, aluminum, or plastic strip set on edge that controls cracking, makes the pattern, and handles joint movementIn: Terrazzo flooring install and polish
Direct-on-line, full-voltage starting through a contactor with no voltage reductionIn: Motor starting methods
Donor antenna
The antenna or feed that captures the outside signal the system rebroadcasts, off-air from a tower or fed by the carrierIn: DAS and public-safety radio
A pressurization test that measures enclosure leakage and predicts how long the room will hold the agentIn: Clean agent room integrity
Door operator
The motor and linkage that opens and closes the doors. The number one source of callbacks, so the highest-return item in most modsIn: Elevator modernization and maintenance
Pre-action that admits water only when both detection operates and the sprinkler piping loses air; the data center defaultIn: Dry-pipe and pre-action trip test
Double riser
Parallel small and large risers with a trap, used on unloading systems to hold velocity across loadsIn: Oil return field guide
Double suction riser
Two parallel risers with a base trap, so oil returns at part load through the small riser and pressure drop stays in budget at full loadIn: Refrigerant line sizing field guide
Double-conversion online / VFI
Load always on the inverter, zero transfer, full isolation from the utilityIn: UPS types and technologies
A clear vertical space between the drain outlet and the receiver flood rim, with no direct connection, required by plumbing codeIn: Commercial ice machine field guide
Draindown
Binder running off the stone in an open-graded asphalt mix when it runs too hot or is held too longIn: Permeable pavement install
Drained area
The horizontal roof projection feeding a gutter, in square feet, plus any wall that drains onto itIn: Gutters and downspouts
Drainfield / leach field
The bed of trenches in native soil where effluent is dispersed and the soil treats it; also called the soil absorption fieldIn: Septic system field guide
Draw-through
Fan downstream of the coil, so the pan runs at negative pressure and a condensate trap is mandatoryIn: Fan coil unit field guide
All names for a large-diameter cast-in-place concrete shaft drilled to a firm bearing layer and reinforced with a steel cageIn: Drilled piers and caissons
Using a remotely piloted unmanned aircraft system to inspect assets that are dangerous, slow, or costly to reach, leaving a visual and thermal record.In: Drone inspection field guide
Drop-on beads are broadcast onto the wet line for night-one return; intermix beads are blended in to surface as the line wearsIn: Pavement marking materials and application
The cured coating thickness in mils, held within the data sheet range and measured per SSPC-PA 2In: Protective coatings and blasting
Dry pipe system
Pipe held under air or nitrogen; a dry-pipe valve trips and water follows when a head opens; used in freeze-prone spaceIn: Fire sprinkler system design guide
The three ways to hold the hole open: an open hole in stable ground, a steel casing, or a fluid that holds the wall by pressureIn: Drilled piers and caissons
Sprinkler system holding pressurized air in the pipe, with water released by the dry-pipe valve when a sprinkler fuses and the air bleeds offIn: Dry-pipe and pre-action trip test
Dry-shake color hardener
Pigment, sand, and cement broadcast on the fresh surface and floated in for surface color and a harder wear surfaceIn: Concrete coloring and staining
A low-resistance ohmmeter that reads the micro-ohm contact resistance across a bolted jointIn: Busway receiving and megger QA
Ductor / low-resistance ohmmeter
An instrument that measures a bolted connection in micro-ohms to find a high-resistance joint that torque alone cannot revealIn: Termination torque QA field guide
Ductulator
A slide-wheel or app version of the friction chart that converts CFM and friction rate into a duct size and velocityIn: Manual D duct design field guide
The design flow in GPM paired with the system head in feet; the point on the pump curve the pump is selected to hitIn: Hydronic pump install field guide
Duty rating
The scaffold's load class, light, medium, or heavy, tied to a load per square foot from the manufacturerIn: Supported scaffold safety
Duty rating / Type
Maximum rated load in pounds, worker plus tools; ANSI Type IAA 375, IA 300, I 250, II 225, III 200In: Portable ladder safety
Driver vehicle inspection report, the walk-around check of brakes, tires, lights, and safety items before a vehicle runsIn: Fleet and driver safety program
DWDM
Dense wavelength division multiplexing, many independent channels of light on one fiber, each on its own wavelength on the ITU gridIn: Data center interconnect (DCI)
Dwell time
The minutes a soft-wash solution sits on the surface to kill the growth before rinsing, without being let dryIn: Pressure and soft washing