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Dehydration – Removal of one or more molecules of water from a chemical compound. Delayed (Chronic) Health Hazard – See “EPA Hazard Categories.” Delivery Pressure – See “Outlet Pressure.” Density – The ratio of the amount of anything per unit volume; e.g., mass of any substance per unit volume at any definite temperature. It is usually expressed in pounds per cubic foot (lbs/ft3). See also “Specific Gravity.” Department of Transportation (DOT) – This is a government agency whose Title 49, Code of Federal Regulations regulates the transport of hazardous materials. Dewar – Vessel which contains cryogenic liquefied gases. Dew Point – The temperature at which the liquefaction of vapor begins; the term is usually applied to condensation of moisture from the water vapor in the atmosphere. Diameter Index Safety System (DISS) – DISS outlet valves are generally used with highpurity products, toxics, and corrosives. Valves equipped with DISS outlet assignment provide a metal-to-metal seal that creates low particle generation, a permeation-free environment, and good leak integrity. Dopant – An impurity usually added in small amounts to a pure substance to alter its properties. DOT ID Numbers – These are product identification numbers, assigned by the Department of Transportation (DOT) to assist members of fire and police departments in using the DOT Emergency Response Guidebook. DOT ID numbers contain two letters followed by four digits. The prefix UN (for United Nations) identifies products recognized throughout the world. Gaseous nitrogen, for example, is identified as UN 1066. Droop – The decrease in outlet set pressure of a pressure regulator which results from an increase in flow rate. Essentially the reverse of lockup. See also “Lockup.” DC - Direct Current. Produced by most motor driven arc welding machines and the newer “rectifier” sets. Direction Of Travel - The direction of travel is the direction in which the deposits are going. The angle at which the electrode is held, in relation to the direction of travel, is the angle between the electrode and the surface of the part not yet welded. Procedure sheets give the recommended angle, and this should be followed as closely as possible. Dehydration– Removal of one or more molecules of water from a chemical compound. Delivery Pressure – See “OUTLET PRESSURE”. Density – The ratio of the amount of anything per unit volume; e.g., mass of any substance per unit volume at any definite temperature. It is usually expressed in pounds per cubic foot (lbs/ft3). See also “SPECIFIC GRAVITY”. Dew Point – The temperature at which the liquefaction of vapor begins; the term is usually applied to condensation of moisture from the water vapor in the atmosphere. Droop – The decrease in outlet pressure of a pressure regulator which results from an increase in flow rate. Essentially the reverse of lockup. Also see “LOCKUP”. Defect - One or more discontinuities that exceed the acceptance criteria as specified for a weld. Depth of Fusion - The depth or distance that deposited weld metal extends into the base metal or the previous pass. Direct Current - Flows in one direction and does not reverse its direction of flow as does alternating current. Direct Current Electrode Negative (DCEN) - The specific direction of current flow through a welding circuit when the electrode lead is connected to the negative terminal and the work lead is connected to the positive terminal of a DC welding machine. Direct Current Electrode Positive (DCEP) - The specific direction of current flow through a welding circuit when the electrode lead is connected to a positive terminal and the work lead is connected to a negative terminal to a DC welding machine. Discontinuity - Any change in a metal’s typical structure. It is the lack of consistence in mechanical, metallurgical or physical characteristics. Discontinuities are found in all metals and welds because they have some degree of inconsistency in them. However, this is acceptable as long as the discontinuities do not exceed the acceptance criteria of the weld or metal in question. If a discontinuity exceeds the acceptance criteria, they are defects and must be repaired. Distortion - The warpage of a metal due to the internal residual stresses remaining after welding from metal expansion (during heating), and contraction (during cooling). Duty Cycle - The number of minutes out of a 10-minute time period an arc welding machine can be operated at maximum rated output. An example would be 60% duty cycle at 300 amps. This would mean that at 300 amps the welding machine can be used for 6 minutes and then must be allowed to cool with the fan motor running for 4 minutes. (Some imported welding machines are based on a 5-minute cycle). Depth of Fusion - The distance from the surface melted during welding to the extent of the fusion into the base metal or previous weld bead.
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