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Control and Instrumentation Glossary

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Duct and Instrument Diagram - the HVAC equivalent of a P&ID
An air distribution diagram which typically includes HVAC equipment; instrumentation; connections to other systems; duct sizes etc
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The suppression of oscillation.
Achieved using viscosity of a fluid for viscous damping, and the induced current in electrical conductors for magnetic damping.
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Distributed Control System
A control system with multiple processors and I/O subsystems connected by a network.
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Dead Time
The interval between the initiation of a change in the input and the start of the resulting observable response.

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The range through which an input can be changed without causing an observable response.

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Decibel (dB)
Unit for expressing a logarithmic measure of the ratio of two signal levels.

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Testing institute and Notified Body based in Denmark.

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Dew Point
Warm gas (e.g. air) can hold more water vapour than a cool gas. If the gas is cooled sufficiently it will reach a temperature where the gas becomes saturated and condensation will form. At this temperature the gas is said to have reached its dew point. Further cooling will allow further moisture to condense.

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An insulating material that can sustain an electric field but resists passage of electric current.

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Dielectric Constant
A number that expresses the degree of non-conductivity of different substances

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DIN - Deutsche Industrie Normen
German industrial standards

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An oscillation of small magnitude often introduced to overcome the effects of friction; hysteresis or clogging

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Double Block and Bleed
Double Block and Bleed consists of the closure of two block valves in series with an intermediate bleed valve used to provide isolation of process from maintenance personnel
Can be constructed from individual valves or in a single purpose built unit.
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Undesired change in the input-output relationship over a period of time.

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The UK's Department of Trade and Industry

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Name given to a family of stainless steels which have a near equal mix of austenite and ferrite.
Duplex stainless steels exhibit high strength and excellent corrosion resistance.
Further Reading
Stainless Steel Guide

Dynamic Range
Ratio of the largest to the smallest signal level a circuit can handle.
Usually expressed in dB
Further Reading


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