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Glossary of terms used on this site

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Term Definition
D'Arsonaval Movement

A meter movement based upon the permanent magnet DC motor principle: a small coil of wire supported on jewel bearings or taut band between the poles of a permanent magnet. The magnetic field of the DC current passing through the coil interacts with the magnet's field, causing rotation of the coil and an attached pointer against the restoring force of coil springs.

DAC (Digital-to-Analog Converter)

An electronic device, often an integrated circuit, that converts a digital number into a corresponding analog voltage or current.

Damping

The manner in which the pointer of an analog instrument settles at its steady indication after the applied electrical energy is changed. Usually expressed as percent over-shoot.

Data Acquisition

A term commonly used for equipment or software that gathers data from devices through a communications channel.

Data Base

A large amount of data stored in a well-organized manner. A data base management system (DBMS) is a program that allows access to the information.

Data Highway

The proprietary data network used by Allen Bradley PLCs to communicate information to and from other PLCs on the network or to and from host computers attached to the network.

dB (Decibel)

20 times the log to the base 10 of the ratio of two voltages. Every 20 dBs correspond to a voltage ratio of 10, every 10 dBs to a voltage ratio of 3.162. For instance, a CMR of 120 dB provides voltage noise rejection of 1,000,000/1. An NMR of 70 dB provides voltage noise rejection of 3,162/1.

DC

Direct current; an electric current flowing in one direction only and substantially constant in value.

Dead Band

The range through which an input signal can be varied, upon reversal of direction, without initiating an observable change in the output signal. Dead band is the name given to a general phenomenon that can apply to any device. For the valve assembly, the controller output (CO) is the input to the valve assembly and the process variable (PV) is the output as shown in figure 1-1. When the term Dead Band is used, it is essential that both the input and output variables are identified, and that any tests to measure dead band be under fully loaded conditions. Dead band is typically expressed as a percent of the input span.

Dead Time

The time interval (Td) in which no response of the system is detected following a small (usually 0.25% - 5%) step input. It is measured from the time the step input is initiated to the first detectable response of the system being tested. Dead Time can apply to a valve assembly or to the entire process.

Dead Volume

The volume of the pressure port of a transducer at room temperature and ambient barometric pressure.

Deadband

The region through which an input can be varied without initiating a response.

Debug

To find and correct mistakes in a program.

Debye-H¸ckel Equation

Used in relating the activity coefficient (fi) to ion strength (see Activity coefficient):where I is the ionic strength, A and B the temperature-dependent constants (see Table A.5), Zi the valence of the ion (i), and  the ion-size parameter in angstroms.

Decibel (dB)

The standard unit for measuring relative strength of a signal parameter such as power or voltage. dB= 10 log (Pout/Pin); dB=20 log (Vout/Vin). 20dB corresponds to a voltage ratio of 10. A NMR of 70dB provides voltage noise rejection of 3162/1.

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