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Glossary Term: DIN
Abbreviation for the standards institution of the Federal Republic of Germany.
Glossary Term: DCS
Digital or Distributed Control System. DCSs historically refers to larger analog control systems traditionally used for PID control in the process industries, whereas PLCs were used for discrete or logic processing. However, PLCs are gaining capability and acceptance in doing PID control while the DCS has come to mean the…
Glossary Term: Digital valve
A single valve casing containing multiple solenoid valves whose flow capacities vary in binary sequence (1, 2, 4, 8, 16, ...); to regulate flow, the control device sends operating signals to various combinations of the solenoids; applications are limited to very clean fluids at moderate temperatures and pressures.
Glossary Term: Digital/Discrete I/O
Input or output that senses or sends either “on” or “off” (1 or 0) signals to the field. For example a discrete input would sense the position of a switch as energized or de-energized. A discrete output would turn a pump or light on or off.
Glossary Term: DP (Differential pressure) transmitter
A transducer designed to measure the pressure difference between two points in a process and transmit a signal proportional to this difference, without regard to the absolute pressure at either point. Often used to measure flow by the pressure difference across a restriction in the flow line or to measure…
Glossary Term: Differential gap
The smallest increment of change in a controlled variable required to cause the final control element in a two position control system to move from one position to its alternative position.
Glossary Term: Diaphragm valve
A valve with a flexible linear motion closure piece that is forced into the internal flow passageway of the valve body by the actuator.
Glossary Term: Diagnostic coverage
A measure of a system’s ability to detect failures. This is a ratio between the failure rates for detected failures to the failure rate for all failures in the system.
Glossary Term: Diaphragm
A sensing element consisting of a thin, usually circular, plate which is deformed by pressure differential applied across the plate.
Glossary Term: Design pressure
The maximum allowable working pressure permitted under the rules of the relevant construction code. See also pressure, design.
Glossary Term: Derivative control
Change in the output that is proportional to the rate of change of the input. Also called “rate control.”
Glossary Term: Decision table
A table of all contingencies that are to be considered in the description of a problem, together with the actions to be taken. Decision tables can be used in place of flow charts for problem description and documentation.
Glossary Term: Dead time
The interval of time between initiation of an input change or stimulus and the start ofthe resulting response.
Glossary Term: D Diagnostics
Some safety rated logic solvers are designated as having capital D diagnostics. These are different from regular diagnostics in that the unit is able to reconfigure its architecture after a diagnostic has detected a failure. The greatest effect is for 1oo2D systems which can reconfigure to 1oo1 operation upon detecting…
Glossary Term: Dangerous failure
A failure of a component in a safety instrumented function that prevents that function from achieving a safe state when it is required to do so. See failure mode.
Glossary Term: Cross talk
The unwanted energy transferred from one circuit, the disturbing circuit, to another circuit, the disturbed circuit. Typically signals electrically coupled from another circuit.
Glossary Term: Coriolis flow meter
A mass flow meter which measures mass flow of a fluid by determining the torque resulting from radial acceleration of the fluid. The name comes from the Coriolis effect that describes the accelerating force acting on any body moving freely above the earth’s surface, which is caused by the rotation…
Glossary Term: Consequence
The magnitude of harm or measure of the resulting outcome of a harmful event. One of the two components used to define a risk.
Glossary Term: Common mode failure
A random stress that causes two or more components to fail at the same time for the same reason. It is different from a systematic failure in that it is random and probabilistic but does not proceed in a fixed, predictable, cause and effect fashion. See systematic failure.
Glossary Term: CPU
central processing unit: The part of a computing system that contains the arithmetic and logical units, instruction control unit, timing generators, and memory and I/O interfaces. This is typically a very complex element which requires Type B classification for SIL hardware fault tolerance requirements according to IEC 61508.
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