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Glossary Term: Overrange limit
The maximum input that can be applied to a device without causing damage or permanent change in performance.
Glossary Term: Overrange
In process instrumentation, of a system or element, any excess value of the input signal above its upper range value or below its lower range value
Glossary Term: OSI
Open system interconnection. A seven layered model of communications networks defined by ISO. The seven layers are: Layer 7 Application: provides the interface for application to access the OSI environment. Layer 6 Presentation: provides for data conversion to preserve the meaning of the data. Layer 5 Session: provides user to…
Glossary Term: Orifice meter
A plate with a calibrated sharp edged hole in it. The plate is positioned across the flow stream in a pipe for measuring fluid flow rates. It typically has differential pressure taps positioned near the orifice and a calibrated calculation element to convert the measured pressure difference into a flow…
Glossary Term: On-off control
A simple form of control whereby the control variable is switched fully on or fully off in response to the process variable rising above the set point or falling below the set point respectively. Cycling always occurs with this form of control.
Glossary Term: Offset
1. A sustained deviation of the controlled variable from set point. This characteristic is inherent in proportional controllers that do not incorporate reset action. 2. Offset is caused by load changes. 3. The steady state deviation when the set point is fixed. NOTE: The offset resulting from a no load…
Glossary Term: Occupancy
A measure of the probability that the effect zone of an accident will contain one or more personnel receptors of the effect. This probability should be determined using plant-specific staffing philosophy and practice. See effect zone.
Glossary Term: Nozzle
1. A short flanged or welded neck connection on a drum or shell for the outlet or inlet of fluids; also a projecting spout through which a fluid flows. 2. A streamlined device for accelerating and directing fluid flow into a region of lower fluid pressure. 3. A particular type…
Glossary Term: Noise
1. In process instrumentation, an unwanted component of signal or. See “interference, electromagnetic”. 2. Any spurious variation in the electrical output not present in the input. 3. An unwanted component of a signal or variable which obscures the information content. 4. Random variations of one or more characteristics of any…
Glossary Term: NC / (NO)
Normally Closed (Normally Open) 1. A switch position where the usual arrangement of contacts permits (prevents) the flow of electricity in the circuit. 2. In a solenoid valve, an arrangement whereby the disk or plug is seated (open) when the solenoid is de-energized. 3. A field contact that is closed…
Glossary Term: NIOSH
(US) National Institute of Occupational Safety and Health
Glossary Term: NEMA standard
Consensus standards for electrical equipment approved by the majority of the members of the US National Electrical Manufacturers Association.
Glossary Term: Needle valve
Its essential design feature is a slender tapered rodlike control element which fits into a circular or conoidal seat. Operating the valve causes the rod to move into or out of the seat, gradually changing the effective cross sectional area of the gap between the rod and its seat. Typically…
Glossary Term: MWP
maximum working pressure. See Pressure, maximum working
Glossary Term: Multiplexing
The transmission of a number of different messages simultaneously over a single circuit.
Glossary Term: MTTFS
Mean Time to Fail Spurious - The mean time until a failure of the system causes a spurious process trip.
Glossary Term: MTTR
Mean Time to Repair – The average time between the occurrence of a failure and the completion of the repair of that failure. This includes the time needed to detect the failure, initiate the repair and fully complete the repair.
Glossary Term: MTTF
Mean Time to Failure - The average amount of time until a system fails or its “expected” failure time. Please note that the MTTF can be assumed to be the inverse of failure rate (lambda) for a series of components, all of which have a constant failure rate for the…
Glossary Term: Modulation
1. The process or the result of the process by which some characteristic of one wave is varied in accordance with some characteristic of another wave (AM, amplitude modulation; PM, phase modulation; FM, frequency modulation). 2. The action of a control valve to regulate fluid flow by varying the position…
Glossary Term: Mode (Low Demand)
(also demand mode per IEC 61511) when demands to activate the safety instrumented function (SIF) are infrequent compared to the test interval of the SIF. The process industry defines this mode when the demands to activate the SIF are less than once every two proof test intervals. The low demand…
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