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Module: Principals / Introduction

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Glossary

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closed-loop control

A continuous monitoring and adjustment cycle to control a single process variable.

control loop model

A simplified diagram showing the flow of information along an instrumentation loop.

feedback control

Action the controller will take depending on feedback from the system.

indicated variables

Variables that are not controlled or manipulated but are measured and monitored.

manipulated variables

Variables that are directly adjusted by the controller to correct a controlled variable.

open-loop control

A control that does not use feedback to determine the actions of a process.

absolute pressure

Gauge pressure plus the local atmospheric pressure.

atmospheric pressure

The pressure exerted by the air around you which is approximately 14.7 psi.

bellows element

A pressure sensing element that uses the elastic deformation of a series of capsules that expand or contract with changes in pressure.

bourdon tube

A pressure sensing element that detects changes in pressure by the elastic deformation of a flattened tube that expands or contracts.

compound gauge

A gauge that measures pressure both above and below atmospheric pressure.

gauge pressure

Absolute pressure minus atmospheric pressure.

manometer

A hollow transparent tube partially filled with a liquid and used for measuring static pressure at medium intensity.

meniscus curve

The interface between the air and the liquid in a chamber caused by surface tension.

P = F / A

Pressure equals force divided by area.

parallax error

An error occurring when you view a pointer position from the angled viewpoint instead of from eye level.

pressure

One of the most common measurements in process industries, typically expressed as pounds and area in square inches resulting in pounds per square inch.

pressure transmitters

Devices that measure pressure through the use of a sensor and output the measurement to a control loop.

infrared digital thermometers

An instrument that analyzes the infrared energy emitted from objects to measure temperature.

resistance temperature detectors

Temperature sensing elements constructed of fine wire wound around a non-conductive material.

thermistors

Very small resistors made of a ceramic/metal oxides mixture sensitive to very small changes in temperature.

thermocouples

Electronic sensors containing two different metal conductors that are joined together to form a junction.

thermowells

Pressure-tight sheaths inserted into a process steam to allow temperature sensors to detect the process temperature without coming into contact with the process fluid.

Bernoulli’s principle

As flow increases, pressure decreases.

bluff body

The part of a vortex flow meter that is placed across a fluid stream.

coriolis

A mass flow meter that measures the rate of a fluid travelling through a moving tube.

flow

The measurement of fluid movement.

flow rate

The measure of the units of a fluid flowing past a specific location over a period of time.

fluid

A substance that has the ability to flow, either a liquid or gas.

laminar

A fluid path which has a smooth flowing pattern.

nutating disc

A common type of positive displacement flow meter.

orifice plate

A metal disc that constricts a pipe’s diameter to accelerate a fluid.

pitot tube

An L - shaped instrument inserted into a process pipe with its open end facing upstream.

Reynolds number

A representation of whether a flow profile is laminar or turbulent.

rotameter

A variable area flow meter that uses pressure to measure flow.

turbulent

A fluid path which is continuously changing.

ultrasonic flow meter

A meter that emits high frequency sound waves that reflect off particles in the flow stream.

vena contracta

The smallest cross section and the point of lowest fluid pressure after an orifice plate.

venturi tube

A tube with a smooth cone-shaped inlet and outlet

viscosity

A measure of the internal friction of the molecules in a fluid.

weigh feeder

A device used to measure and control the flow of solid materials.

bubbler system

Device that uses a purge gas to measure pressure exerted by a liquid to determine level.

displacer

Instrument used to measure liquid-liquid interfaces.

head pressure

The pressure exerted by a liquid.

innage

The measurement of a fluid's height from the bottom of the tank to the surface of the material.

level gauge

Device used to visually measure level through a viewing chamber connected to a vessel.

level measurement

The determination of the height of a process material's surface in reference to a zero point.

ullage

The measurement of height from the surface of the process material to the top of the vessel.

zero suppression

Transmitter signal offset calibration to a positive number.

alarm

A switch that indicates abnormal ranges for process variables.

bypass switch

A device used to override the normal operations of a system.

flow switch

A switch that uses paddles placed in a fluid to detect flow.

HOA

A three position toggle switch for Hand/Off/Auto

limit switch

A switch which detects the position of an object by direct contact.

normal state

The condition of minimum stimulus for the sensed variable.

proximity switch

A switch that detects the presence of an object without physically touching it.

relay

Device used to pass on information between instrumentation.

shutdown switch

A switch used in fail safe system designs to deactivate a circuit.

switches

Sensors that trigger an action in a process.

vibration switch

A switch used on velocity-based equipment to provide early warnings for equipment wear or impending failure.

signal conversion

A calculation that equates process signals from on signal type to another.

smart transmitter

A transmitter with built-in microprocessor circuits.

transducer

A device which changes a signal from one form to another.

transmitter

An instrument that outputs sensed variable information in a format that is suitable for a controller.

bumpless transfer

Changing to automatic or manual mode without the controller changing the output signal.

controllers

Instruments that make decisions to control a process variable.

dead time

A measure of time between a manipulated variable signal change and when the resulting response can be observed in the controlled variable.

distributed control systems

Computer-based systems that are inter-connected to reduce the potential of a catastrophic failure.

error

The difference between the target and the measured process variable.

gain

A term describing the proportional relationship between the magnitude of the controlled variable signal and the magnitude of the manipulated variable signal.

hunting

Cycling of the controlled variable signal above or below the set point.

hysteresis

A lack of responsiveness to a change in the direction of the manipulated variable signal.

lag time

The period of time, after the dead time period, that it takes a controlled variable to reach equilibrium following a manipulated signal change.

loop tuning

The act of adjusting the proportional, integral, or derivative mode parameters to optimize control for a particular process and its response dynamics.

offset

The difference between the controlled variable and the set point when the manipulated variable signal has not been changed.

oscillation

The controlled variable signal cycling above or below the set point as the process returns to a steady state.

process equilibrium

The condition of a process when the system is balanced and is running in a steady state.

programmable logic controllers

Computer-based systems that use a single microprocessor to manage multiple control loops.

proportional band

The percentage of the controlled variable signal required for the controller to send its full output signal.

automated control valves

Valves that restrict fluid flow to manipulate a controlled variable as part of a control loop.

discreet control valves

Valves used where the desired flow is either on or off.

louvers

Device used as a final control element to modulate air flow in a process.

valve positioner

Device that compares the valve stem position to a control signal and adjusts pressure to the actuator diaphragm or piston.

bias control

The addition of a value to the measured variable signal to determine the controller’s output signal.

cascade control

Two controls loops combined so that one controller provides a set point for the other control loop.

feedforward control

Taking corrective action before a process variable deviation occurs.

ratio control

Maintaining a fixed ratio between two or more variables regardless of flow rate changes.

split range control

Two or more final control elements used to control a process.

alarms

Auditory or visual signals warning personnel of upsets in a process.

Emergency Shutdown Systems

Devices responsible for shutting down equipment in extremely abnormal situations.

interlocks

Two or more control loops interacting with one another to take either a process or a safety action.

permissives

A set of conditions that must be met before a piece of equipment can be started.

simulated input signals

Test signals sent to a controller to help isolate or eliminate signal problems.

troubleshooting

A logical, systematic process of thinking used to repair malfunctioning systems.

Learning Aids

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Principals - Learning Aid

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Principals Flash Cards

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Pressure - Learning Aid

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Pressure Flash Cards

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Temperature - Learning Aid

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Flow - Learning Aid

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Level - Learning Aid

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Position Indication - Learning Aid

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Signal Transmission - Learning Aid

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Controllers - Learning Aid

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Final Control Elements - Learning Aid

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Advanced Process Control Strategies - Learning Aid

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Emergency Shutdown - Learning Aid

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Emergency Shutdown Flash Cards

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Troubleshooting - Learning Aid

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Principals

Topics

1 . Introduction
2 . Automated Control Systems
3 . Process Variables
4 . Instrumentation Functions Part 1 of 3
5 . Instrumentation Functions Part 2 of 3
6 . Instrumentation Functions Part 3 of 3
7 . Control Loop Model

Instrumentation

Topics

1 . Principals
2 . Pressure
3 . Temperature
4 . Flow
5 . Level
6 . Position Indication
7 . Signal Transmission
8 . Controllers
9 . Final Control Elements
10 . Advanced Process Control Strategies
11 . Emergency Shutdown
12 . Instrumentation Troubleshooting