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Limit Switches

Limit Switches

A limit switch detects the physical motion of an object by direct contact with that object. An example of a…


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Controller Output Current Loops

Controller Output Current Loops

The simplest form of 4-20 mA current loop is the type used to represent the output of a process controller,…


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Logic Programming in PLCs

Logic Programming in PLCs

Although it seems each model of PLC has its own idiosyncratic standard for programming, there does exist an…


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Control Valve Actuators

Control Valve Actuators

The purpose of a control valve actuator is to provide the motive force to operate a valve mechanism. Both…


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Phasors, Phase Shift and Phasor Algebra

Phasors, Phase Shift and Phasor Algebra

Phasors are to AC circuit quantities as polarity is to DC circuit quantities: a way to express the…


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Cascade Control

Cascade Control

A simple control system drawn in block diagram form looks like this: Information from the measuring device…


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Diagnosing Feedback Control Problems

Diagnosing Feedback Control Problems

Negative feedback systems, in general, tend to cause confusion for those first learning their fundamental…


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Basic Feedback Control Principles

Basic Feedback Control Principles

Feedback Control System Components Before we begin our discussion on process control, we must define a few key…


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DC Motor Speed Control

DC Motor Speed Control

DC electric motors generate torque by a reaction between two magnetic fields: one field established by…


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Digital PID Controllers

Digital PID Controllers

The vast majority of PID controllers in service today are digital in nature. Microprocessors executing PID…


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Valve Positioners

Valve Positioners

The reason why a pneumatic control valve’s stem position corresponds linearly to the amount of air…


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Practical PID Controller Features

Practical PID Controller Features

In order for any PID controller to be practical, it must be able to do more than just implement the PID…


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VSD and VFD Motor Drive Features

VSD and VFD Motor Drive Features

Modern DC and AC motor drives provide features useful when using electric motors as final control elements.…


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Integral (Reset) Control

Integral (Reset) Control

Imagine a liquid-level control system for a vessel, where the position of a level-sensing float sets the…


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Non-contact Temperature Sensors

Non-contact Temperature Sensors

Virtually any mass above absolute zero temperature emits electromagnetic radiation (photons, or light) as a…


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Digital Representation of Numerical Data

Digital Representation of Numerical Data

Process measurements are often of an analog nature: the temperature of a furnace, the rate of fluid flow…


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Instantaneous and Time-overcurrent (50/51) Protection

Instantaneous and Time-overcurrent (50/51) Protection

Perhaps the most basic and necessary protective relay function is overcurrent: commanding a circuit breaker to…


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AC Motor Speed Control

AC Motor Speed Control

AC induction motors are based on the principle of a rotating magnetic field produced by a set of stationary…


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AC Motor Braking

AC Motor Braking

There are several different methods useful for causing an AC induction motor to brake, or slow down: DC…


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Interposing Relays in PLCs

Interposing Relays in PLCs

In addition to directly performing logic functions, electromechanical relays may also be used as interposing…


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Pneumatic PID Controllers

Pneumatic PID Controllers

A pneumatic controller receives a process variable (PV) signal as a variable air pressure, compares that…


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Analog to Digital Conversion and ADCs

Analog to Digital Conversion and ADCs

In order to provide an interface between the internal (digital) world of a computer and the external (analog)…


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Auxiliary and Lockout (86) Relays

Auxiliary and Lockout (86) Relays

An important type of “accessory” relay, especially for legacy electromechanical protective relays,…


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Using Manometers for Measuring Pressure

Using Manometers for Measuring Pressure

A very simple device used to measure pressure is the manometer: a fluid-filled tube where an applied gas…


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Colorimetric and Potentiometric pH Measurement

Colorimetric and Potentiometric pH Measurement

pH is the measurement of the hydrogen ion activity in a liquid solution. It is one of the most common forms of…


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Energy in Chemical Reactions

Energy in Chemical Reactions

A chemical reaction resulting in a net release of energy is called exothermic. Conversely, a chemical reaction…


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Kirchhoff’s Current and Voltage Laws

Kirchhoff’s Current and Voltage Laws

Two extremely important principles in electric circuits were codified by Gustav Robert Kirchhoff in the year…


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Phasor Analysis of Transformer Circuits

Phasor Analysis of Transformer Circuits

Three-phase AC is the dominant mode of electric power generation and distribution in modern societies.…


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True Mass Flowmeters

True Mass Flowmeters

Many traditional flowmeter technologies respond to the volumetric flow rate of the moving fluid.…


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Safety Instrumented Functions and Systems

Safety Instrumented Functions and Systems

A Safety Instrumented Function, or SIF, is one or more components designed to execute a specific…


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Flow Measurement from Differential Pressure

Flow Measurement from Differential Pressure

Take for instance the problem of measuring fluid flow through a pipe. To the layperson, this may seem to be a…


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Practical PID Controller Features

Practical PID Controller Features

In order for any PID controller to be practical, it must be able to do more than just implement the PID…


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Level Sensor Accessories

Level Sensor Accessories

Disturbances in the liquid tend to complicate liquid level measurement. These disturbances may result from…


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The HART Digital/Analog Hybrid Standard

The HART Digital/Analog Hybrid Standard

A technological advance introduced in the late 1980’s was HART, an acronym standing for Highway…


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Relating 4 to 20 mA Signals to Instrument Variables

Relating 4 to 20 mA Signals to Instrument Variables

A 4 to 20 mA current signal represents some signal along a 0 to 100 percent scale. Usually, this scale is…


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Phasors, Phase Shift and Phasor Algebra

Phasors, Phase Shift and Phasor Algebra

Phasors are to AC circuit quantities as polarity is to DC circuit quantities: a way to express the…


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Comparison of  Op-Amp Circuits With Analogous Pneumatic Mechanisms

Comparison of Op-Amp Circuits With Analogous Pneumatic Mechanisms

Self-balancing pneumatic instrument mechanisms are very similar to negative-feedback operational amplifier…


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Continuous Float Level Transmitters

Continuous Float Level Transmitters

Perhaps the simplest form of solid or liquid level measurement is with a float: a device that rides on the…


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FOUNDATION Fieldbus H1 Protocol Data Link Layer

FOUNDATION Fieldbus H1 Protocol Data Link Layer

Like so many other industrial data networks, FOUNDATION Fieldbus is an “unswitched” or…


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Null-balance Voltage Measurement

Null-balance Voltage Measurement

A number of different sensors used in instrumentation generate DC voltage signals proportional to the process…


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