Understanding 4 to 20 mA Loops

David A. Snyder, P.E.

Course Outline

This 5 PDH course discusses the uses and limitations of 4 to 20 mA current loops.  These types of analog signals are used in almost every domestic industrial facility for analog input and analog output loops.  The main limitations of 4 to 20 mA analog output loops are the maximum analog output voltage, the total loop resistance, and the minimum load voltage requirement. The main limitations of 4 to 20 mA analog input loops are the loop power supply voltage, the total loop resistance, and the minimum transmitter voltage requirement.  HART communications on 4 to 20 mA analog signals is also discussed.  Most of the commonly-encountered process measurements, such as temperature, pressure, level, and flow, are explained with regard to how they are represented by 4 to 20 mA signals.  The three major types of modulating control valve characteristics, namely quick-opening, linear, and equal-percentage, are discussed, with detailed explanations of how the 4 to 20 mA signal affects the actuator position and resulting Cv of these valves.  There are more than 80 illustrations and dozens of examples to help increase the reader’s understanding of the text.  Many examples show how to obtain the answer using two methods – basic math calculations and looking at a simple graph.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

The following learning objectives are included in this course:

• Understand the difference between analog and discrete signals;
• Become acquainted with how a digital HART communications signal is added to a 4 to 20 mA analog signal;
• Calculate the limitations of an analog output control loop;
• Understand the concept of an operating region defined by a 4 to 20 mA transmitter’s operating parameters and limitations;
• Determine the operating point of a transmitter with respect to its operating region at a certain loop power supply voltage and total loop resistance;
• Understand how the 4 to 20 mA transmitter catalog cut-sheet defines the operating parameters and limitations and how to show them graphically.  Conversely, understand how to derive the operating parameters and limitations when only a graph is given on the transmitter catalog cut-sheet;
• Understand the difference between 2-wire (loop-powered), 3-wire, and 4-wire transmitters;
• Become acquainted with common decisions with regard to wiring and cable types for 4 to 20 mA loops, such as wire gauge (circular mil), color codes, conductor resistance, conductor stranding, and grounding of shields;
• Understand how loop isolators can be used to minimize the total loop resistance on an analog input loop;
• Calculate what a given current value of a 4 to 20 mA analog input (measurement) signal actually means in terms of temperature, pressure, level, or flow;
• Understand the difference between quick-opening, linear, and equal-percentage valve characteristics;
• Calculate the percent of maximum Cv for a given current value of a 4 to 20 mA analog output (control) signal sent to a modulating valve;
• Understand how variable speed drives are controlled by the 4 to 20 mA analog output (control) signal; and
• Become acquainted with how a 4 to 20 mA analog input (measurement) signal can be brought directly to a variable speed drive, bypassing the PLC or DCS, in some applications.

Intended Audience

Anyone who is involved in the specification, design, installation, or maintenance of 4 to 20 mA loops, instruments, and modulating control valves will benefit from this course.

Benefit for Attendee

Upon successful completion of the course, the reader will have a thorough understanding of 4 to 20 mA analog input loops and analog output loops, including their uses and limitations.  The reader will be familiar with the real world parameters and quantities that are represented by a 4 to 20 mA signal.

Course Introduction

Most industrial processes have at least one analog measurement or control loop.  An analog loop has a signal that can vary anywhere in the range between and including two fixed values, such as 1 to 5 VDC.  Other names for analog loops include modulating, throttling, and continuously variable loops.  Questions such as “How is differential pressure used to measure flow?” and “What does an analog input current of 10.5 mA mean with regard to the real-world temperature in the process?” will be answered in this course.  In addition, the relationship between the load limit graph and maximum resistance formula on transmitter catalog cut-sheets will be illustrated, and examples will be given that will explain how the loop power supply voltage and total loop resistance can be plotted as the operating point on graph of the transmitter’s operating region.

Course Content

The course content is in a PDF file:

Understanding 4 to 20 mA Loops

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Course Summary

The common-place 4 to 20 mA loop makes it easy to transmit a measured parameter from the instrument in the field to the control system in another location, or to send a control signal from the control system to a modulating control valve or variable speed drive in the field.  These 4 to 20 mA signals are useful and simple, but they do have some limitations, as discussed and quantified in this course.  In addition, a direct relationship is given between the analog input signal and the measured parameter, such as pressure or temperature, and between the analog output signal and the Cv of the most common types of modulating control valves.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.