Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: Switch or Transmitter
Share
Notification Show More
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > Control Systems > Switch or Transmitter

Switch or Transmitter

Last updated: November 13, 2018 9:33 pm
Editorial Staff
Control Systems Instrumentation Design
No Comments
Share
8 Min Read
SHARE

For decades, process instrumentation specifiers have faced the decision whether to use a switch or a continuous transmitter for a given application. Either type of instrument can be used to effectively control industrial processes and protect equipment and personnel — and each has associated pros and cons. Application specifics typically drive decision-making, dictating which approach is most effective from performance, cost and life cycle support perspectives.

At its most basic, a switch acts in a binary fashion, changing state when a pressure, temperature, level or other process variable crosses over some predefined threshold. If the process variable in question can be allowed to vary in the course of normal operation, a simple mechanical switch linked to an on/off valve or pump can effectively and reliably control the process at hand, keeping a tank from running dry or a temperature from climbing too high.

Transmitters, on the other hand, continuously measure and communicate their assigned process variables over a range of values. A transmitter can facilitate on/off control actions similar to those of a switch through configurable discrete outputs within an associated controller. But a continuous transmitter teamed with a modulating control valve or pump with variable speed drive also can be used to implement more subtle (albeit more expensive) control strategies — such as a proportional-integral-derivative (PID) algorithm — to maintain the process variable at a specific value or setpoint.

A third option is the integrated or hybrid switch-transmitter, which combines a continuous transmitter and solid-state switch within a single instrument housing. This approach effectively combines a number of the advantages of both.

Process Requirements

When choosing among these three alternatives, the instrument specifier must first answer three key application-specific questions:

  • In the course of normal operation, can the process variable in question be allowed to vary between lower and/or upper limits, or is more precise setpoint control required?
  • Beyond the control action itself, is there a need for local display or remote communication and alarming of the process variable in question?
  • What are the consequences if the process variable does move outside its intended range? Will equipment damage, excessive downtime or an unsafe condition ensue?

Whether or not precise setpoint control is needed will determine whether a switch or transmitter output is needed to provide the necessary control functionality. If control over a range of setpoints is required, a continuous transmitter will be needed. Just how precisely the process variable must be measured — and controlled — will determine how accurate (i.e., how expensive) the transmitter will need to be.

If there is a need to display or communicate the process variable in question, application of a transmitter also is indicated. A switch by its very nature does not provide a measurement signal output to drive a local display or remote communications. It simply changes state when its threshold is crossed, initiating a discrete (on/off) control action.

Other considerations being equal, however, today’s switch technology can provide extremely reliable, cost-effective control when the process variable need only be controlled within upper and/or lower bounds without the need for remote visibility. The installed costs of a switch are generally less than those of a transmitter of comparable quality. Further, the switch can be expected to last on the order of 40 years whereas a transmitter, because of its electronic componentry, might face a considerably shorter life.

A switch also has the inherent advantages of simplicity: it comes factory-calibrated, is relatively simple to install, requires no power and connects directly to the final control element. No communication with a host controller is required, which can mean response time in as little as 5 milliseconds. Transmitters, on the other hand, typically communicate with final control elements through some sort of host controller — adding complexity, potential points of failure.

When Safety Matters

If adverse consequences such as equipment damage, excessive downtime or an unsafe condition are likely to ensue if the process variable moves beyond its intended range, risk analysis and safety considerations come to the forefront. In these types of applications, the transmitter holds one advantage over the switch in that it’s easier to tell if a transmitter is not working. This is both because of its continuous communications output and the extensive self-diagnostics built into today’s electronic transmitters. While normally quite reliable, there’s no obvious or automatic way to tell if a switch deployed in a protective (non-control) application has failed. To help alleviate this possibility, manual proof tests performed at designated intervals are needed to ensure ongoing switch integrity.

Increasingly, if a process variable excursion represents a significant safety risk or other adverse consequence, a continuous transmitter teamed with a back-up safety switch provides the most robust combination of protection and control, although periodic proof tests of the switch still are needed to ensure continued protection. Additionally, the use of intentionally diverse underlying sensor technologies decreases the likelihood of a common-cause failure of both devices, providing further assurance that if the transmitter and/or controller fails, the process will be brought to a safe state.

And while a separate transmitter and switch provides more secure operation than either instrument by itself, this approach does entail the purchase of two instruments, two process penetrations, and all the attendant effort and expense they represent. For this type of application, the hybrid transmitter/switch represents a more cost-effective yet secure solution: a continuous transmitter and solid-state switch installed together in a single device.

Indeed, the hybrid switch-transmitter addresses all three key application considerations raised above. Like a standalone transmitter, it can provide continuous setpoint control over a range of values as well as drive a local display or remote communications. Further, because the integrated switch and its output are functionally independent of the transmitter and its output, this single instrument configuration provides a more cost-effective alternative for applications where a continuous measurement is needed together with a functionally independent layer of switched protection.

Also Read: Instrumentation Design Gudelines

Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

Overview of PID Control terms
Centrifugal Compressor Start Permissive and Interlocks
What is Hammer Effect in Gauges ?
How to Create a Project in LabVIEW?
Transmitter Minimum Span, Accuracy & Turndown
Load Cell Design
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
Leave a Comment

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

What is a Mimic Panel ?
Cable Screen shall not be Grounded at Field Device ?
How to Avoid Bad Grounds ?
Motor Control Circuits
Control Valve Split Range Example
Basics of 4 to 20 mA analog Signals
4-wire Passive versus Active Transmitters
What is Inter Discipline Check (IDC)?

Keep Learning

Instrument Process Datasheet

Why is an Instrument Process Datasheet Required?

Types of Noise in Electronics

Types of Noise in Electronics

Location of Measurement Displays

Location of Measurement Displays

Level Transmitter trend

Identify Level Control Loop Tuning Problems

Signal Isolator

What is a Signal Isolator? Principle, Advantages, Disadvantages

DCS Program to Maintain Draft in Furnace

DCS Program to Maintain Draft in Furnace

Pull-up switch circuit.

Pull Up and Pull Down Switch Circuits

Advantages of Redundant System

Redundant Automation Systems – Need and Advantages

Learn More

PLC Program for Car Parking

PLC Program for Entry and Exit Control of Car Parking

Yokogawa DCS

Yokogawa DCS and SIS System Architecture

Instrumentation and Control Course

Free Instrumentation Course for Trainee Engineers

stepper motors Animation

Fundamentals of DC Motor Animation

3-way directional control valves

Directional Control Valves Working Principle

Delete Siemens CPU Memory

How to Delete the Siemens CPU Memory?

Capacitance Level Switch Working Principle

Capacitance Level Switch Working Principle

Industrial safety Objective Questions and Answers

Top 10 Industrial safety Objective Questions

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?