The Book of Instrumentation
In 2018, Visaya’s content team got together and decided to create a book of instrumentation!
This article will be under construction until the end of 2018, and we’ll fill it with content throughout the year. Don’t worry, we’ll organize it all and keep it readable (I hope).
Stick with us during this challenge in 2018 (along with my own challenge to lose weight, but that’s a topic for another blog)!
As a student, if I wanted to read more about the topics I was studying, I had to visit the school library. In general, technical books are painfully hard for a student to afford. I still wonder why those books are so expensive.
Now we have a bunch of process automation websites out there, but they all seem the same. Some have great articles, but most are biased, making it hard to find neutral information on topics such as measuring principles or Industry 4.0.
And many authors still share their insights the old-fashioned way, offline. Of course, online you have to share in a different way or risk losing your readers when they don’t find what they want. We developed Visaya’s content with those challenges in mind.
First off, besides all the technical stuff, we answer the questions people ask Google. Second off, we provide all you need to know about a vendor’s device before choosing it for your application. Third off, we find fun and interesting ways to talk about our nerdy automation stuff to make it worth reading at home as well as at the plant.
Since the start of 2017, we’ve posted more than 600 articles covering different topics and answering a wide range of questions. Now, we want to condense those articles into a free digital book, The Book of Instrumentation. It will allow you to find everything you need to know about instrumentation at your fingertips. Then we can go forward in other topics.
This book will cover the basics of the process automation world, measuring devices, digital protocols, final elements, and control systems.
Hope you enjoy it!
Table of Contents
- Chapter 1 – The automation world
- Chapter 2 – Pressure measurement
- Chapter 3 – Level measurement
Chapter 1 – The automation world
Let’s start from the beginning, defining automation! You’ve heard about automation at least once in your life, because this broad topic covers things in your daily life as well as in big industries. Even though your daily automation differs dramatically from industrial automation, they follow the same principle.
Okay, so what does automation mean?
We can define automation as the procedure to make things run without humans to control them all the time. Automation can control many kinds of processes and machinery. For this to happen, you must measure a certain physical or chemical variable. A control system can compare the variable with a setpoint, then actuate a final element to control it.
You can control any kind of process – electrical, pneumatic, hydraulic, and so on. You’ll also find automation in different industry segments – water treatment, food production, mining, and more. We have a section here at Visaya where you can view lots of content on different industries.
Then we have building automation for heating and cooling, fire suppression, access, security, and so on. And now, with digitalization and the Internet of Things (IoT), we have so many smart devices at home. Alexa and Google Home can control lamps, coffee machines, heaters, and other stuff in the average home.
Within this range of topics, we’ll share things related to process automation like application types, field networks, control systems, and frequently asked questions.
The new and old automation pyramid
I’m sure you hear about the automation pyramid once a day, right? This topic will review automation as it is, but also as it may become with IoT, cloud computing, and Industry 4.0 stuff. If you already know about the pyramid, don’t leave yet, because we’ll cover the NEW architecture.
A good chunk of automation discussion on the internet centers around the structure of the pyramid now with all the smart sensors and whatnot. We have all kinds of field sensors to measure and detect certain process variables. These smart and not-so-smart sensors send their data to the control system through standard communication protocols.
Once all this data reaches the control system, it uses the data to control and actuate the final elements. The control system maintains the quality, quantity, security, and schedule of the process.
The automation pyramid provides an easy understanding of all automation levels in the process, from measuring to management.
You can read all about it in our article on the old and new automation pyramid.
Okay, now that we have the basics on process automation, what’s the difference between a technician and an engineer? Will I do the same work as an engineer if I’m a technician? Will I make the same amount of money? Hold on! We’re getting to that!
Both technicians and engineers can create and modify automated processes. However, you could call the technician the “doer” in this picture. Technicians get more hands-on experience than engineers and focus on a job’s practical elements. They provide assistance in certain areas and perform the daily tasks required to keep processes running smoothly.
Click on the link to read our post on the automation technician.
With so many types of automation, how can you choose where you’ll fit best? And what exactly does an automation engineer do? I’m sure these questions have haunted many a young STEM hopeful.
An automation engineer sets the automation of a process. You must understand the process and its needs to select good devices to monitor, measure, and control all the necessary variables. This work includes choosing and programming a control system for the process.
At this stage, you have protocols, the languages spoken between measuring devices and control systems. Four main protocols cover most applications – analog, HART, PROFIBUS PA, and FOUNDATION Fieldbus. More have begun to take hold, like EtherNet/IP and PROFINET.
Click this link to read the entire article about the automation engineer.
So you graduated an engineering course with a bachelor’s degree. What’s next? The job market, right? When you start looking for a job, one question which will certainly come up is, “What’s your salary expectation?”
In school you learn all about the physics, electronics, and logic behind process automation. However, who talked about how much all that knowledge is worth when you’re trying to enter the market?
To give you a idea of an automation engineer’s salary, we at Visaya did a little research on how much you should earn, based on data from websites such as PayScale, Indeed, SalaryList, Glassdoor, and SalaryExpert.
In this research we looked for both recently graduated engineers, with little to no previous experience, and engineers with more than 5 years of experience.
Click this link to read the entire article about the automation engineer salary.
How do you learn process automation?
The path – or paths, I should say – to become an automation engineer is not as straightforward as the one for a civil engineer or chemical engineer, for example. In many countries, you won’t find any automation engineering degrees. So what should you study if you want to work in process automation? Which course should you go for? Is university the only way?
Process automation requires a large set of skills. You need the physics and mechanics behind the process and its measuring devices. And you should know about the electronics of the transmitters as well as the wiring and communication between the field devices and the control system. Then you also have the programming of the control unit.
As you can see, we can break down these parts in three main courses when it comes to university degrees: mechanical engineering, electrical/electronic engineering, and software engineering or computer science.
These three choices can lead you to become an automation engineer. The difference is the focus you will have in each one. In the end, you will probably learn a bit of all of them.
For those of you not so keen to spend four years or more in a university, you have the technical schools and associated degrees to give you a start.
Click this link to read the entire article on how to learn process automation.
What are measuring devices for?
Earlier in this book, we talked about the old and new automation pyramid. I hope you haven’t forgotten already. In the first level, the pyramid base, we have the measuring devices. But what are they used for?
Well, if you paid attention when you read the “what is automation” part of this book, you probably already know, but let’s make it clear.
Measuring devices read a certain process variable and send the data to a control system. That system analyzes the data, decides what action to take, and then sends commands to the actuator to bring the process value closer to your setpoint.
Process automation has many kinds of measuring devices. However, they can be divided into five main categories: pressure, level, flow, temperature, and analytics.
We’ll talk more in depth on each of these categories in the next chapters of this book.
Click this link to read the entire article on measuring devices.
Piping & Instrumentation Diagram (P&ID)
The P&ID is a graphic representation of your whole process. It uses standard symbols and annotations of all the instrumentation, piping, and system components of your process. It is a very important tool to use if you need to manage a physical process.
When you are in the planning stage of your process, it will help you to see how everything goes together even before you set a single storage tank or field instrument in place.
After your process is up and running, it can help with maintenance, plant safety, and training.
In order to standardize the instrumentation symbols used in the P&ID the International Society of Automation (ISA) created the ANSI/ISA’s S5.1-1984 (R 1992) standards.
These standards help engineers to communicate instrumentation, control, and automation goals consistently.
Click this link to read the entire article on P&ID and the ISA 5.1.
Chapter 2 – Pressure measurement
Pressure is a primary variable in process automation. That means you can also measure other variables, like level and flow, using pressure measurement.
Back in your school days, you probably learned that pressure equals the value of a certain force applied in a certain area. Process automation has three types of pressure measurement – differential, gauge, and absolute pressure.
For differential pressure (DP) measurement, you need to measure the difference of pressure between two points in your process. You have high- and low-pressure ends in the transmitter. And with DP measurements, you can also calculate level and flow.
Gauge pressure is a specific type of differential pressure. Here the low-pressure end of the transmitter vents to the atmosphere. Therefore you measure the difference between the atmospheric pressure and the pressure in your process.
Absolute pressure works similarly to gauge pressure, but instead of atmosphere for the low-pressure chamber, you have a vacuum.
Click this link to read the entire article on what pressure is.
The pressure transmitter measures the pressure in your process. It can also measure more than pressure. If you combine it with a primary element, for example, you can measure flow. If you use it in a tank, you can find the level of the tank’s contents.
The transmitter itself consists of an electronic board connected to a sensor. There are many kinds of sensors – piezoelectric, capacitive, resonant silicon, and more. The transmitter reads the pressure detected by the sensor and transforms it into a standard output signal we can read.
Click this link to read the entire article on pressure transmitters.
If you want to measure level using pressure, then you need some math. Sure, the transmitter can do it for you, but you’ll need to understand what it’s doing.
Measuring level with pressure uses the Pascal equation as its basis. For this equation, pressure (P) equals the liquid’s density (ρ) times acceleration due to gravity (g) times the liquid column’s height (h), or P = ρ * g * h. With this formula, if you know the density of the fluid, you can calculate the level.
If you want to read more about level measurement based on pressure, then check out our definitive guide to pressure transmitters. It has an entire section dedicated to level.
Unlike measuring level with pressure, when you want to read the flow of a liquid in a pipe, you need more than a formula. Here you have a bit more to consider:
- The primary element creates a restriction on the flow and thus a difference in pressure before and after it.
- The installation structure – impulse lines, tubing, valves, and other mechanical bits – sends the pressure from the primary element to the transmitter.
- The transmitter reads this pressure and transforms it into numbers you can read.
A little more complicated than level, right? But don’t worry, we’ll go over all of them briefly. And if you want to read more, we have a section in the definitive guide to pressure transmitters dedicated to flow.
First, let’s start with the math and physics. Just as level measurement uses the Pascal formula, flow uses the Bernoulli equation as its basis.
With the Bernoulli equation, once you know the characteristics of the liquid, you can find the DP between two points in the pipeline. With the DP, you can calculate the flow.
However, to create this pressure difference, you’ll need a primary element. The primary element restricts the area in which the liquid flows, forcing a pressure drop in that section. The market offers many types of primary elements, such as orifice plates, pitot tubes, and venturi tubes.
If you want to learn more about this topic, you can read our article on differential pressure transmitters in flow measurement.
Types of pressure device on the market
When we talked about what pressure is, we gave you a hint of some of the different pressure devices on the market. We talked about gauge, absolute, and differential pressure, right? Well, guess what? Each type of pressure has its own type of device! We also gave you a hint on how they work, but let’s recap here.
The DP transmitter is the most common type out there. No surprise, because with a DP transmitter you can measure level and flow as well as pressure. The sensor of a DP transmitter has two chambers, the high- and low-pressure ends. The sensor reads the pressure in these chambers, and the transmitter converts the sensor’s data to a number we can read.
Two other types are the gauge and absolute pressure transmitters. They resemble DP transmitters in that they also have two pressure chambers. However, the process applies pressure just in one of them, the high-pressure end. The low-pressure end in the gauge device has the atmosphere as a reference, and absolute has a vacuum.
Besides these three types, you can also find fancy options, like electronic DP transmitters, to ease installation in confined spaces, and wireless gauges, which look and feel like analogic manometers but have wifi technology built in.
We talk about all these transmitters in our definitive guide, if you want to read more.
If you haven’t fully explored our content here at Visaya, we have two formats focused on answering questions from you, our marvelous audience.
In the #WishIknew format, we answer basic questions about process automation in a quick and useful way. The #WishIknews brief you on how devices work, installation, calibration, and more.
For basic questions about pressure measurement, we made a number of #WishIKnews and put the following seven in this article:
- What is pressure measurement?
- How do you measure pressure?
- What is absolute pressure measurement?
- Differential pressure measurement?
- Static pressure measurement?
- How do you measure level with a pressure transmitter?
- What is the difference between hydrostatic and differential pressure transmitters?
Some of these questions we answered here as well.
Our Q&As answer questions sent from our community. If you have a question about process automation, drop us a line and we’ll answer it! We picked two Q&As for you to check out.
I want to use the Smar LD300 I have on a new tank. However, the device is configured for the old tank. How do I set it up for the new tank?
First, find your lower range value (LRV) and upper range value (URV). This calculation will depend on your tank type and installation. To learn more, you can review our definitive guide to pressure transmitters.
When you have these values, you need to set them on your transmitter. So connect your handheld to your LD300, then find the configuration tab.
Under this tab, you’ll find “Range.” And in that menu, there’s “Configure Range.” Here you can choose “Without Reference,” “Low with Reference,” or “Up with Reference.” Once you set the values, you’re good to go!
You can also use the local configuration to set the LRV and URV without the handheld. It’s more complex, but you can read the entire Q&A here.
I have a Rosemount wireless pressure gauge that I was using without its wireless connectivity. Now I want to set it up to communicate with my new wireless gateway. Can you tell me how to do it?
You’ll need your gateway, network ID, and join keys. You’ll also need a handheld with the proper device description installed.
After you remove the device cover, connect the handheld to the “comm” port at the bottom of the dial. Then use it to access the menu.
On the menu, click “Configure,” then “Basic Setup” and “Join device to network.” Now, add the network ID and join keys. The gateway should automatically connect. And there you go!
Chapter 3 – Level measurement
We have many different ways to measure level in a process. Although you can use a differential pressure transmitter, we already covered that in the pressure chapter of this book. So now, let’s move on to some other ways to measure level.
Time of flight
Before we dig into the different kinds of devices, it would help to know the concept behind them, the time-of-flight (ToF) principle. And you may know this one better than you think! You’ve heard it referred to as echolocation or sonar or radar. Let’s break it down with the bat example.
In order for a bat to find its prey, it sends out ultrasonic waves, sounds too high for us to hear. When these sounds reach the prey, they bounce back and the bat senses them. With this information, the bat can tell where the prey is located and even its size.
OK, but how do bats relate to process automation?
The time of flight concept works similarly. The device sends out electromagnetic or mechanical waves. When these waves hit a surface – the product inside a tank, for instance – they bounce back to the device. With this information, it can determine the level inside a tank. Now we’ll get into the types of devices that use this concept.
The non-contact radar level transmitter, also known as free-space radar, sends waves from the device on no guided path, just out in the open. If you want to know how this makes a difference, keep reading. It’ll make more sense in the next section of this chapter.
When it comes to non-contact radars, you need to focus on two aspects, frequency and antenna size. Both parameters affect your beam angle and spread.
Most radars in the market work in one of four frequency bands. You have C-band at six gigahertz (GHz), K-band at 26, X at 10, and a few on W at 75 to 85.
The antenna size determines your beam angle. Bigger antennas will give you smaller beam angles for the same frequency. However, if you have higher frequencies, then you can achieve low beam angles with smaller antennas. Check out this table to see what I mean.
You definitely need to know your beam angle if your process has low tanks or tanks with interference, or if you need to install your radar close to the tank wall.
The VEGAPULS 64 gives you a great example of a tiny beam angle. In fact, we have a video testing whether it can measure the level of a water bottle!
Guided wave radar
Just like non-contact radars, the guided wave radar works on the ToF principle. However, here a probe guides the wave. So when should you use a guided wave radar instead of a non-contact radar?
Depends on your process conditions. If you have a product with a low dielectric constant or high turbulence in your process, you may want a guided wave radar.
When it comes to choosing one, you’ll need to remember to pick a probe that will work in your process conditions. If you have a turbulent process, then you’ll need to anchor your sensor. And your probe should not touch any metal, such as the tank wall or a mixer.
There are three main types of probes – single element, twin element, and coaxial. Manufacturers will have tables to help you choose the right one for you. However, we’ll give you a brief overview.
Coaxial probes cover a vast array of applications, including those with low dielectric constants. Twin-element probes have good measurement range, and you can get either a flexible or rigid one. In viscous or sticky products, you should probably go for a single-element probe.
If you want to learn more about this topic, read our article on guided wave radars.
This time-of-flight device uses mechanical waves instead of electromechanical like the radars. While radar waves travel at the speed of light, the waves of an ultrasonic level transmitter travel at the speed of sound.
Ultrasonic have fairly easy setups. Moreover, they work in applications with density, dielectric, or viscosity changes. On the downside, foam, steam, vapor, and turbulence can cause problems. And they don’t work in vacuum at all. You also need to remember that an ultrasonic has a dead band, a range in which it doesn’t work. You have to factor that into your calculations for your measurement range.
If you want to learn more about this topic, read our article on ultrasonic level transmitters.
One of the newer technologies, this device also uses waves that travel at the speed of light like radars. One advantage of laser devices is that they work regardless of the dielectric constant of your product. If you’re measuring vapor level, for example, it will perform better than an ultrasonic level transmitter.
On the other hand, if you have fine particles in your tank’s atmosphere, then you should probably use something other than a laser. The particles will affect your signal and therefore your readings.
If you want to learn more about this topic, read our article on laser level transmitters.
So far in this chapter, we presented devices that use the ToF principle. The capacitance level transmitter is the first that doesn’t.
As the name suggests, the device works as a capacitor. Here, the probe acts as one of the capacitor plates, and the metallic vessel acts as the second plate. The material between them is the insulating material, which has a relative dielectric constant (K). The capacitance (C) will depend on the area of your plates (A), the distance between them (d), and the dielectric constant (E).
With all these variables, the transmitter can calculate the capacitance of the sensor and thus the level in the vessel through the following formula:
C= E(K A/d)
- C = capacitance in picofarads (pF)
- E= constant
- K = relative dielectric constant
- A= effective area of the plates
- d= distance between the plates
And how does the capacitance reflect the level of the tank? Well, when the tank level is zero, you have just air as your insulating material. As soon as your product reaches a certain level, it becomes the insulating material, changing the capacitance read by the sensor. Ta da!
We said earlier that the metallic wall of the vessel works as one capacitor plate. However, what happens if you have a plastic or nonmetallic vessel? Well, then you need a probe with a concentric tube to work as the capacitor plate. These sensors are called double-rod electrodes.
If you want to learn more about this topic, read our article on capacitance level transmitters.
So far we’ve seen two working principles for level measurement, ToF and capacitance. However, we have other options on the market. Displacer and servo devices work on Archimedes’ principle, also called the law of buoyancy. If you don’t remember this one, then we’ll review for you.
Archimedes’ principle states that fluid exerts an upward force on a body immersed in it, whether partially or completely submerged. This force will equal the weight of the fluid the body relocated upward at the center of mass of the relocated liquid.
You’ll find displacers in different sizes, but most have a cylindrical shape and a regular cross section. You normally install one in a bypass and connect it to a spring to limit its movement. When the level of your tank changes, the buoyant force measured by your sensor will also change. The transmitter will read the force, translate it to a known level measure, and send the data to your control system.
If you want to learn more about this topic, read our article on displacer level transmitters.
Servo level transmitters work very similarly to displacers in that they also use Archimedes’ principle. In fact, the servo transmitter has a displacer in it! So how do they differ from displacer transmitters?
In the servo device, the displacer is connected to a measured wire around a wire drum. The device calculates levels using the distance between the wire drum and the displacer when it touches the product surface.
It detects the surface by monitoring the change in the displacer weight as it begins to float. The drive motor adjusts the displacer’s position as the level changes. You can calculate this distance between the wire drum and the displacer with the number of rotations of the drum it took until the displacer made contact with the liquid’s surface.
If you want to learn more about this topic, read our article on servo level transmitters.
Unlike the other level devices we’ve discussed so far, the vibration fork won’t give you a continuous measurement. It mostly works as a level switch to prevent tank overfill. It can also measure density, but that’s another topic. Let’s see how the vibrating fork works.
This device uses the principle of the tuning fork. If you ever used a tuning fork to tune a musical instrument, you’ll get this one pretty quickly. When you hit the fork, it oscillates in a certain frequency and emits the sound you want to replicate on your instrument.
The vibration fork has two prongs which oscillate at its natural frequency. A piezo-crystal assembly built into the device makes the prongs vibrate.
If you have a vibration fork installed in your tank and there’s no liquid in contact with it, it vibrates at its natural frequency. As soon as liquid touches the fork, the frequency changes and the sensor picks it up. So you can use vibrating forks as on/off level switches.
If you want to learn more about this topic, read our article on vibration forks.
When we talk about silo levels, we usually focus on one of three popular options on the market. Two of them have been around for a while and we talked about them already – lasers and radars. But the most modern device and the third option is the 3D level scanner.
The scanner works similarly to a radar or laser, using the ToF principle. However, instead of just one antenna, this device has three, one for each dimension.
It sends low-frequency acoustic waves to multiple points on the product in the silo. The product reflects the waves back to the scanner’s three antennas. The three antennas can then triangulate the signals that return to them and create an X, Y, and Z graph with the data.
If you want to learn more about this topic, read our article on 3D level scanners.
Now that we’ve learned all about level measurement, let’s have a look at some of the frequently asked questions which we’ve answered here at Visaya.
Nowadays I see a lot of recommendations to use radar for level measurement. Would it benefit me to switch from a pressure transmitter to a radar level meter?
The answer to this question will depend on what kind of process you have. In some applications, a differential pressure (DP) transmitter can work better than a radar device. We need to consider the advantages and limitations of both.
The DP transmitter doesn’t measure level directly. It derives the calculation from the pressure measurement, which is a relation between the height and density of the fluid. Therefore, if your product changes density, a DP transmitter won’t provide the most accurate measurement. And if you install your pressure transmitter using capillarity, temperature will also affect your reading.
The radar transmitter, on the other hand, can measure level directly without density influence. However, it has a more complex setup. Radar transmitters also depend on the dielectric constant of the liquids to work.
Therefore, you’ll have to make a decision based on your specific application.
I work in a cement company and we have a solid level application. We have two solutions to implement in our level measurement, a radar or a 3D scanner. Why invest more money in a 3D scanner for my application?
Both devices will give you the level measurement you want. However, the 3D scanner will give you access to more information. The choice here comes down to how you store your product in your storage vessel.
When you have a solid product, you may have trouble finding the exact volume of the tank because of the behavior of the product inside. The 3D scanner gives you a better overview of the tank level, which makes easy to calculate the volume of the product. And any buildup on the tank walls can cause problems. But the 3D level scanner can show you if you have buildup in your tank.
In most applications, you’ll still find radars instead of 3D scanners, mostly because scanners need more software and engineering to give you all the benefits of the technology.
Radar devices have worked fine in these applications for the last decade. However, you’ll get more data to work with if you choose a 3D scanner. So is this investment worth it for you? Only you can decide!
Can I use capacitance level transmitters in plastic vessels?
Capacitance level transmitters usually use the vessel wall and the probe as the capacitor plates, with the product as the dielectric. But you have specific conditions you must follow for this sensor.
If you have a medium with a conductivity higher than 100 micro-siemens per centimeter, for example, then the factory can calibrate the device for you. However, if the conductivity falls below one micro-siemen per centimeter, you’ll need to calibrate the device locally. You should also avoid using capacitance level devices between 1 and 100 micro-siemens.
When it comes to plastic vessels, you won’t have the vessel’s wall as the second plate of the capacitor. Thus, you’ll need to create one. Some vendors offer transmitters with two probes. You can also install a concentric tube in the tank to create this second plate. Look into your options before choosing this solution.