What you need to know about measurement and measuring devices
If you’re new in the automation world, or if you just want to learn more about measurement and measuring devices, this article can tell you a bit about the base of the automation pyramid.
In my career I’ve often needed to explain these things. Of course, explaining to people without the same technical background makes it harder, but I’m a professional. I took a “technical training for non-technical people” course! It taught me how to present to HR people, financial types, and many others. Making connections between my topics and daily life helped tremendously.
Many of these people I presented to worked in automation companies but barely knew what their products/services could do. So I had to clarify and then show them how relevant measuring is – to them and their industry!
Measure, control, and actuate
First off, I explained the basic concept of measure, control, and actuate in a process. I promise, if you’ve ever taken a shower, then you know this concept. And boy, I hope you’ve taken a shower at some point in your life.
When you start the shower, you need to check the temperature of the water, right? You stick your hand or foot or whatever in the water to find out how hot it is. And voilà – you deployed the measure step!
At this point, you need to decide whether the temperature is right. Here, your control system – your brain, in this case – analyzes what you felt and compares it to what you want (setpoint). But what happens if the water feels colder or hotter than your setpoint?
Then time to actuate! So you use your other hand to adjust the shower. After that, the process happens again until you find your setpoint and finally get in the shower.
This explanation creates a relationship between process control and daily life. The same cycle happens in automation, where you need to measure (field devices), control (PLC, DCS), and actuate (valves, pumps, etc.). Now you can explain the cycle to anyone who takes a shower!
In the automation world, we have physical and chemical variables that we must measure to control processes and produce the desired results. Field devices use different working principles to measure the process variable and deliver the data to a control system.
So let’s review some of the most basic principles we use in automation.
For pressure, we’ll talk about tires. First off, we have to calibrate the tire. A properly inflated tire rides better than a flat tire, right? When you need to calibrate a car or bicycle tire, you can use a machine to measure the pressure and fill the tire until the pressure reaches the setpoint programmed in the machine.
You need this measurement to avoid underinflating the tire, because it doesn’t ride as well, or overinflating it, because it may blow up. And no, don’t try this at home, kids.
Speaking of blowing up, many industries must control the pressure in pipes and tanks and such to avoid a similar situation. Believe me, you want to be a control freak in these cases. But you can also use pressure to measure slightly less dangerous variables, flow and level.
Many homes have hot-water tanks so you don’t have to take freezing-cold showers. (You may have a tankless heater now, but I’ll bet you used to have a hot-water tank, so just play along with the rest of us, okay?)
So how do you avoid overfilling that tank? For that, you have a simple but effective level device. Most tanks have a level switch to close or open the water pipe to fill the tank. When you have enough water, the supply stays closed. And when you use enough hot water to drop below a certain level in the tank, the supply opens again. Once it reaches the “enough” level, it closes again. Look, it’s the cycle!
Industry tanks often store or mix materials for the production process. And they use similar measuring devices to avoid overfilling or running dry.
Okay, now somehow the water that goes to your hot-water tank gets measured to go on your water bill, right? For that, water companies have fairly simple flow meters. They measure your usage for those companies to read and charge you for later.
As you consume this water, those flow meters measure everything that passes through the pipe. Sometimes you pay for air as well, but we’ll skip that discussion today. Just a fact I thought you might find interesting.
On the industry side, if you send process material through the pipes, then you need to know how much is coming. Many controls use flow as the primary variable to keep production going.
I know this is a basic topic, but how many of you look at these ideas this way? And we have still more process variables – pH and oxygen and many others. So we must have measuring devices in the industrial world to track these variables so that our processes run properly. How many other variables can you explain this way?