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I am a longtime member of the A-List—more correctly called “The Automation List”—one of the longest running email discussion lists for automation issues on the Internet.
Every few days, somebody posts to the A-List a problem with a level measurement, or asks how to do a level measurement. The expertise of these posters ranges from dead newbie to experienced automation professional. What I have noticed is that often the answers are well-intentioned, but wrong. They are wrong because they tend to immediately recommend a product, rather than provide an analysis of the application.
Here’s an edited version of one recent exchange.
"I need a level sensor for a deep-well level application. It is 300 meters.”
“Use an ultrasonic sensor.”
“Use a radar sensor.”
“Use a guided-wave radar sensor.”
“Use a down-hole level transducer of the ‘torpedo’ variety.”
That last was mine, and it was wrong too.
What, me wrong? You bet. And I was wrong exactly the same way the other people who answered the question were.
We instantly began to recommend our favorite product, rather than investigate the application. There could have been all sorts of qualifying issues that the original poster didn’t get around to telling us. For example, this could have been an oil well. There could have been projections in the well that would deflect a radar or ultrasonic beam. In fact, 300 meters is far too long a range for either, but you see what happened. We all jumped on our favorite hobby horse sensor and rode into battle with it.
And we wonder why level applications are hard.
Figure Out the Application First
Before you do anything else, you have to have the application parameters. Most of us get so practiced with instrumentation design that we seem to start with the last ISA S20 instrument specification form we worked with and just plug it in. But the S20 forms were not designed to be application selection forms. You start with the sensor or transmitter. That’s backwards.
Many years ago, I hit on the idea of trying to define a range of applications and superimposing many types of level measurement devices and techniques on each range. I called it “The Level Measurement Continuum,” (See Figure 1 below) and I’ve written about it before. But I haven’t found a better method of finding your way through the thicket of applications.
FIGURE 1: THE LEVEL MEASUREMENT CONTINUUM
The measuring device you choose will be a function of the difficulty of measuring the particular application... and its cost. (Click the image to open an enlarged PDF of this chart.)
As you can see, the continuum ranges from applications that are “too easy to mess with” to “too hard to do.” Yes, there are still level measurement applications that are impossible to do with today’s technologies, just as there are level measurements that are too easy to bother with. In those cases, somebody can occasionally just go over and check where the level is.
“Too Easy to Mess With”
You can think of applications like this: sumps, wet wells with high freeboards, tanks where level is maintained by an external force like a throttling valve, or a flow control system. Most of the time, you needn’t measure the level, unless you are obsessive about such things. While nearly any device will work on an application like this, whether you actually install one will depend on the value of the measurement data. If it is worth more than the cost to install and maintain the sensor and data transmission, you should probably go ahead and install the sensor.
This equation is likely to change somewhat with the coming introduction of inexpensive wireless level transmitters, as the cost of installation will drop substantially without wires or power connections. In fact, the second wireless transmitter project going into BP’s wireless demo plant at the Cherry Point Refinery in Blaine, Wash., is level transmitters in the tank farm that were never economically practical before.
Nearly Any Device Will Work…
These are the applications where it is easy to see how it can be done: clean water; open-channel flow in a primary device; still, clean, non-fuming liquids in an atmospheric pressure tank.
This last one sometimes gets you in trouble. Often liquids that do not fume in cold temperatures do at higher temperatures. Note that “oils and emulsions” are very far up the continuum, almost to the “few devices will work” line because many hydrocarbon-based liquids, emulsions and tars give off vapor blankets that vary in thickness, depending on the temperature in the tank. The higher the temperature, the thicker the vapor blanket. This gives ultrasonic sensors fits, but can also affect other technologies.
Few Devices Will Work…
I know I skipped the part about “Many Devices Will Work….” I’ll be back to it. I’m leaving it to almost last because it is the most difficult.
Truth is, in applications such as interface measurement, agitated tanks, vessels with internals or other clearly difficult applications, a red flag pops up, and you automatically pay more attention to the application than those in the category “Many devices will work….”
This is good news. These difficult applications are seriously hard to do. Notice that there is no divider between “Few devices…” and “…may not be measurable….” That’s because any of these applications can be so burdened by qualifying issues that they become impossible. A simple interface measurement may be doable with a differential pressure device. Now make it an interface at 1200 ºC. See what I mean?
Consider a simple water level measurement, as was done in the containment units at the Hanford reservation. Simple, except the radiation levels were enormously high, and the devices needed to work maintenance-free for 300 years.
For such applications, you need to gather as much information about the physical parameters as you can without regard for whether you think the data is meaningful, and give it all to your supplier’s most experienced engineer.
And that supplier should have more than one measurement technology. It is too easy for a supplier, in the heat of trying to help you (and incidentally make a sale), to make his device fit. Notwithstanding this tendency, here’s where you’d better bring all the resources of your supplier—or more than one—to bear.
Many Devices Will Work…
Sure they will. You can come a cropper here really fast. Reaching for the level transmitter in your hip pocket for every application is so easy that too often you cross the border into dangerous territory before you know it. These applications should be standardized on a specific technology, or even a specific level transmitter model.
As you approach the middle of the chart, the level measurement continuum is not linear. There’s a slippery and fairly deep slope right there in the middle of the chart that can be seen with powders and granulars. It is easy to shoot a level into a stationary tank containing granular material—unless, of course, there’s an angle of repose or rat-holing. Add vibration to get rid of the first two, and you may need to rethink your entire measurement method.
It Ain’t Level…
You must always remember that you aren’t measuring level. You’re measuring pressure, pressure differential, distance, capacitance, admittance change or change in received radiation. Sometimes you have to infer a measurement that isn’t really there. For example, when you are measuring the level inside an agitated tank that is vortexing, there IS NO level! What you are trying to do is predict where the level will be when the agitator is turned off!
And that’s why measuring level is hard.
[Editor's Note: Click the Download Now button below to open an enlarged PDF version of the chart referenced in this article.]
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