One of the characteristic differences between solids and liquids is their tendency to either hold their shape (solids) or spread out evenly (liquids). This isn’t exactly news, but we need to remind ourselves of this reality every time we think about taking a level measurement.
Emerson’s Mark Menezes brings it up again in an article for Processing magazine titled Is Volume Measurement the Real Goal? He suggests users taking level measurements in tanks and vessels where solids are stored really don’t care about the level by itself, but also the volume of the contents, and for the following purposes.
In any application where dry bulk materials are stored in tanks, silos and other vessels, users need to know how much is inside each container for many reasons:
• Inventory management of raw materials, intermediate production and finished product
• Monitoring and controlling process capacity to allow for manufacturing flexibility
• Maintaining a safe environment for people, equipment and the environment.
When dealing with liquids, having a single level measurement in a vessel is enough because liquids spread evenly and seek their own level. Solids don’t, so a single spot measurement in a vessel is not enough because there is no way to tell what the surface of the material looks like. Maybe it’s smooth and level, but more likely it has peaks and valleys. In most real-world environments, there is no practical way for an operator to look inside and see, so problems can escalate:
To understand how this affects operations, consider the following example: The level at one location in a vessel may be 50 percent, but directly underneath the filling point a peak may have formed that reaches 100 percent. Looking at only the peak suggests additional material cannot be added so the vessel is effectively full. However, directly over the emptying point, a “rat hole” may have formed, so when an operator opens the drain port, nothing comes out. At that point the level is effectively zero percent because no more material can be removed.
So as Mark points out, two operators may look at the same vessel from different ends and get vastly different measurements. Nobody has any idea what is really inside the vessel. The key is to stop thinking about level and deal with volume. But how? He says the three main technologies are radar, ultrasonic and acoustic.
All three can work in the right conditions, but the question is whether they deliver the necessary breadth to get an accurate picture of the contents. A single spot measurement, regardless of the technology used, can’t do the job. How do we see the entire surface? Just as we need two eyes for depth perception to judge distance effectively, measuring an irregular surface also requires multiple measuring points:
To create a 3D model of the surface of a solid instead of a single level, the device needs to obtain x-, y- and z-axis coordinates at multiple points on the surface. Similar to a GPS receiver, this triangulation requires at least three separate broadcast frequencies and receiving antennas. Once a detailed map of the surface has been obtained, total volume is calculated from the known vessel shape. Overall resolution and surface detail depends on the number of measuring points. Higher resolution provides a more precise volume calculation but needs more processing power, which sometimes requires external data processing.
There are acoustic level instruments, such as the Rosemount 5708, able to take these detailed surface “pictures” using multiple sensors. Each instrument has three sensors looking from slightly different vantage points, and when the three views are combined, it is possible to get a detailed model of the surface.
The chosen technology – radar, ultrasonic, acoustic or other – should be used to measure levels at multiple points on the surface and create a map of the surface from wall to wall within the vessel. With sufficient detail, this map can provide accurate volume calculation and best visibility regardless of peaks and valleys. Installing such capability requires capital investment, but the returns gained through more effective inventory management can recover those costs quickly.
So the critical question isn’t which technology to use, but how should the sensors be deployed. The answer doesn’t have to be complex, in fact it can often be done with a single unit such as the 5708.
The article contains more details to help guide your decision-making process, and you can also get help from others who might be working on the same problem. You can probably find many, right here in the Emerson Exchange365 community. Look for the Level Group to interact with your peers and our experts.