By Chris Pedullo
While portable tank systems can be very accurate, two simple design factors that have the most influence on system accuracy and productivity are often overlooked during the system's design. These design factors are almost always overlooked due to the misconceptions of proper portable tank weighing. The recommendations in this article offer tips to help avoid the two most common misconceptions with portable tank weighing.
Accurate portable tank systems are cost-efficient and save money by increasing production throughput and reducing wasted material; this is especially true over a long-term period. The ideal portable tank system must offer the highest accuracy possible when moving freely from one location to another. Combining these two design properties—high accuracy and portability—adds an extra level of difficulty when trying to achieve a top performing system.
The First Misconception: A four-load cell portable system is better than a three-load cell portable system
One of the misconceptions with portable tank systems is the idea that a four-load cell system offers higher accuracy when compared with a three-load cell system. Normally, four-legged systems work fine when they are stationary. This is true only if all four of the tank's load cells are under compression. Mechanical shimming compensates for any out-of-level discrepancies, allowing four-legged stationary systems to perform as intended.
However, under careful analysis, it becomes evident that four-legged systems never evenly distribute a tank's weight among all four legs (picture a four-legged chair on an uneven kitchen floor pivoting back and forth on opposite corners). When transported to a new location, a four-legged portable tank system can distribute its weight in such a way that only three-load cells are in compression. Therefore, the overall weight signal—an average of three normal weight signals combined with a fourth abnormally low signal—will certainly produce inaccurate weight readings and cause frustration and confusion.
Figure 1: Recommended frame design for a three-legged portable tank
(click image to enlarge)
A three-legged portable tank system is superior to a four-legged portable tank system since all three of its load cells are always in compression. This guarantees a normal weight signal. Figure 1 illustrates a correct three-legged system attached to a three-load cell system. In summary, a three-legged design is always preferred over a four-legged design when used with portable tank systems.
A portable three-legged system offers the following advantages compared with a four-legged system: increased system accuracy and reduced equipment costs. Lower equipment costs—including one less load cell and one less mounting kit—and higher system accuracy are two incentives that should convince a designer to select a three-load cell system over a four-load cell system.
Converting a Four-Cell System to a More Accurate Three-Cell System
Figure 2: Correct four-legged portable tank system (double-frame design)
(click image to enlarge)
A better solution for a four-legged portable tank weighing system is illustrated in Figure 2, which turns a four-legged portable tank into a three-point load cell system. The four-legged tank is attached to a subframe, which "sandwiches" three load cells in between a base frame and the tank's subframe. This creates a three-point load cell system and ensures that all three load cells are in compression at all times. A three-point load cell system offers all the advantages as previously described for a three-legged portable tank attached directly to a frame. This design helps guarantee that the load cells are not affected by the changes in a floor's properties when the tank is moved from one location to another.
Second Misconception: Plates are better than frames when designing an accurate portable tank system
The second common misconception with designing a portable tank system is the idea that plates provide higher system accuracy when compared with tubular frames. In order for a portable tank system to offer the highest accuracy, it must have a rigid foundation that does not flex or bow, regardless of external influences. Realistically speaking, no system is perfectly rigid, but a system that limits this amount of flex will increase its chances of having the highest system accuracy. Portable tank systems can perform as well as stationary systems if they follow this fundamental rule.
Figure 3: Incorrect frame for a portable tank system
(click image to enlarge)
Tank systems that are mounted to a flat steel plate, as shown in Figure 3, have stability and rigidity issues that do not produce accurate results. A thick steel plate may seem like a rigid, stable surface that does not flex when a load is applied to it, but this is not the case. Figure 3 shows the forces of a three-legged portable tank system and the flex that is created.
With a high percentage of a weighing system's accuracy being contributed to the mechanical design of a weighing system, the need for a proper foundation is critical for accurate tank weighing. No matter how thick the plate is, a significant amount of flexing will occur and system accuracy will be reduced. Bear in mind, load cells are highly sensitive force sensors that can detect a change in weight of one thousandth of a millimeter in deflection. For this reason, it is important to have a rigid foundation both above and below the mounting kits of the load cells.
Conclusion: Proper design ensures an accurate and economical solution to a portable tank system
The preferred design of a portable tank system is shown in Figure 1. As with all tank-weighing systems, the legs of the tank should be kept as short as possible to increase stability.
A three-legged system is the recommended design for portable tank systems. Portable tank systems that are already built with four legs should use the double-frame design that "sandwiches" three load cells, as shown in Figure 2. This design will always offer higher system accuracy than a four-legged tank that is directly mounted to a frame with wheels.
Also, a flat steel plate frame is not an acceptable substitution for either a four- or a three-legged tank system. Hollow tubular square or rectangular frames are more rigid and provide the most support. Following the portable tank design recommendations described in this article will help ensure your portable tank system will have its highest system accuracy and cost-effectiveness.
Chris Pedullo is application engineer, process weighing and controls, Sartorius Mechatronics, Brentwood, NY. He can be reached at 631-254-4249 or via e-mail at email@example.com.