I cannot tell you how many manufacturing facilities currently have tank systems with load cells installed that is no longer operational. Most of the times the load cells are left in place and the electrical cables connecting to the controls are simply cut. Production managers, quality control managers, and maintenance engineers alike have encountered weigh cells that never worked properly. Why? Why have so many switched from a supposedly high accuracy load cell solution to old fashioned level probes or sight glasses? There are several causes. I will touch on the worst culprits.
OK, I am an engineer and I must admit before I specialized in load cell solutions I couldnt care less what type of load cell was specified under a tank during project design phase. I would pick a load cell from the local catalog; mount one cell under each leg bingo, a perfect weighing system. The only science involved was finding the vessel dead-weight, estimating the product specific gravity, and then calculating the total weight. It was an easy design, simple integration drawings, then it was time to move on: Westinghouse engineering. Make it fit, then plug it in.
Poor specification of load cell systems may not be engineering incompetence. Rather, it could be a lack of education of how load cells work and a lack of appreciation of how real life factors affect these installations. A review of some simple mechanics, installation tips, and a few real life lessons found in this article may help design engineers more effectively integrate load cells. Quality control managers, production managers and others also may gain some insight into correcting their existing tank or hopper installations.
Understand the mechanics
The heart of the weigh cell is the strain gauge. Although there are several different types of weigh cells, I will focus on the shear beam load cell. The alignment of the force vectors is important; these sensors cannot discriminate against true vertical load and side/torque loading. You want to measure the true vertical load, i.e., the weight within the tank not the shear loads. However, real world conditions or improper installation often cause the load cells to see much more than vertical loading. These other factors are what cause poor repeatability in weighing that ultimately lead to abandonment of load systems. Before learning about the factors that cause inaccurate tank weighing it is important to learn how a shear beam actually works.
A shear beam load cell is a precision-milled piece of metal that includes several strain gauges. These gauges are positioned at a 45° angles to measure the steel shear strain.
Figure 1 shows the principle positioning of the strain gauges on the shear beam load cell.
Figure 1. The shear beam weigh cell is highly dependent on the material of construction.
Strain gauges are zigzag-shaped strip conductors, etched from thin metal film. They operate on the principle that the electrical resistance of a conductor will increase when it is subjected to mechanical stress. This mechanical stress is caused by tiny changes in the film length and cross-section. The resulting voltage change is very slight; to measure it accurately, a Wheatstone bridge or some other familiar amplifying circuit is used. The letter F in Figure 1 shows how force is introduced to the shear beam cell.
The body of the load cell is usually constructed of mild steel, stainless steel alloy or aluminum. A typical load cell will undergo thousands of weighing cycles, so the composition of the metal is very critical in producing reproducible weighing results. A shear beam load cell is actually measuring the shear strain of steel, so if the steel itself isnt monolithic throughout, errors in weighing will occur. Metal selection also affects the life cycle of the cell; body failures have occurred. In general, its critical that load cell manufacturer obtain steel and aluminum that is high grade to produce the most reliable installation.
Not all cells are the same
Bending beam and shear beam load cells are the industry standard for bench and floor scales. Next time you have a free moment, lift up the pan and look underneath a scale. More than likely, you will find some type of bending beam load cell. If these types of load cells are used in bench scales then why cant they be used in vessel weighing tasks?
Scales and balances are self-contained units that rarely encounter the extraneous side forces and environmental influences that vessels do. Yes, it is true that scales are subject to environmental factors, however not to the extent of process vessels. Bending beam load cells are perfect for environments where static weighing take place; they arent intended for a process vessel. Weighing a vessel requires a load cell and mechanical support system with a high tolerance of non-vertical loading and a capability for producing reproducible measurements.
Start with the tank
First things first, a weighing application will never be successful without proper attention to the tank. Vessels are usually designed to safely hold a quantity of material accounting for such factors as wind, earthquakes and other forces of nature. The idea that the vessel will need to weigh a material is generally an afterthought in its design.
The most important factor to consider when designing for a weigh cell is the construction of the weighing frame. The frame is where the load cells will be mounted. Whether the cell is a shear beam, bending beam or compression type does not matter. This structural steel is mechanically connected to all the load cells under the vessel to ensure that they cannot move in different directions independently of each other. Because were only interested in vertical compression forces the design must resist external forces that could cause the load cells to tilt.