Using load cells to accurately measure the content of tanks seems like a good idea. Yet at many facilities, they have been disconnected. They are left in place and the electrical cables connecting to the controls are cut because they never worked properly. Why? Several reasons. Ryan Titmas, product manager at Sartorius North America, Bohemia, N.Y., a manufacturer of weighing and liquid handling systems, outlines "the worst culprits."
Let's begin with a focus on the shear beam load cell. The alignment of the force vectors is important; these sensors can't 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 cause the poor repeatability in weighing that ultimately leads to abandonment of load systems.
How a Shear Beam 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° angle to measure the steel shear strain. They are "the heart of the weigh cell," says Titmas.
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 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 to producing reproducible weighing results. A shear beam load cell is actually measuring the shear strain of steel, so if the steel itself isn't monolithic throughout, errors in weighing will occur. In general, it is critical that the load cell manufacturer obtain steel and aluminum that is high-grade to produce the most reliable installation.
Titmas points out that bending beam and shear beam load cells are the industry standard for bench and floor scales. Bending beam load cells are perfect for environments where static weighing takes place; they aren't 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 the capability to make producible measurements.
Start with the Tank
A weighing application will never be successful without proper attention to the tank. 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 can't move in different directions independently of each other. Because we're only interested in vertical compression forces, the design must resist external forces that could cause the load cells to tilt.
Figure 1 shows the best type of installation. Notice how the load cells are placed in the middle of the beams. Each cell is situated between two frames. This frame network ensures the load cells will move together, creating a more repeatable weighing environment. In Figure 1, the load cells are mounted on top of a bottom frame assembly that provides a rigid foundation. The weight of the vessel is being transferred to the load cells and then to the bottom frame. This transfer of force occurs with minimal deflection. Although the load cells can be mounted directly to the floor without the bottom frame, the floor must allow little deflection and settling.
"The load cells are measuring deflection down to thousands of a millimeter, so if your foundation deflects, you aren't going to achieve acceptable accuracy. Most architectural designs allow a floor deflection that would be unacceptable for weigh cell operation," says Titmas.
Figure 2a illustrates another typical design problem found in tank weighing. It shows load cells installed on the end of very long tank legs. In this example, errors are due to the legs moving in different directions. Measurements become unrepeatable as extraneous deflection causes variability in movement.