How to Use Load Cells for Weight Measurement

Weight measurement is critical in many areas of industry, health care, and research. Within the manufacturing industry, the product is often purchased by weight. The manufacturer needs to make sure that they are providing the correct amount of material. They also need to make sure that they know the shipping weight, both in terms of calculating shipping costs, and to avoid overloading the shipping containers. Similarly, the recipient of this material needs to confirm that they are receiving the total amount that they purchased.

During the manufacturing process itself, weight is often required for correctly formulating a product. If this product is a chemical formulation, or a pharmaceutical, various ingredients have to be precisely determined. If it is a packaging line, appropriate weights must be dispensed. If it is paint formulation, the correct amount of pigments must be used.

Load Cell Overview

To answer the question about the use of load cells, we must first understand what a load cell is. In the most basic format, a load cell is a transducer. It is a device which is used in a scale, and accepts a load of any weight within its specified range. It converts this weight into an electrical signal, which is sent on to the processing section of the scale. This section of the scale will generally convert this signal into usable information, which may be shown on the scale display, or sent out to other computers or monitoring devices.

Load cells can use a number of different technologies. Hydraulic load cells use an oil filled compartment. As weight is applied, the pressure of the oil changes. By measuring this pressure, the amount of the weight can be determined. Vibrating wire load cells apply the load to a wire. The frequency of vibration is related to the load. The most popular type of load cell utilizes resistive strain gauges that are bonded to a metallic bending beam. The change in resistance is proportional to the amount of load. A new ultra precision technology, called SAW, or Surface Acoustic Wave load cells, harnesses delay lines fabricated on lithium niobate substrates. The frequency of the delay line will vary with the weight applied to the load cell.

Load Cell Configurations in Scales and Industrial Weighing Systems

Load cells are available in a number of different designs. One of the most standard is called a Single Point Load Cell. In a simplified description, the scale consists of base and a platform. The load cell is sandwiched between the two. This type of scale is often designated as a Bench Scale. Any load on the platform must pass through the load cell to the base. The load will affect the electrical signal from the load cell, which will then be processed to calculate the applied weight.

Another common configuration is used in Floor Scales, or Platform Scales. Generally, these scales have a higher capacity and much larger platform than a Bench Scale. A single point load cell cannot be used in this case, because the base and platform would bend far too much from torsional loading. Instead, four load cells are used, with one in each corner of the scale. This can be implemented in a similar manner to the Bench Scales, with the load cells sandwiched between the base and the platform. Some designs bypass the base entirely, with the load cells resting on the ground directly through their leveling legs. Of course, a platform is still required.

When the load is placed on the platform, it will now be distributed between the four load cells. The exact share of each load cell will be dependent on the placement of the load on the platform. Of course, the weight readout should be the same, irrelevant of where on the platform the load is placed. That means that the output of each load cell should be almost identical. But it is almost impossible to manufacture load cells with virtually identical outputs. To correct this, many scale manufacturers provide an internal circuit board with connectors and variable resistors to trim each load cell to the exactly the same output. Some higher quality scales, such as Arlyn Scales, pre-match the load cells during the scale fabrication process. This allows the variable resistor circuit board to be eliminated, which also eliminates one of the major sources of failure of the scale.

This type of load cell is used in scales that may have capacities ranging from about 500 lb, all the way to 30,000 lb and more. In fact, the same concept is used in truck scales and rail scales.

Yet another configuration for load cell use is called an “S” type load cell. In fact, this load cell looks very much like an S. There is a threaded hole on the top of the load cell, and another threaded hole on the bottom. This type of load cell is often used in hanging scales or crane scales.