Weighing applications are present in almost every industry. Load cells and scales are the predominant solution for these applications.
What is a Load Cell?
The term “load cell” is often used in the discussion of industrial scales or similar weighing devices. In the simplest sense, a load cell is a force transducer. A transducer is a device that changes energy from one format to another. Most transducers change a physical attribute, such as flow rate or pressure, into an electrical signal.
A force transducer converts force into an electrical signal. In the case of a load cell, the force being measured is the force of gravity acting on a mass applied to the load cell. The heavier the mass, the greater the output from the load cell. It is therefore measuring weight.
A load cell is a generic term. Different designs of load cells are used for different methods of applying the weight to the load cell. For example, a single point load cell is often used in bench scales and cylinder scales. The bottom of the load cell is attached to a base plate whereas the top is attached to the scale platform.
Types of Load Cells
There are a number of different technologies that are used to build load cells. Some have been used for many years. One is a much more recent, break through technology developed by Arlyn Scales
Hydraulic Load Cells
This type of load cell has been used in different formats for well over 100 years. It typically includes an oil filled piston connected to an oil filled tube. The opposite end of the tube could be attached to a dial device, or to an electronic pressure transducer. When a load is placed on the piston, the pressure of the oil will change. This increase of pressure could cause the dial to move, proportional to the weight on the piston. Alternatively, a pressure transducer could convert the measured pressure into an electronic signal, which could then be used for any purpose. The accuracy of a hydraulic load cell is only fair, and is often adversely affected by changes in temperature and other environmental conditions.
Strain Gauge Load Cells
The most common currently used load cells are of this type, and have been available for more than 80 years. They rely on a special type of resistor called a strain gauge. It can be envisioned as a very long, extremely thin piece of wire. It has a resistance that can be measured. Typically, it may be 350 ohms, or in other cases, 1000 ohms. Actually, this long wire is formed into a very compact, coiled format. Often the pattern is photo etched on a very thin, very temperature stable foil.
The resistance of any piece of wire will increase if it is stretched and becomes longer. That is also the case with the strain gauge. If it is stretched, the length of the pattern becomes slightly longer. If it is compressed, it will become slightly shorter.
In most high accuracy load cells, four strain gauges are bonded onto a carefully machined piece of metal, called the spring element. This element is designed to be an excellent spring. It bends in proportion to the amount of load that is applied. It will return to its original position when the load is removed.
The strain gauges are positioned so that two of them will be in tension when the element is loaded, and the other two will be in compression. They are wired into a bridge circuit so that a differential voltage is developed. This voltage is extremely small, and is usually amplified to a more usable level.
Good accuracy can be achieved with carefully designed and manufactured strain gauge load cells. Most industrial weighing requirements can be met, although others require higher accuracy and performance.
Ultra-Precision SAW Load Cells
Certain industrial processes require higher accuracy than is obtainable from strain gauge load cells. They include formulation of chemicals, paints and dyes, electronics, among others. Pharmaceuticals and biopharmaceuticals are two very specific examples. Furthermore, any operation requiring a large fixed weight with small increments or deletions, are also good candidates for higher precision.
Arlyn Scales has developed a new weighing technology that provides the best of all worlds. It is known as SAW, which stands for Surface Acoustic Wave. It relies on a semiconductor based process to use high frequencies to determine exceedingly accurate displacement of the load cell, when a load is applied.
The SAW load cell utilizes a transmitter transducer and a receiver transducer. The transmitter sends an acoustic wave along a substrate which induces a similar wave on the receiver substrate. The amount of time required for this signal to travel from the transmitter to the receiver is dependent on the distance between them. When a load is applied to the spring element of this load cell, the distance between the transmitter and receiver will change by a very small amount. This will cause a change in frequency of the transducers. Measuring this frequency will provide an extremely accurate determination of the distance between the transducers, which is exactly proportional to the load applied.
During the past many years, there have been only small, incremental improvements in weighing accuracy. SAW technology has suddenly introduced a more than ten times improvement in accuracy. Often, when one thinks of ultra-precision, one also thinks of ultra-delicate. SAW load cells are actually the opposite, being more rugged and abuse resistant than strain gauge load cells.
Choosing the Right Load Cell for Your Needs
There is usually a straightforward answer to the technology required for your load cell needs. Strain gauge is appropriate for most situations. Ultra-Precision SAW scales will handle all of your higher accuracy requirements.
If you do need the best accuracy, you will then need to determine if a bench type SAW scale is suitable, or if a large Ultra-Precision scale is needed. There are even Intrinsically Safe scales available if required.
Reach out to Arlyn Scales Today
It is often difficult to decide which load cell and which scale will best solve your weighing issue. We have experienced sales technicians who have seen almost every imaginable situation. Give us a call and let us help.