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A load cell is used to determine mass: The weight force exerted by the sample is converted into an electrical signal by the load cell. Load cells are therefore the most important components of an electronic scale. One of the most widely used technologies is based on strain gauge technology: an analog load cell consists of a measuring element (so-called spring body) made of steel or aluminum, on which a strain gauge (Wheatstone bridge) is mounted. Each electronic scale has an integrated load cell that ensures that a weight can be measured.
DMS load cells are the most widely used, but there are other technologies for weight determination in weighing technology such as EMFC, electromagnetic force compensation, in which mass determination takes place completely without friction loss. Generally, load cells are installed in industrial plants, such as filling plants, weighing silos or checking the filling level of tanks.
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FAQ
What types of load cells are there?
There are many different types of load cells, depending on the application they are designed for. Bending beam or shear beam load cells are often used, for example, in platform scales. Compression load cells, on the other hand, are usually installed under a structure (container, silo, etc.) that is loaded with a weight from above and are often designed for higher loads. With tensile load cells, on the other hand, a weight is attached to the load cell. Load cells are the most important component of a scale, be it a platform, floor scale or bench scale.
How does a load cell work?
Although the material of the spring body is stable, it is also minimally elastic. This means that when a weight is placed on the scale, this force acts on the spring body. The term spring body derives from the fact that the material deforms minimally as a result of this effect, but returns to its original shape - or "springs" as soon as no force is applied to it. This is an elastic deformation of the measuring element. This change is detected by the strain gauges mounted on the load cell. A strain gage, or "DMS" for short, is an electrical conductor. Because it is firmly attached to the spring body, any force acting directly on the measuring strip extends or shortens it. The electrical resistance increases when the measuring strip is stretched, but decreases when it is compressed. This change in resistance changes the output signal of the load cell and thus makes it possible to determine which force is acting on the load cell - in other words, which weight is currently being weighed.
In the next step, the electrical signal is transmitted to weighing electronics or weighing terminals (which could also be evaluation and display devices). The measured weight can then be read on the display of a display device, indicator or weighing controller.
Important features of load cells
In addition to the various types of load cells, these differ in the manufacturing materials used, mostly aluminium and stainless steel. In addition to the material, the environment in which the cell is used must also be taken into account: Especially the ambient temperatures are an important factor: Every material changes due to heat or cold, which also applies to load cells and strain gages. In order for the exact weight to be displayed, the load cell must be able to compensate for external interference.
Accuracy is particularly important when weighing: load cells are classified into different accuracy classes. These are divided into classes A to D, where A is the best possible class. Strain gage load cells are mostly produced in classes C and D. Depending on the area of application, the requirements for the required accuracy vary: For example, the pharmaceutical industry requires different accuracies and process measurement uncertainties than for instance recyling or retail.
How are the strain gauges arranged within a load cell?
The structure of a single point load cell is characterised by two parallelogram links, which are fixed to both sides of the base body. Two strain gauges are glued to each of the parallelogram links. The thin points deform the parallelogram links into an S-shape - the basic rectangular shape becomes a parallelogram. This parallelogram link structure ensures that off-centre loads are compensated for and that the load is always applied vertically. This means that platform scales up to a certain size can be realised with just one load cell.
What are the test specifications for single point load cells?
For single point load cells, there is no test specification for measuring the corner load error in OIML R60, as load cells are tested here with point load application. For this reason, the maximum platform size is specified in a single point load cell data sheet in accordance with OIML R76, i.e. the test specification for non-automatic weighing instruments.
For scales with single point load cells, however, a corner load check is necessary. This procedure is independent of the number of load cells under the platform and is carried out in accordance with OIML R76.
The test is carried out with a load of one third of the measuring range. The eccentricity is defined by dividing a rectangular load plate into four equal quadrants and placing the weight in the centre of each of these quadrants. The measurement error must remain below one calibration value digit. If the corner load test shows that the error limits are exceeded, the load cell must be replaced.
How are strain gauges protected from environmental influences?
It is common practice to encapsulate the strain gauges of aluminium cells with silicone to protect them against environmental influences. This potting compound is applied over a large area on the respective link of the parallelogram guide. However, silicone has the disadvantage of being permeable to water vapour. Most carrier films (such as acrylic resin, epoxy resin, phenolic resin, polyamide) are hygroscopic, as are some of the adhesives used. The carrier film swells when it absorbs water, which results in expansion and thus causes a measurement error.
One way to improve this is to encapsulate the strain gauges in metal. This can be realised in the area of the parallelogram links or with an additional deformation body. Cells are known here that have an internal ring that is coupled for load absorption and fixation and arranged inside the parallelogram links.
The parallelogram takes on the task of absorbing the torque under eccentric load. The deformation body is optimised exclusively for the strain gauges and strain measurement. The deformation body is then welded and is water vapour-tight thanks to this metallic encapsulation.
Encapsulated stainless steel cells are predestined for ambient conditions with high requirements for the degree of protection up to IP69 and aggressive media. The foil of the strain gauges is completely encapsulated in metal, which means that there is no influence of humidity on the measurement result. The connection cables are usually connected using a glass feed-through in order to fulfil the requirement for hermetic sealing.
For the user, this means that aluminium/stainless steel load cells with silicone encapsulation should primarily be used in dry applications encapsulated in a housing or if the influence of moisture plays a subordinate role. In addition to the IP protection, the cleaning agents that may be used must also be taken into account; a higher protection class usually indicates wet cleaning with chemical additives. Even anodised aluminium cells only offer limited protection here, as certain areas, such as threaded holes, cannot be anodised.
How long has DMS technology been around?
As early as 1856, Lord Kelvin discovered that the electrical resistance of copper and iron wires increases when they are subjected to tensile stress. He thus laid the foundation for strain gauge technology, which is the basis for today's single point load cells. However, Edward E. Simmons and Arthur C. Ruge are considered the fathers of strain gauges. Interesting fact: Ruge submitted the patent for a strain gauge in 1938 to MIT, where he worked. Their judgement: As this development was deemed to be "commercially unprofitable", he was allowed to claim the patent and the income from it for himself alone. Today, strain gauges are considered an indispensable component worldwide for everything to do with industrial weighing.
What weighing indicators are available?
Our load cells enable you to determine weight values reliably and with high precision. In order for these values to be efficiently integrated into your production processes, the signals must be recorded and forwarded to the control system. Various weight transmitters, weighing indicators and weighing controllers are available for this purpose. Weight transmitters, which are usually housed in control cabinets, forward the measured values. Weighing indicators, on the other hand, also offer a display on which the weight values can be read directly. Weighing controllers also enable the control and automation of weighing processes in order to optimise the entire procedure.
How-to Guide Single point load cells
Highly accurate measurement results with only one load cell
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