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How to Select a Right Load Cell with Low Capacity

The growing demand for products, including kitchen scales, body scales, postage scales and handheld scales, has fueled promotion of both the production capacity and quality of low capacity load cells. In order to allow more clients to select the right load cell for themselves based on the integrated performance of the scale, reduce the cost, improve product quality and enhance product competitiveness on the basis of ensuring the integrated performance, this paper mainly provides several methods on how to choose the right low capacity load cell (taking the load cell with a double-hole parallel-beam structure for example) based on some commonly-used performance parameters.
1. Capacity (Rated load)
Proper setting of the load cell capacity can improve the integrated machine's performance on the whole. The following equation can be adopted to make the choice:
W denotes the capacity of the load cell.
G denotes the maximum weighing of the electronic scale.
G0 denotes the tray weight of the electronic scale.
0.8 is the safety coefficient.
For example, for an electronic scale with maximum capacity 5kg, if its tray weight is 0.5kg, you should choose the single point load cell with capacity 7kg. (Generally, the maximum capacity is 3-6kg. The tray weight of the electronic scale should be smaller than 0.5kg. The tray weight of the 15kg electronic scale should be smaller than 3.5kg.)
Single point load cell with capacity 7kg
Some users can directly choose the capacity of the load cell as the maximum weighing value of the scale. This method is suitable to electronic scales whose capacity is above 10kg. However, there should be margin for those whose capacity is below 10kg. If the electronic scale (low capacity) is improperly designed in terms of overload limit, the machine might fail to get started because overload deformation of the load cell caused in the installation, debugging or customer use process. When modifying the four-corner eccentric load error of the load cell, if the user technical indexes lay no specific emphasis on the calibrated load, you can use one third of the load of the load cell nominal value for calibration according to national standards. Under the condition, you can consider adding the weight of the tray. If factors in this aspect are not taken into consideration, the chosen load cell might result in increase of the four-corner eccentric load error of the whole machine.
2. Resistance value
Currently, the load cell resistance can be divided into two types 350Ω and 1,000Ω. To a strain gauge of 350Ω, since the resistance value is small and the power consumption is large, the temperature increase caused by self-heating of the strain gauge after electrification is high. In terms of load cells similar those for palm scales, their load cell volume is small and the sensitive region is too thin. The radiating conditions of the strain gauge are poor, which might result in zero-temperature drift of load cells. The large drift of the rated output temperature is increasingly prominent and hard to control. Load cells with serious drift might be unable to be eliminated after zero-point input following start of these load cells, which will then impair the scale precision. Therefore, load cells whose capacity is smaller than 2kg usually have a small cross-section. The resistance of load cells used to provide power for dry cells should not choose the 350Ωtype under general conditions but choose 1,000Ω.
The low capacity load cell usually features a double-hole parallel-beam structure. If the capacity of a load cell is between 3kg and 1,000kg, its cross-sectional dimensions are relatively large and the strain gauge's dimensions are large, the resistance can be set to be 350Ω. On the market, there are load cells with a large resistance, such as 2,000Ω and 3,000Ω. However, the larger the resistance value is, the higher the manufacturing cost of the load cell will be.
3. Output sensitivity
Output sensitivity refers to the load cell's output value of inputting every 1V voltage when the load cell is receiving the rated load. (mV/V)
Due to limitations of the sensitivity gauge strain sensitivity coefficient, elastic materials' strain capacity (limit) and machining, the sensitivity is usually set to be the following values, 2.0, 1.8, 1.5, 1.2, 1.0, 0.8, 0.5, or 0.4 mV/V. As to load cells whose capacity is below 500g, the sensitivity is usually set to be around 0.5mV/V. When the capacity increases to 0.5kg to 2kg, the sensitivity is usually set to be around 1.0mV/V. When the capacity increases to 2kg, the sensitivity is usually set between 1.2mV/V and 2.0mV/V.
500g to 2kg single point load cell
In the practical manufacturing process, when the thickness of the general load cell's sensitive area is smaller than 0.5mm, serious deformation of some parts might be easily caused, thus increasing the cost of machining and reducing reliability and stability of the load cell. At the same time, when the 0.02-grade load cell is made, the first-pass-yield will drop. As a result, the load cell's overall cost will rise. To cope with problems of the kind, Change the external dimensions of the load cell (elastomer) or reduce the output sensitivity of the load cell, and increase the circuit gains as compensation.
Usually, the scope of the sensitivity tolerance ranging from ±10%FS to ±20%FS can be already satisfied relevant requirements. Though sensitivity normalization can control the load cell's sensitivity within ±0.1%FS, it might increase the load cell's manufacturing cost. Most users turn to the calibration increase function in the secondary meter to meet the use requirement.
4. Zero-point output
Most people think that, the lower the zero-point output of the load cell is, the more convenient it is to use. It is even believed that this performance index is a symbol of the high-quality load cell. This idea makes sense to some extent but is not fully true. In the early period, circuits could be distributed via adjustment of the regulating resistor on the input end of the amplifier. Since the stability of this method is poor, the former zero-point output should be ensured to be as small as possible. With development of hardware and software, the regulating resistor can be changed into the fixed resistor, and realize zero clearing upon starting. Therefore, there is no need to control the zero-point output within a small scope. Meanwhile, restricted by the A/D switching circuit, the superposition value of the zero-point output and the full-scale output should not exceed the regulating scope of the A/D switching circuit, and the zero-point output should not be left unlimited. Therefore, the zero-point output scope should be identified according to the above comprehensive factors. It is more suitable to set the zero-point output to be around ±0.3mV/V. The load cell with a high accuracy is usually set around 2% FS. The higher the accuracy is, the higher the manufacturing cost of the load cell will be. The zero-point output can also be chosen according to the regulating scope of various meters. There is no fixed rule therefore. The zero-point output selection should be reasonable, economic and stable.
5. Zero-point output temperature drift and rated-output temperament drift
These two indexes are indexes reflecting the temperature performance of load cells, which should be decided according to changes of the use environment of load cells, accuracy of load cell controllers and temperature performance of load cell controllers.
As to the zero-point temperature drift, since the ambient temperature changes are slow in the practical use process of most electronic scale and it takes a shorter time to get started, the zero-point output of the excitation starting can be automatically tracked and eliminated. Therefore, the accuracy of the scale and the compensation cost of the load cells should be both considered to finally choose proper compensation accuracy. When the comprehensive accuracy of load cells should be around 0.05%FS, the temperature performance is usually set around ±0.1%FS/10℃. When the comprehensive accuracy of load cells is around 0.02%~0.03%FS, the temperature performance is usually set to be around ±0.02%~±0.1%FS/10℃. When the accuracy is low or the use environment is favorable (such as the infant scale for the medical use), there is no need to compensate this performance, and the temperature performance is around ±0.3%~±0.8%FS/10℃.
The rated output temperature drifts because of the elastic modulus of elastic materials changes with the temperature. As a result, the load cell's output sensitivity will change with the temperature. The performance belongs to a system error. When the elastomer materials, load cell structure, strain gauge and manufacturing process are fixed, you just need to connect the compensation resistance with a large resistance temperature coefficient to the input end of the load cell. In this way, the rated output temperature drifting can be easily controlled within certain scope. At present, the rated output temperature drifting of general load cells should be controlled within the scope of ±0.02%FS/10°. If the accuracy should be further improved, the linearized compensation can be employed, which is to adjust the rated output temperature drifting to be ±0.016%FS/10℃ with the temperature compensating plate and the fixed resistance reasonably connected in parallel. This can also well explain the increasing cost resulted from this method.
6. Four-corner eccentric load errors
To modify the four-corner eccentric load errors of load cells with the general accuracy, load cell manufacturers usually adopt the tooling which is the same to the size of the client's practical scale disk. The client just needs to install the load cell and the load cell controller in a full set to basically achieve the required accuracy. Since the four-corner eccentric load errors will happen after reinstallation of errors, electronic scales with a high requirement of accuracy relies on proper filing adjustment after the load cell installation. Therefore, the space for filling adjustment of eccentric load errors should be reserved for the scale body design.
Major factors influencing the four-corner eccentric load errors include whether the capacity of the load cell is properly selected, the size of the installation moment, stiffness of the scale body bottom disk, size of the scale disk, etc.
In the practical process of modification, modification is conducted at one third of the weight load of the full capacity. The filling should ensure uniformity up and down to avoid the partial filling to be too large, thus causing strain concentration to influence the zero-point output performance of the load cell.

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