Weighing System
What is the meaning of a weighing system?
A weighing system is a measuring system used to determine the weight of objects or quantities through scales or other related applications.
History for weighing process.
The oldest weighing tools was discovered about 2000 BC in Pakistan where a balance using plates and stones to weigh goods for trading. In ancient Egypt about 1878 BC balance appeared in the old Egyptian sculptures which show the god Anubis weighing the heart of a dead person to determine whether he is good or not. In China about 300 to 400 BC a wood balance and bronze weights. After that, the weighing process was developed over time till reaching the 17 century AD when the weighing balance was replaced by the spring scale the spring scale uses the gravity effect to make the movement of the spring on a round or straight scale with a pointer responding to the object weight. This type of scale is still used today. In the 20th century, the modern digital was invented based on the variation of an electrical resistance due to the weight of the object. In 1980, the first digital scale was recorded.
What is the strain gauge?
From the sensor definition which is converting the physical quantity to an electrical quantity, the strain gauge is a sensor that converts the weight to an electrical signal proportional to the applied weight. it is fabricated as a winding arrangement of etched metal wire on a high thermoplastic polymer insulation called polyimide. The common material used for the metal wire is the copper-nickel alloy.
How the strain gage is working?
The idea of strain gage is that when a force that represents the weight of an object is applied to the strain gage, the strain gage will stretch and deform causing a change in the dimension of the metal wire. As known electrical resistance is a function of its physical properties which are length L and cross-section A. The equation of the resistance is as follows:
R = σ L/A
When the length L and cross-section A change, the resistance will change accordingly which means the value of R will be equal toR (new) = R + ΔR
The strain gauge is connected to an electrical circuit designed to deal with the results of the resistance change. This electrical circuit is called the Wheatstone bridge. When voltage is applied to the Wheatstone bridge, the output voltage of the middle points of the bridge is equal to zero at balance (at no load). When a load is applied, the used strain gauge will be affected and as a result, the resistance will change. The bridge will be unbalanced and the output voltage at the middle points of the bridge will change accordingly.
What is the load cell?
The load cell is an electronic device used to measure the weight of an objective. The load cell is the industrial form of the strain gauge and its electrical circuit is used for weighing. The size of the load cell depends on the size of the object that will be weighted.
What are the types of load cells?
- Compression load cell.
- Tension load cell.
- Bending beam load cell.
- Shear beam load cell.
- S-Type load cell.
- Single point load cell.
- Canister load cell.
- Ring torsion load cell.
What are the applications of the weighing system?
The applications where the scales or weighing system are vary based on the physical properties of the material and its size.- House kitchens where small scales are used during the cooking process.
- Body weight measurements to calculate, monitor, and control, weight gain and weight loss.
- Markets to calculate the cost and quantities of products.
- Medical labs for sample weight determination.
- Industrial processes labs like physical and chemical labs.
- Quarries, which it used to monitor the loading/unloading and costs of the quantities of the materials.
- Monitor and control the materials' feeding rate in the feeding systems and calculate the materials' weight variation in the industrial processes.
What are the advantages of the weighing system?
Accurate especially for the sensitive application to the weighing error.Easy to monitor and follow because of the simplicity of the scales at home and in labs.
Healthy tools for body physical condition monitoring and control.
easy to detect the reliability and the device's wrong reading (by using a known weight).
How is the calibration done for the weighing system?
For the weighing system and because of the sensitivity of the results used in the lab or in all product fabrication as well as sample results in the lab, the measured weight value should have a minimum tolerance and errors. That is why we should test the scales periodically to confirm the error is within the accepted designed range and best and also the performance of the weighing system is in perfect condition.
Although most scales have some tools like tare adjustment which adjusts the reading of the scale to zero when no load is applied to it or the container is empty, we should first test the scale by using a standard known weight to measure the scale and calculate the error% between the standard and the measured value. If the error is within the scale-designed error range, the scale is OK and we repeat the testing process to confirm the reliability of the system.
If the error is out of range and not accepted, we should proceed with calibration. The calibration process uses the same standard known weight and adjusts the scale parameter to the new values to eliminate the values. The adjustment may be done automatically based on the type and the design of the scale. After that, we proceed with a test for more couple weight to ensure the performance after calibration.
In some scales used in commercial and training, the calibration should be done through authorities which issue a certificate confirming that the scale is healthy and accurate.
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