How Do Weighing Scales Work?
Old balance scales used to use a series of weights to try and make the two sides of the scale level. For example, you would add objects that you knew the weight of until you achieved balance and could work out the weight of your item. Older physician scales, or ‘beam balance scales,’ also required a little manual work by sliding the counterweight along until the balance beam is completely horizontal. However, we are now able to measure weight automatically.
How Do Modern Weighing Scales Work?
The majority of scales now work by using devices known as load cells to measure how much an object weighs, whether you are using it for medical, industrial or retail purposes.
A load cell is a force gauge that is made up of a transducer that is used to create an electrical signal whose magnitude is in direct proportion with the force being measured.
Essentially, when any weight is placed on to a scale, the load cell bends slightly, which causes the electrical signal that runs through the load cell to change. This signal change is due to the amount of electrical resistance the bending causes to the strain gauge inside the load cell.
The signal is then read by an electronic device, often a digital weight indicator, and transformed into a digital weight value. The value is then displayed for reading.
Regardless of the complexity of the scale and weight indicator, all scales work in this similar way whether you’re using scales at home to keep an eye on your weight, using retail scales to weigh fresh produce or using accurate medical scales.
Are Weighing Scales Accurate?
Modern weighing scales have achieved a high level of accuracy, however, re-calibrations will be necessary over time in both analogue and digital models.
As with most daily use items, weighing scales are subject to wear and tear over time due to the delicate mechanisms that operate the machinery.
Analogue weight scales can be easily tested and readjusted at home, however, digital scales will require professional attention - particularly industrial models. Marsden provides a professional scale repair and reverification service to help keep your scale in working order.
How to check your weighing scale accuracy
Testing the accuracy of your weighing scale is a simple process.
First, take an item that states its weight on the packaging - a full bag of sugar is the simplest option.
Place the item on your scales.
Check the weight displayed on the scales against the weight stated on the product packaging.
You will know your scale needs readjustment if the displayed figure does not match the weight of the product.
The Four Types of Load Cells
Some differences between types of scales and how they work is due to the different types of load cells. These include pneumatic, hydraulic, strain gauge (as mentioned in the above example) and capacitance.
Pneumatic Load Cells
Pneumatic load cells deal with air pressure and consist of an elastic diaphragm attached to a platform surface where you place the object to be weighed. There’s also an air regulator that limits the flow of air pressure and a pressure gauge. The scale works by using pressurised air to balance out the weight of the object, and the amount of air required is therefore used to determine how heavy the object is. The pressure gauge then converts the reading into an electrical signal.
Hydraulic Load Cells
The hydraulic load cell works by using a fluid such as water or oil. Scales with hydraulic load cells work like pneumatic load cells except they use a piston and pressurized liquid rather than air. When an object is placed on the platform, the piston applies increased pressure to the contained liquid which is proportional to the applied weight. After calibrating the pressure, you are able to take an accurate measure of the weight and use the pressure reading as an analogue gauge or convert it to an electric signal.
Strain Gauge
The strain gauge is the most popular load cell used in working scales. The electrical resistance when under strain is proportional to the strain placed on the cell when weighing an object which makes it straightforward to calibrate into an accurate measurement. This accuracy is also due to the four strain gauges inside the strain gauge load cell being in a ‘Wheatstone Bridge’ configuration. This is an electrical circuit that measures unknown electrical resistance and provides very accurate measurements when bent under the weight of an object.
Capacitive Load Cells
Capacitive load cells work on the ability of the system to store charge. This load cell consists of two parallel flat plates that have a current applied to them. When the charge is stable, it is then stored between the plates. The amount stored or the capacitance depends on the size of the gap between the plates. So, when an item is placed on the plate, the gap will decrease and create a change in the capacitance, which can then be calculated into weight.