Testing the accuracy of a weighbridge can reveal a different reading when weights are placed on the scale, compared to when the weights are taken off. Hysteresis is the measure of that difference in indication, when the same test point is approached with increasing or decreasing weight.
How is hysteresis identified?
Typically, calibration of a weighbridge will involve testing at five points within the weighbridge’s capacity using calibrated weights, starting from zero. If the scale’s capacity is 1000kg, for example, it will be tested every 250kg, thereby providing five readings between zero and capacity. The tester adds weights to the scale one by one, and checks the reading each time. The weights are then removed one by one, with all the weight readings recorded on the calibration certificate.
When readings at the same point differ on the way up compared to on the way down, hysteresis is identified and can be measured according to how much the readings differ at the same test point.
Why is weighbridge accuracy so important?
Weighbridges determine the mass of a truck and the load it is carrying. The difference between the mass of the unloaded truck (tare weight) and the mass of the truck when it is loaded (gross weight) enables the net weight to be calculated. For traders of commodities such as secondary raw material, this weight determines the value of the shipment they are buying or selling.
Once the truck has gone over the weighbridge, both unloaded and loaded, the verified mass of the shipment is recorded on a weighbridge ticket, along with other details such as the identifier of the truck, which is usually its registration plate. According to UK regulations, the weighbridge ticket needs to accompany the load on its journey, usually in the form a written, hard copy document.
Weighbridge accuracy is therefore essential, and trade-approved weighbridges need to be tested, adjusted and certified on a regular basis, in a process known as calibration. If testing reveals levels of inaccuracy that breach industry standards, possibly in the form of eccentricities (where the reading changes according to where the load is placed on the weighing scale) or hysteresis, the weighbridge needs to be fixed to remain compliant.
Linearity and repeatability
Inaccurate weighbridge readings can also be captured by measuring linearity and repeatability. Linearity issues mean that the weighbridge does not measure equally accurately throughout the range. There may be errors in the middle of the range, for example, making the readings nonlinear. Repeatability issues are when the same load is measured several times in identical conditions, and the result is not always exactly the same.
Other meanings of hysteresis
Hysteresis was coined by a Scottish physicist and engineer, James Ewing, who originally used it to describe the behaviour of magnetic materials. The concept of hysteresis spread, and is now widely applied across aerodynamics, hydraulics and computer interface design, albeit in different ways and with different meanings.
In economics, for example, hysteresis is sometimes used to describe the effect of traumas such as Covid-19 on economic systems. Hysteresis can be observed when the trauma has passed and economic conditions return to normal, but various economic metrics do not. For example, the permanent effect that lockdowns had on people’s tendency to work from home, or to leave the workforce, are potentially measurable examples of economic hysteresis.
