Why does your weight seem to differ if your bathroom scales are placed on carpet instead of a solid wooden floor?
鈥� Bathroom scales operate on the principle that your weight compresses part of a mechanism, such as a spring. If the scales are sitting on a soft surface such as carpet, the surface acts as a spring in its own right. In this situation the reading will be lower than it should be, since your weight goes into compressing both the mechanism and the surface beneath. Being inflexible, a hard floor has a negligible effect on the reading.
鈥淏eing inflexible, a hard floor has a negligible effect on the reading produced by the scales鈥�
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Neil Ayre, Kalgoorlie, Western Australia
鈥� I found that my scales gave readings that were reproducible to within 100 grams 鈥� as advertised on the packaging 鈥� but only if consistently placed in the same position on a hard, level floor. Reproducibility declined if the floor was uneven. And with the scales on a carpet, its readings of my weight could vary by a couple of kilograms.
The scales have load sensors in each of the four corners. Each sensor is glued to a plastic foot which sits within a cylindrical hole and rests on a rubber pad, as shown in the diagram above.
I noticed that when stepping on the scales I always put some sideways force on the plastic feet, causing at least one of them to twist and rub against the sides of the hole. As my full weight bore down, friction with the floor would prevent the plastic foot from recentralising. Instead it remained in contact with the sides of the hole, affecting the reading.
My solution was to put the scales in a wooden box as shown. The base of the box had to be thick enough 鈥� 15 millimetres proved sufficient 鈥� not to bend when I stood on it, and large enough for the scales not to touch the sides. I added a lid (which obviously needs to be removed before using the scales) to keep out dust, spiders and so on.
Anyone building such a box should ideally glue the rubber pads to the base to prevent the scales sliding into the sides of the box. Thus equipped, the scales will give reproducible readings whatever the floor type.
Brian Longson, Dorchester, Dorset, UK