R Avis, of Waitaki Girls High School, asks :-

Why do bathroom scales indicate a lower weight when used on carpet than when used on a hard floor?

Ian McKenzie, a Senior Calibration Technician with GEC Avery (NZ) Limited, responded.

There are two types of personal (bathroom) scales available, a low cost mechanical dial and medium cost with battery operated Digital Readout.

Both rely on a mechanical lever system, connected to a spring which extends under your weight, to transfer the force to a dial or electrical loadcell display.

Because the personal scale lever system and housings are manufactured of light weight pressed steel, for ease of carrying and cost, they are subject to flexibility. The forces applied will not be accurately transferred when used in abnormal conditions.

The scale is designed to sit on all four feet. Depending on the height of the corner feet, if placed on carpet the whole housing may also be partially supported by the carpet and the body will flex differently than when used correctly, for which it has been calibrated. Thus the indicator will not register your correct mass.

With the many anomalies surrounding the use of personal scales in the home situation it is imperative the scale is used on a hard level surface with the base resting on all four feet only. If the scale must be used over deep carpet then place it on a solid piece of timber or chipboard.

It should also be used in a dry bedroom rather than in a bathroom which is often full of moist air which can corrode vital components.