The Secret of Weight Management Or Why you Weigh More on Thick Carpet

The effect of thick carpet on the accuracy of bathroom scales was investigated using two analogue scales and two digital strain gauge scales. A mass range of 0Kg – 104Kg was used. Loading history was found to have a significant effect on analogue scales, and the experiments were designed with this in mind. The mean spring constant of a Hanson analogue scale system was found to be 12.13±0.30KNm; this compares with a value of 36.52±1.34KNm for a Hanson strain gauge system. Of the soft surfaces studied, only thick carpet (10-12%) and foam (8-10%) caused greater than 2% increase in analogue scale readings. These surfaces did not cause a substantial effect when placed only under the corners of the scale. Supporting the base of the scale on a hard surface caused a 13% increase in readings; by contrast, tilting the scale and moving the centre of mass on the scale had very little effect. Deflection of the base of the Hanson analogue scale was studied across the mass range using a DTI. The most deflection was found at the rear of the scale, for both analogue (−2.896±0.021mm) and digital (−0.752±0.012mm). Deflection of the scale base was found to be principally due to stress incurred from supporting the weight at the corners of the scale. Readings on a hard surface with the base supported showed no excess deflection of the test plate compared to hard surface readings. Since little movement of the test spring was found, it was concluded that there must be no substantial change in moment of the lever system when the base is supported. An excess downwards motion of the top plate was discovered when the base of the scale was supported, which implied excess movement of the levers (and thus test plate). Sharpening the levers was found to reduce the increase in Hanson analogue readings on carpet to 2.60±0.29%; thus, lever movement was found to be a principle cause of the increase. Thick carpet was found to cause an excess of +0.87±0.03mm vertical movement in the mechanism converting test plate movement into dial rotation; it was concluded that this movement was the principle cause of the increase in readings still present with the sharpened levers. Thick carpet was found to have virtually no effect on the digital scales studied, causing an increase in readings of 0.295±0.008% (Hanson) and 0.133±0.008% (Salter); this meant the strain gauge system was by far the best practical solution to the problem.