| dc.description |
Modifications to a commercial platform weighing machine, to provide a continuous record of changes in weight of a standing load on the platform, are described. With this instrument, weight changes of up to 55 lb in a maximum load of 560 lb can be recorded to an accuracy of 0.5%, i.e. less than 5 oz, which is the accuracy of the weighing machine itself.An earlier instrument, although satisfactory for short periods of operation, was found to be unsatisfactory for measuring weight changes over a time in excess of a few hours, owing to random changes of up to 2 lb in full-scale calibration of 50 lb. This difficulty has been overcome in the present instrument by using photoelectric cells to detect the vertical movement of the steelyard, and a servooperated mechanism to control the restoring force. The output, which is directly proportional to the change in weight on the platform, is in a form which is suitable for being recorded on a potentiometric recorder. It is substantially independent of normal ambient temperature changes; the calibration changes by 0.01% per ° F change of ambient temperature. This error can be allowed for if the ambient temperature changes are known. |
