Influence of the Mechanical Error on Performance of Beta Thickness Gauging and the Prediction of System Operation Time

Abstract

The thickness gauge systems based on beta radiation were widely used in the industry for online, continuous and non-contact measurements. One of the important parameters in beta thickness gauge is the accuracy which the system was evaluated based on it. The effective factors in the measurement accuracy of these systems consist of the mechanical and detection sections. In the mechanical section, the detection system (source and detector) was moved throughout the factory line (without sheet) in order to measure the mechanical error. In the detection part, the detection set-up was stopped in its parking lot and the biaxially oriented polypropylene sheets with different thicknesses were placed between the source and detector to determine the contribution of detection error. In this research, a beta thickness gauge with a 147Pm source (half-life of 31 months) was evaluated in biaxially oriented polypropylene sheet production lines in order to determine the mechanical and detection errors. The contribution of the mechanical error due to system motion and the contribution of the system’s detection error were measured individually. The total error is the sum of the acquired errors of two mechanical and detection sections. In this study, the 2sigma (standard deviation) criterion was used as a system error. The mechanical error was calculated 0.12 by moving of system and without the sheet. Also, the detection error was determined 0.1 by placing the sheet between source and detector and stopping of the system. The total error was estimated 0.16 using the error correlation relation. The acquired results showed for producing of biaxially oriented polypropylene sheets with 15-µm thickness and the maximum acceptable error of  ±0.2 µm, the system can be used for 40 months at the precision measurement time (zero time). If the mechanical error is reduced by 50%, the using time of the radioactive source increases to 16 months (56 months). The reducing of the mechanical error can minimize cost, radiation safety, and shielding requirements.