在对小径管环焊缝进行倾斜透照椭圆成像时，先要计算出射线机射线源焦点相对于焊缝中心平面的偏心距。设计了一种由曲线图和诺模图组合而成的偏心距图，依据给定的参数值，用直尺可快速准确地从偏心距图上获取偏心距值。该偏心距图为现场实际检测工作提供了便利，同时也可作为快速验证工具，用以对操作指导书中偏心距规定值是否准确进行快速核验。

In the case of oblique exposure ellipse imaging of the girth welds of small diameter tube, the offset distance between the focus on the flaw detector and the plane of the weld centers should be calculated firstly. A chart for calculating the offset distance has been designed. The chart was composed of the curve graph and the nomograph. With the given parameter value, the precise figure of the offset distance can be obtained quickly by a ruler. The chart made the on-site testing work more convenient. Meanwhile, it can be used as a fast verification tool for quickly verifying the specified result in the operation instruction.