The air piston gauge (APG) has been established at the National Physical Laboratory, India (NPLI) for Pascal realization since 2002. The APG at NPLI has been considered a transfer pressure standard because it has been calibrated against the primary pressure standard, i.e. ultrasonic interferometer manometer. As per existing international protocol, the APG establishment as a primary standard, the effective area (A _e) of piston–cylinder (p–c) assembly and masses must be directly traceable to SI units. We have calculated A _e and associated uncertainty of p–c assembly using the theory of rarefaction gas dynamics, which is based on dimension measurements. The value of A _e is obtained for varying temperatures and radii of piston and cylinder in their respective uncertainty limits. The variation of the cylinder's inner radius within its uncertainty limit of 0.7 m includes the expected effective area, i.e. 3.356 775 (5) cm². The effective area’s expected values are obtained for the cylinder’s radius of 10.338 00 (2) mm, which is approximately 0.65 m away from the value obtained from dimensional measurements and well within the uncertainty limit. Therefore, to get the effective area of 3.356 775 (5) cm², the uncertainty of the cylinder’s radius should be at least one order less (similar to piston’s radius) than that of the present value. The precision in dimension measurement of the cylinder's internal radius is the critical parameter for effective area calculation of p–c assembly.

自 2002 年以来，印度国家物理实验室 (NPLI) 已建立空气活塞压力表 (APG) 以实现帕斯卡。 NPLI 的 APG 已被视为传输压力标准，因为它已针对主要压力标准（即超声波）进行校准干涉仪压力计。根据现有的国际协议，APG 建立作为主要标准，活塞-气缸 (p – c) 组件的有效面积 ( A _e )和质量必须直接追溯到 SI 单位。我们使用基于尺寸测量的稀薄气体动力学理论计算了A _e和 p - c 组件的相关不确定性。A _e的值 是针对活塞和气缸在其各自的不确定度范围内的不同温度和半径而获得的。圆柱体内半径在其 0.7 m 不确定性限制内的变化包括预期的有效面积，即 3.356 775 (5) cm ²。圆柱半径为 10.338 00 (2) mm 时获得了有效面积的预期值，该值与从尺寸测量中获得的值相距约 0.65 m，并且完全在不确定性范围内。因此，得到有效面积为 3.356 775 (5) cm ²，气缸半径的不确定性应至少比当前值小一个数量级（类似于活塞半径）。圆柱内径尺寸测量精度是 p - c 组件有效面积计算的关键参数。