To have someone pour it directly by hand would certainly not be accurate enough, and if one were simply to pierce a hole in the bottom of a tank and let the water flow out through it, the changing level of water in the tank would alter its pressure, and this would cause the rate of flow to vary.
用人工直接打水，絕對無法精準；在水箱上挖一個洞，由洞口流出水來，也會因為箱內水位高低變化，水壓不同，造成水的流速有異。