There are two methods to control shading ratio. The first method is multiple sampling: adding small amounts of samples for multiple times and observe the changing of shading ratio. When reach the pre-set value, then stop adding samples and start test procedure. The second method is sampling with the fixed amount. For powder samples of the same grade, the amount of the sample to achieve the optimal shading ratio is firstly determined by experiments. Subsequently, the same amount of samples measured by balance can be used for each measurement. It can achieve the required optimal range of shading ratio at one time.