Sampling of Granule Blends

A common way to express the randomness (or the degree of mixing) of a particle blend is by measuring the statistical variation (standard deviation) in the composition of several separate samples. For a completely random mixture it has been shown that  

Using the above equation, the theorehcal sample size needed to represent the bulk composihon of a blend with a given standard deviation can be calculated. Data in the table below are normalized to the relahve standard deviation based on the component concentrahon (P) to allow for easy comparison (relative standard deviation = standard deviation / P x 100). The number of particles is also expressed as grams. It can be seen in the table that the lower the %component in the blend or the greater the confidence in the composihon (lower the acceptable relative standard deviahon), the greater the number of particles are required in the sample. 

The required sample size for a granule blend is important to consider relahve to the intended use practices for the product. The sample size required to represent the blend composition must be less than the amount an applicator would be expected to measure for use. In other words, a blend containing 25% of one component and requiring a 20-30 g sample to accurately represent the blend composition would not be a good candidate for a product that would be used in back pack applications where the user would need to sample a 1-g aliquot. However, an application in a ground sprayer at a product rate of 1 ounce per acre 

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