You buy 20 tonnes against a clean COA, and three weeks later the message comes: 'every big bag runs differently.' The paperwork looked fine. The shop floor says otherwise. Both can be true at once.
The problem
A customer buys a larger quantity, the overall COA looks good, and yet individual big bags or sub-lots process differently. The average was right; the uniformity was not — and the average is what most people buy on.
Why it happens
Average values for a whole lot do not automatically mean every sub-quantity is identical. A lot can be blended from sub-lots, produced across a transition, or — in wide-spec and certain spot lots — carry a real distribution of properties. The COA describes the lot as a whole; your machine sees one big bag at a time. The wider the specification, the more that distribution can matter.
How to handle it properly
Treat lot structure and uniformity as part of the purchase, not an afterthought:
- Understand the lot structure — is it one production run, or blended sub-lots? Is it wide-spec or a transition lot?
- Sample representatively — sample across the lot, not one bag — averages hide spread.
- Homogenise where needed — blending a large enough production batch can matter more than chasing a better COA.
- Match spread to tolerance — a tight part needs a tight, homogeneous lot; a forgiving part can take more spread.
Key takeaway
A good average COA does not guarantee every big bag runs the same. For wide-spec and spot lots, understand the lot structure, sample it properly and homogenise when the part needs it — uniformity can matter more than the headline numbers.
Professional spot buying isn't just the COA — it's sampling, lot management and homogenisation.
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