关键要点
- Sampling error usually exceeds laboratory error by a wide margin.
- Increments must be spread across the whole lot, not taken from accessible points.
- Seal, split and retain samples for any result you may need to defend.
Increments, composite and reduction
Take many small increments distributed across the lot, combine them into a composite, then reduce by riffle divider or quartering to laboratory size. Each step must be documented; an undocumented reduction step is where representativeness is most often lost.
Bulk versus bagged
Bulk cargo is sampled during flow — on a moving stream at loading or discharge, which gives the best representativeness. Bagged cargo is sampled from a defined proportion of bags selected across the stack, not from the bags nearest the door.
Sealing, splitting and retention
Split the composite into at least three sealed, labelled parts: buyer, seller and umpire. Record lot identity, date, location, sampler and seal numbers. This is what converts a result into contractual evidence.
操作步骤
- Define the sampling plan and increment count before the lot arrives.
- Take increments across the full lot, ideally from a moving stream.
- Combine increments into a documented composite.
- Reduce to laboratory size using a defined method.
- Split into sealed buyer, seller and umpire samples.
- Record and retain sampling documentation with the lot file.
需要核查的内容
- Written sampling plan referenced in the contract
- Increment count and distribution recorded
- Sealed retention samples held with documented seal numbers
需管理的风险
- Samples taken only from accessible surfaces or the first bags
- Segregated cargo sampled at one point giving a misleading result
- No retention sample available when a dispute arises
常见错误
- Treating one grab sample as representative of a container
- Skipping documented reduction and sending an oversized sample
- Failing to seal and witness samples on contested cargo
常见问题
How many increments are enough?
It depends on lot size and material heterogeneity. Recognised sampling standards give increment counts by lot size; the contract should reference the standard applied rather than leaving it to judgement.
Who should take the sample?
For contested or high-value cargo, an independent surveyor witnessed by both parties. For routine receipts, trained internal staff following the documented plan.