Test-Weight Apparatus

Test-weight apparatus produces a packing-density measurement, and packing density depends on exactly how the material was packed. A fixed funnel geometry controls how the grain stream spreads as it falls; a fixed drop height controls the momentum the kernels carry as they land; a fixed filling rate avoids either rushing the fill or letting it settle unevenly; and the stroking action removes the excess in a standardised way rather than leaving an arbitrary heaped or compacted surface. Each of these choices measurably changes the mass that ends up filling the container, which is why grading standards specify apparatus design and filling technique in close detail rather than leaving them to operator discretion.

Kilograms-per-hectolitre apparatus, used across most of the world, and pounds-per-bushel apparatus, used in the United States, employ different container volumes, different funnel and drop-height specifications, and different filling conventions. As a result, a figure obtained on one system is not simply the same physical quantity expressed in different units — the two procedures do not measure identically, and the relationship between them is captured in a published conversion table rather than a straightforward arithmetic factor. A converted figure carries additional uncertainty relative to a figure obtained by direct measurement on the target system's own apparatus.

This distinction is a genuine, practical friction in international grain trade: contracts that specify a test-weight requirement should state which measurement system and apparatus the figure refers to, since a converted number and a directly measured number are not interchangeable for contractual purposes.

• Wetter grain packs differently than drier grain of the same variety and condition, so a test weight taken before drying is not the test weight the same lot will show after drying, independent of any change in the grain itself.
• Fines and dockage — small particles, broken material, and foreign matter — fill the interstitial space between whole kernels differently than whole kernels do, so an uncleaned sample can read differently from the same lot after cleaning.
• Vibration during filling, or during transport of a filled measure before weighing, can further compact the grain and raise the apparent figure above what an undisturbed fill would show.

Because of this sensitivity, a test-weight figure is only comparable to another figure, or to a contractual specification, when both were determined on samples in a comparably specified condition — a routine and well-documented source of buyer–seller dispute when that condition is not made explicit.

Because the apparatus responds to how kernels of a given shape and surface texture pack together, as much as to the intrinsic density of the kernel material itself, the resulting figure does not disclose what is happening inside the kernel. Two lots can share the same test weight while differing in protein, starch, or internal soundness, and a shrivelled but otherwise sound lot can pack, and therefore weigh, differently from a plump lot of similar internal composition. The reading is a physical-condition screen, not a composition measurement — see Test Weight for what it is commonly, and incorrectly, assumed to indicate.