Estimating seed quantity
Several methods are used to estimate the number of seeds within collections. If fruits are to be stored, then the number of seeds per fruit must be recorded.
Weighing small seeds on a seven decimal place balance
For most collections, a four or a seven decimal place balance (dependent upon seed size) is used to weigh five samples of 50 seeds or fruits. If seeds are exceptionally small (e.g. those of Orchidaceae and Orobanchaceae), a sample of 250 seeds is used. Once the weight of the sample has been measured, the remainder of the collection is then weighed and the total quantity of seed calculated. The total is deliberately underestimated to ensure that the collection is not exhausted earlier than expected.
For small collections (less than about 300 seeds), they are counted individually.
If the collection is clean and has suitably sized seeds/fruits it may be counted by a seed counting machine, but this method is too slow for very large collections.
X-ray analysis is sometimes required to determine the proportion of empty or infested seeds.
The determination is recorded on the Seed Bank Database (SBD) which calculates the final seed quantity.