Measure a model filter’s particle reduction
The idea
State the measured goal: reducing a specified kind of harmless visible particle in equal-volume samples.
Step by step
- State the measured goal: reducing a specified kind of harmless visible particle in equal-volume samples.
- Calculate the decrease relative to the starting count, using the same counting method before and after.
- Keep the conclusion narrow: a particle-count test does not establish that water is safe to drink.
Example
A model filter is tested with harmless visible particles. An equal-volume sample has 100 counted particles before filtering and 20 after. What percentage reduction is shown?
Answer
80%
The reduction is 100 − 20 = 80 particles. Divide by the starting count and multiply by 100: × 100 = 80%. This tests counted particles, not all possible water contaminants.
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Use paper drawings or invented counts to compare two filter models. Identify one other property that would need a different test.
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