Clean the sample well and zero it
DailyA film on the prism is a measurement error you cannot see.
What you need
- Lint-free cloth or lens tissue. Distilled water. The cap it came with.
What goes wrong
- Wiping with a paper towel, which scratches the prism. Zeroing against tap water, whose dissolved solids are the thing you are trying to measure. Reading a hot sample, which reads differently from a cooled one.
The procedure
- Wipe the well with a lint-free cloth and distilled water after every sample, before the coffee dries.