The coffee refractometer needs to be cleaned and calibrated regularly.
Samples must be managed in a way that prevents water evaporating but also allows them to cool to room temperature when reading will be more accurate.
CO2 content is expected to be higher in dark roast coffees, while H2O content is expected to be lower in dark roast coffees.
The effect of these two presences in roasted coffee is to increase extraction yield values.
Scientific literature reports a 1.3-2.3% H2O content in low temperature, slow roasts (specialty).
When calculating extraction yield, the mass extracted from the coffee grinds will reduce the liquid retained.