RO water to calibrate refractometer?

Answer:

Yes Garry

 I use RO Water too which is pure enough.

Refractometers always read the SG at a water temperature of 20C regardless of the tank temperature and therefore to calibrate the refractometer properly the instrument must be at 20C during calibration and not the water.

When you then take a reading the REFRACTOMETER will always measure the SG as if the water is at 20C as the small amount of water that you add to the slide will have no impact on the temperature of the equipment and will soon change to the same temperature as the refractometer.

What ATC (auto temperature compensation) means is that once calibrated at 20C you can use the refractometer in higher ambient temperatures where the instrument is hotter or colder.

Once you get your reading you need to compensate for the temperature as most people quote an SG at 25C.

 From the chart above you can see that water at 1.025 and 25C has an SG of 1.0264 at 20C. As your refractometer measures at 20C then you need to look for a reading od 1.0264 on your refractometer to achieve this

 Hope that was not too confusing

 Regards

Stuart