The refractometer is calibrated to read correctly at 20°C. The refractive index of the measured sample will vary with temperature. Therefore it is important, that measurement with the refractometer takes place at 20°C.
The accuracy of the instrument may be checked using a simple test.
Use distilled water. Apply a sample of distilled water and check that the R.I. reads 1,333 (+/- 0,002)
This sample can be performed in regular intervals as a part of security quality-control program. The refractometer has not to be send in. The check can take place in your company.