It is important you realise the benefits and limitations of both instruments.
A hydrometer measures the density of a solution. A refractometer measures the refractive index.
At the start of fermentation, the wort is a solution of saacharides (complex wort sugars) and water. After fermentation commences, there is less sachaarides, ethanol and water. Ethanol is less dense than water, so a hydrometer is not very effective at trying to make an effective measurement of two fluids that are effecting the hydrometer in different ways. The ethanol is making the hydrometer sink, the dissolved sachaarides is making it float. This sg reading is known as the apparent attenuation.
The refractometer will show a much higher brix as it is not affected in the same way as the hydrometer by the ethanol. It shows the real attenuation.
For example, at the start of fermentation, sg on the hydrometer is 1.050, the refractometer shows 12.5 brix. Everyone is happy as we know that 1 brix is equivalent to 4 sg points.
At the end of fermentation, the hydrometer shows 1.010, which equates to 80% apparent attenuation. The refractometer shows 5 brix, which gives the real attenuation of 60%.
It is then that you realise that the hydrometer is not very good at showing what is really happening in the finished beer. There is alot more unfermented material in beer, and the balance between malt flavours, hops and alcohol takes on a new angle.
Any instrument is only as good as its last calibration. You may find that if you use it regularly, the calibration doesn't change much. If you only pull it out every blue moon, the ambient temperature may have changed and a calibration is due.
Follow big d's suggestion of condensed steam. I use some of the water that collects under the lid of the kettle.
So that you become familiar with the use of your refractometer and so you can relate it back to your old sg readings, do regular calibrations and side by side readings with both the hydrometer and the refractometer. You will quickly become happy with your shiney refractometer.
