Thanks for the great question. Testing the alcohol level of a wine is a subject that always seems to have some confusion among home winemakers.
A refractometer can not be used as an alcohol tester for wine. It will not test the alcohol level. A refractometer will only test the sugar level of a wine must or finished wine. This is no different than what a wine hydrometer can actually do. They both measure the sugar in a wine, not the alcohol.
By comparing two sugar level readings, one taken before the fermentation and another after, you can determine how much alcohol was made. This is because wine yeast consume sugar and turn some of it into alcohol. If you know how much sugar was consumed by the wine yeast, you can then determine how much alcohol was made.
This principal is exactly the same for a refractometer as it is for a hydrometer. Neither are alcohol testers, but they will allow you to calculate the alcohol level of a wine must or finished wine by comparing a current sugar reading (brix) with a beginning reading.
What makes the refractometer extremely useful — and more handy than a hydrometer in some cases — is that you can take accurate sugar readings with very small liquid samples — just a couple of drops is all that is needed. This makes it ideal for checking the ripeness of the grapes while out in the vineyard. You only need to squeeze the juice from a single grape to see how sweet the grapes are becoming. This is very valuable when trying to determine when to pick your grapes.
Alternately, the hydrometer needs enough sample for it to float. This could take as much as 4 or 5 ounces of wine or must. A hydrometer jar is also needed to hold the sample. So as you can see more time and effort is involved to take a reading with a hydrometer. This pretty much rules out taking a sugar reading on the fly as you might with a refractometer.
Gary, to answer your question more directly, a refractometer is a great tool for any winemaker to have. It is very handy, and it provides a quick way to get a sugar reading almost anytime, anywhere. But a wine refractometer is not an alcohol tester. It will not directly give you the alcohol level of your wine. This can only be done by comparing a beginning reading (before fermentation) with a current reading.
Happy Wine Making,
Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.