The most precise method for determining alcohol in wine is by distillation. This method is based on removing and collecting all of the alcohol contained in exactly 250ml of wine. The distillate is diluted back to exactly 250 ml. using distilled water, and the alcohol concentration is then determined by measuring its density using an alcohol hydrometer, usually calibrated at 20C and in % v/v. Other methods include ebulliometry, which uses the depression of the boiling point of water in the presence of alcohol. This method requires more expensive equipment than distillation and is also dependent on the accurate measurement of the change in boiling point of wine compared to water.
The Vinometer, as the blurb says is a piece apparatus that consist of an open, graduated glass tube with a small filling resevoir. The vinometer's reservoir or funnel is filled with a small amount of wine being tested until some wine exits out at the other endIt is then turned around and placed on a flat surface, filling reservoir side down, and allowed to self drain.The alcohol concentration level is read of the vinometer's scale, on the capillary, at the top of the liquid being measured, in %v/v alcohol.
The level of the liquid is determined by the modifying affect that alcohol has on the interfacial tension between water and glass and the opposing surface tension of water. The greater the alcohol concentration the less marked the liquids capillary action and the lower it will sit in the tube of the vinometer
The vinometer is calculated on the basis of pure alcohol and water solutions, whereas wine is not a pure water/alcohol solution and the accuracy of a vinometer can not be relied upon.
Sugar interferes with the interfacial effects that a vinometer's action relies on and hence the method can only be applied to dry wines. Similarly other components in wine can also interfere with accurate measurements.
So I will need to check out my vinometer by diluting pure alcohol with distilled water so that I have some idea of how accurate this little instrument can be. But seeing as how that will have to wait until next week, I just couldn’t resist the temptation of checking one wine. The wine is a Spanish Tempranillo from Rioja (2000 Valenciso Reserva). The label says 13% alcohol. The vinometer gave the same reading (two measurements) of between 14-15%, so let’s call it 14.5%. That is quite within the law as below 14% a wine is allowed 1.5 points above or below the value stated on the bottle.