An alcoholometer is a measuring device which is used to estimate the alcohol by volume in a fluid. The device does not measure the presence of alcohol directly, instead measuring the relative density of the fluid when compared to water to determine how much alcohol may be present in the mix. A number of factors including dissolved sugars and other impurities can throw off the reading, making it an estimate rather than a firm measure, but in the right circumstances, the device can be a very useful tool for people making or working with alcohols.
The alcoholometer is a specialized type of hydrometer. It has been calibrated specifically with alcohol in mind, allowing people to take readings from the device to measure presumed alcohol content in various products. These devices work best with distilled spirits, as they are less likely to contain impurities. With things like wine and beers, the reading can sometimes be significantly off, and the accuracy of the reading can also be complicated by a low alcohol content.
The alcoholometer consists of a tube, much like a thermometer, which is lowered into a sample of the fluid. The reading is taken by looking for the meniscus of the fluid and seeing which line on the alcoholometer it matches. Depending on how dense the fluid is, the meniscus will settle at varying points on the alcoholometer. The device can be used at various stages of the brewing and distilling process to measure and monitor the production process.
As can be seen, the device really measures density, not alcohol content, although it has been calibrated with alcohol in mind. Anything which makes the fluid more or less dense will throw the reading off. Home brewers sometimes use such devices to arrive at a ballpark estimate of alcohol content, also relying upon their experiences with previous batches and on other factors.
Knowing the alcohol content of a fluid can be important for a number of reasons. Alcohol sold for consumption is usually required to be labeled with a disclosure of the alcohol content, as this information is believed to be beneficial for consumers. Alcohols used in various industrial processes also need to be tested to confirm that their alcohol content meets the needs of the application. For example, if an alcohol is being used to sterilize, an alcohol content which is too low could cause it to be less effective, which could potentially be dangerous.