As distillers know, accurately measuring proof levels is integral to producing great spirits. Furthermore, different kinds of spirits require specific proof ranges in order to be sold legally; tax regulations also depend on these measurements; accurate measurements therefore require significant care in taking place.
Adjusting the proof of spirits begins after distillation has taken place. Spirits come off of a still at much higher proof levels than what would normally be bottled, so they must be diluted down before entering barrels for ageing. Alan, one of the founding partners at Branch & Barrel Distilling Company explains that many factors can impact how a spirit is proofed such as type of distillation process used and temperature.
Once a spirit has reached its final bottling proof, it should be left for some time for its ingredients to fully mix, allowing water and alcohol to fully fuse and create its final taste and character. Some distillers may even allow their product to rest at higher proof than listed on its bottle, an approach known as barrel or cask strength.
Proof is determined by comparing its alcohol content against a standard, either by volume or weight. Distillation is one of the more frequently employed techniques, although other methods may also be applicable.