The Gauging Process

Author - Anthony Gannon is a final year student on the brewing and distilling course at IT Carlow. He has written the following piece on the gauging process which we recently used at Flying Tumbler. 

Gauging is the method of determining the final %ABV and bulk volume of a whiskey cask. It is important to gauge a cask after the completion of the maturation period because the composition of the whiskey within the barrel will have changed over the years it has been in storage.

The two main components of the whiskey, water and alcohol, will begin to evaporate. This is because the wood of the cask is porous and allows for oxygen to enter the cask, resulting in a reduction of the bulk volume and quantity of alcohol within the cask.

This is called “The Angels Share” and it is a necessary evil in the maturation process. The cask must be allowed to respire because the same process that causes evaporation of the spirit also drives the necessary chemical reactions that result in the spirits maturation, this is what makes our whiskey great. The typical cask used in Irish distilleries is an ex-Bourbon cask. These casks are made of oak and usually have a volume between 190-200 litres. On average in Ireland there is a 2% loss of spirit per year in a typical ex-American Bourbon cask. This is due to the climate of Ireland. So our miserable weather is actually good for something! In more humid countries the loss can be even greater. For example, in Kentucky and Tennessee, where the climate is comparatively warmer, the concentration of spirit actually increases due to the greater evaporation of water.

Historically the gauging process was performed by excise officers, called ‘Gaugers’ for the purposes of tax collection. This process was done by using a bung rod which is a long thin ruler. The diameter of the cask head, its widest section and the length between the two points are measured. The bung rod is then dipped through the bung hole until it reaches the far side of the barrel. It is then removed and the high watermark on the rod is measured. These values taken together can be used to calculate the total volume of liquid within the cask, also called bulk volume. Today there are more modern methods of measuring the bulk volume of a cask.

Once this is done the alcoholic content of the liquid, called the percentage Alcohol by Volume (%ABV) is measured. Historically this was done using a hydrometer however today more modern and accurate methods are used.
By subtracting the %ABV from the bulk volume we are left with the quantity of Alcohol within the liquid. This is called LPA (Litres of Pure Alcohol).

The reason why we look for these values is because they allow us to determine the quantity of water needed to reduce the %ABV of the liquid in the cask to its final volume. This is called ‘cutting’ and it is performed once the barrel has been emptied/disgorged.  Blending can take place before or after cutting depending on the preference of the distillery. A blended whiskey is made up of varying proportions of different types of Irish whiskey. Once this has been done the whiskey is then ready to be bottled and this is the final product you see on the shelf in a pub or an off license.

As part of the cutting process, we will be using Reverse Osmosis filtered water. This is the purest, highest quality water available. As part of our project with Flying Tumbler we will be undertaking all of these steps.