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I have had a miserable time trying to get reliable measurements for surface area to volume ratios in large barrels. I’m going to name some of the problems and discuss solutions to them. I include some measurements here and hope to help others avoid some of the frustrations I have had trying to pin down the numbers.
- Problems with finding SA/V measurements:
- Very few people are interested in precise measurements of interior SA/V, so very little is written about it. Much of the writing you do find is about generalities (i.e. “Small barrels have a higher SA/V so your spirit will age faster ha ha”) and are not helpful.
- There are a few websites with charts showing interior surface area of a range of barrel sizes, for example from Deep South Barrels, Red Head Barrels, Tall Ship Barrels, et. al, but upon inspection their numbers are insane. To give you some idea of how insane they are… to match their claim that the SA for a 53 gallon barrel is 6535 in2, a cylinder would have to have a 3.8” radius and be 22.5 feet long. This is a real issue, because these numbers are readily available, easily found by a Google search, and dangerously incorrect. If you were to follow their numbers, you may damage your spirits. Let's say this again, but in bold: Deep South Barrels and Red Head Barrels surface area to volume charts are wrong. Don't use their numbers for anything.
- Barrels come in many different volumes and geometries. Bourbon barrels are reasonably standardized, but many spirits are aged in hogsheads, butts, barriques, etc. If you are looking to match the characteristics of the spirit you like, it will be tough to get measurements of the surface area to volume ratio that is appropriate for you to aim for.
- The real-world geometry of a cask is a challenge to work with. Nobody wants to integrate that curve. In truth, you can make some fairly simple approximations of the proportions of a cask, and the results you get from your calculations with that approximation will be pretty good. But we will have to keep in mind that the accuracy of our numbers is not especially high. Common methods for approximation are a 1) a simple cylinder, or 2) two truncated cones or conical frustums.
- On the rare occasion that you do find a measurement for a particular barrel, the units are all over the place, which means that comparing apples to apples requires a spreadsheet. I have seen in2/gal, cm2/gal, cm2/L, and even gal/ft2!
- The effective SA/V ratio in a barrel changes fast as the level of the liquid gets lower in the barrel. For example, by my calculations, by the time a bourbon barrel has lost 10% of its volume (often in the first year!), the contact SA/V ratio is 88% of that of a full barrel. I'm not sure what folks will do with this information except keep it in mind when they are aiming for a reasonable amount of wood in their spirits.
I hope this post helps and can be a point of reference for future discussions about aging using oak. Cheers.