UNDERSTANDING PROOFING AND GAUGING 

Navigating U.S. regulations and methods for determining alcoholic strength.
BY TYLER DERHEIM 

Before we dive into the topic of proofing and gauging, it’s useful to establish some background and definitions; while they may be familiar to you as experienced distillers, these will form the vocabulary for this article. These definitions and regulations primarily apply to United States-based producers, though there are comparable terms and guidelines in most jurisdictions. 

Determining Proof 

Proofing is the act of determining the alcoholic strength of spirits. Proof is expressed in degrees (e.g. 100°pf, which is equivalent to 50% ABV). “Proofing down” is the act of adding water to spirits to reach a target alcoholic strength. “Proofing” and “proofing down” are sometimes used interchangeably, which can lead to confusion. “Determining proof” is the language used in the regulations and is a clearer way of putting it. You must use proofing devices and methods approved by the U.S. Alcohol and Tobacco Tax and Trade Bureau (TTB) when your proof measurement is used for tax determination (e.g. when measuring proof in the bottling tank prior to bottling). 

Determining Volume 

Weighing is one of the legally-mandated methods to determine the quantity of liquid in a tank or container for most craft producers. It’s helpful to record the empty (tare) weights of all containers you use for spirits so that you can subtract it from your observed (total) weight to determine the net weight of spirit, and therefore tank content. However, the regulations do require you to ‘zero’ or ‘tare’ your scale prior to measuring spirit weight. Weight must be measured in pounds and then converted to volume (gallons) using the TTB Gauging Manual, Table 4. 

Alternatively, for fixed tanks, you may use the volumetric method. This involves using a sight glass fitted to a tank whose internal geometry has been computed such that you can measure liquid height in inches and convert it to gallons. 

Gauging—Determine Proof + Determine Volume 

Gauging is the act of determining the total alcohol quantity in a tank or container. A gauge is expressed in proof gallon terms. You must have a measure of the proof and the volume to compute your gauge. In simple terms, Determine Proof + Determine Volume = Gauge. Because proof gallons measure the absolute alcohol quantity in your batch, adding water to a batch of spirit does not change its proof gallonage—only its wine gallonage. 

Real-world Example 

Let’s say that you receive a tote of grain neutral spirit and the seals are not intact. Therefore, you must gauge it, as required by 27 CFR 19.407. 

First, measurements. You weigh it on a floor scale and observe a weight of 1,951 pounds. The paperwork accompanying the spirit indicates that the tare weight of the tote is 125 pounds, so the net weight is 1,826 pounds. Now, you can grab a sample of product from the tote and determine the proof using one of the approved methods (e.g. hydrometer and thermometer). In this example, your proofing results indicate 190.3°pf at 60 F. 

These two values are all that you need to compute your gauge. To determine the proof gallons in your 1,826 pounds of 190.3°pf spirit, refer to Gauging Manual Table 4 and look up your proof to find proof gallons per pound. Here, you can see that 190.3°pf spirit has 0.28029 proof gallons per pound. Then it’s just simple multiplication: 0.28029 x 1,826 pounds = 511.81 proof gallons. For extra credit, you can also determine the wine gallons (ordinary gallons) of product using the conversion factor for 190.3°pf (0.14729 gal/lb): 0.14729 x 1,826 pounds = 268.95 wine gallons. Wine gallons may also be inferred based on the proof gallons and proof: 511.81 PG / 190.3° x 100 = 268.95 wine gallons. Using this simple equation, you can rearrange the factors and solve for any of the three, if you know two of them. The three factors are: wine gallons, proof gallons and proof. 

Allowable Gauging Methods for Tax Determination—Determining Proof 

Regulations in the gauging manual (27 CFR 30) call out specific hardware requirements and procedures for determining proof and volume. 

For spirits with under 4g/liter of dissolved solids, your allowable proofing methods include the precision hydrometer/ thermometer method, or an approved benchtop density meter. Hydrometers and thermometers must be frequently tested to ensure their accuracy, per 27 CFR 19.188. Regulations are silent on precisely what “frequently” means; I typically recommend at least annually if not more frequently, particularly for instruments that are used often. 

When you use a hydrometer and thermometer, you must always apply and record any correction factors used, even if the factors are 0. You must also account for the effect of temperature on proof readings using TTB Table 1. This means managing up to three “offsets”—hydrometer, thermometer and temperature correction.

A major upside to determining volume by weight is that you do not need to worry about the effect of temperature on spirit volume: a given liquid quantity of spirit will have the same weight at any temperature.

Additionally, it can be challenging to test a hydrometer’s accuracy—you’ll need another hydrometer/thermometer to compare against, which themselves must be calibrated/tested. Alternatively, you can test hydrometers against a benchtop density meter. But if you have a benchtop density meter, you no longer need hydrometers!

You can see why I typically recommend for distilleries to simply re-buy their hydrometer and thermometer, with traceable calibration/correction factors, once per year. It winds up being cheaper and simpler than trying to re-test your existing equipment. Density meters, while expensive, can have their measurements validated simply by testing pure distilled water in them, and they can last years or even decades with proper care.

Finally, if you are working with spirits that have dissolved solids above 4g/liter, you must account for the effect of the dissolved solids on your proof measurement. For solids loads up to 6g/liter, this can be accomplished using one of the three methods outlined in 27 CFR 30.32: Evaporation method, distillation method or pycnometer method. For solid loads exceeding 6g/Liter, your primary option is to use the distillation method, which involves a small laboratory still and a specific protocol. The alternative to the distillation method is to petition TTB to allow you to use a different method, if you can prove its equivalence or superiority to the distillation method.

Accounting for obscuration loads under 4g/liter is optional; doing so allows you to squeeze a little bit more water into your bottling tank, allowing you to yield slightly more bottles out of the same starting base spirit. This extra effort can be worthwhile for high value spirits.

Allowable Gauging Methods for Tax Determination
Determining the volume in a tank can be achieved in two ways: by weight or by volume.

Table No.4* Gallons Per Pound

When using the weight method, you place a vessel on a scale and press “tare” to zero the reading. Or, if using load cells, you simply ‘zero’ the reading while the tank is empty. Then, you flow your spirit into the tank and observe the reading on the scale in pounds. Care should be taken to ensure your reading is timely and correct: load cell readings are temperature-sensitive and tend to drift over time. Readings should not be taken if many hours or days have elapsed since the scale was zeroed or tared.

A major upside to determining volume by weight is that you do not need to worry about the effect of temperature on spirit volume: a given liquid quantity of spirit will have the same weight at any temperature.

Scales must be tested (and recalibrated if necessary) every six months, per 27 CFR 19.186.

If your spirit is under 6g/liter of dissolved solids, converting weight to volume simply involves looking up the weight per gallon in Table 4, as shown in the example. Once you’ve hit 6g/liter of solids, though, you must employ an alternative method for determining volume based on weight. This alternative method is covered in 27 CFR 30.41 and requires you to measure both apparent proof and true proof; or if your spirit is denser than water, you will have to measure specific gravity and true proof.

When using the sight glass method of determining volume, you use a predetermined “gallons per inch” factor to convert a liquid height to a “gallon” reading. You must then adjust the volume of the spirit to account for the effect of temperature using TTB Table 7, unless your spirit happens to be 60 F at time of measurement. Luckily, the volumetric method is valid for spirits with any dissolved solids content.

One final note on the volumetric method. I often see IBC Totes, drums, dump buckets and other vessels that have “gallon” or “liter” markings on them. Observing the liquid level based on these markings does not qualify as a valid gauge—regulations in 27 CFR 30.51 require that “Volumetric measurements in tanks shall be made only in accurately calibrated tanks equipped with suitable measuring devices, whereby the actual contents can be correctly ascertained.” Plastic vessels, due to their varying composition and bulging when loaded, usually cannot be accurately gauged by volume and should instead be weighed on a suitable scale.

Historical background

Up until 1980—when deregulation hit the industry—gauging was only performed by U.S. government agents. Spirit collection tanks had locks, with keys held only by agents. Proprietors couldn’t even touch their own distillate until unlocked and gauged by the resident agent. But, with the stroke of a pen, gauging responsibility shifted from federal agents to DSP proprietors (Distilled Spirits Tax Revision Act of 1979). This would have been a great moment for the U.S. government to harmonize with the rest of the world and begin measuring proof at 20 C instead of 60 F. Instead, legislators left the Gauging Manual essentially unchanged from its original publication by government chemists in 1938. If you’re ever wondering why the manual and methods seem obtuse, this might help explain it! 

Tyler Derheim is a professional services consultant for FIVE x 5. If you’re looking for help with these requirements, or indeed with most any of your regulatory requirements, FIVE x 5 offers ACSA members a free 20-minute consultation, with no strings attached. For more information, visit Fx5.com/distillery-consulting.