In this episode, we are talking about proofing and gauging spirits as well as the sources of error in your measurement and ways they can be minimized. The interview is with Adam Spiegel of Sonoma County Distilling Company where we talk about making whiskey the old school way.
Proofing starts at the TTB Gauging Manual and the associated tables, at least in the U.S. They’ve got a very detailed set of steps to go through on how to determine the proof of your spirit and as long as you’re able to follow them exactly you’ll be in good shape to get the proof that you are looking for. The first thing that is necessary to proof correctly is the right equipment and basically, three things are required to dilute your spirits; volume, proof, and temperature. In order to dilute accurately, you need all three of those measures to be accurate as well. For this reason, the TTB specifies the hydrometers that you use; certified as accurate by the equipment manufacturer in writing and conforming to ASTM standards, they need to be calibrated to 60 degrees Fahrenheit, a 20-degree proof range per hydrometer and accurate to 0.2-degree proof. This hydrometer needs to be paired with one of four different types of thermometers; Pencil, V-back, and Glass Shell (both earlier and later models). These thermometer types are pictured below with their required accuracy.
1 ° F Accuracy 1 ° F Accuracy ½- ¼ ° F Accuracy
Once you have the accurate hydrometer and thermometer there is, of course, a detailed procedure for how to use them to get the best accuracy. I’m not going to go over that here but if you’re curious to check out the gauging manual. Once you’ve determined the apparent proof of your spirits and then adjusted to true proof you are ready for dilution at this point you figure out how much spirit you have to dilute and add the proper volumes of water. For a quick note volume must be determined by weight, a calibrated tank with certifications of accuracy or an accurate mass flow meter (±0.5%) when using bulk spirits but there is no direct requirement for packaged spirits beyond accuracy in the volume in the package so you are kind of on your own to figure out what your volume is for not bulk spirits prior to packaging.
Once you’ve figured out how much water to add then you need to correct that water for its’ temperature and then add it to your spirit. It takes some time for the ethanol/water mixture to come to a new equilibrium so people typically let it attenuate at least over night if not for a couple of days. This time will also allow for the spirit to cool back to room temperature after it heats up from the dilution. The temperature correction here is mainly so that your equipment is the same temperature as the spirit. If your thermometer is a different temperature than the liquid you can impact the accuracy of your measurement. The way that works is that the glass body needs to adjust to the liquid temperature otherwise there will be a gradient from the bulk liquid temp through the glass to the measurement medium (mercury is what I want to call it but there is zero chance of you having a mercury thermometer anymore). As with most things the TTB has a procedure for how to bring your equipment properly to temperature if there is a difference or you can just wait for the spirit to come back to room temperature. This also affects hydrometers as they can slightly cool or heat the liquid around them and slightly change the density reading. There are other ways that you can affect the reading of your equipment from air bubbles supporting hydrometer to liquid on the hydrometer above the level of the liquid weighing the hydrometer down. Due to these sources of error typically the diluted spirit needs to be checked again prior to bottling and the process repeated.
With glass hydrometers, another source of error is simply reading it correctly. With the glass shell thermometers, I’ve found the same error, now this may be just because my eyes aren’t perfect but between the slight rotation of the hydrometer and reading through the meniscus (you read ethanol at the bottom of the meniscus) I have trouble hitting the 0.05° proof accuracy particularly when they’re closer together. My solution to this was to do a double check with a second person either verifying my measurements or helping each other correct to the right number. The TTB recommends taking a second sample and using that for your double check which is also a great idea.
Aside from basic errors in the equipment and using it correctly there are some other common errors during proofing the biggest one is that one gallon of water plus one proof gallon does not equal two gallons at 25% ABV. Due to how ethanol and water combine there will be shrinkage of volume (also where the exothermic portion comes from and since I really don’t want to do the thermo here I’ll just link to a great paper on the subject of modeling it) This calculation is slightly more complex than I want to express here but there are a bunch of calculators out there that will correct for this for you. I prefer AlchoDens if you are only looking for a proofing calculator though some of the combined packages have the same quality of math they just cost more and you get more. The nice thing about the shrinkage error is that 1 volume of ethanol in 1 volume of water plus two volumes of water shrinks to 1 volume of ethanol in slightly less than 3 volumes of water so your proof will be higher than you want this means that you can just do the work over again and you will typically get there by at least the third try and you won’t overshoot.
The next largest source of error is in your thermometer. If you look at Table 1 of the Gauging Manual you can see how temperature affects the measured proof of a spirit. After diluting let’s say you measured your spirits at 70°F and 80° proof, well you’ve screwed up and overshot but by how much? That works out to 75.5° proof but if you’re off by 1°F and the spirit is really 69°F then it is 76° proof that is already a greater error then you’re allowed from the improper reading of your hydrometer. Your thermometer should be accurate to at least 0.4° F assuming there are no compounding errors. Which unfortunately there are. Since this method is based on volumes temperature occurs in three places in the dilution process; the correction of proof, the volume of the spirit, and the volume of the diluting water. While the volume errors due to temperature are fairly low they still exist. 500 pounds of our example spirit at 70°F works out to 60.0312 gallons while at 69°F it is 59.9673 gallons or roughly a 0.1% error. That error is normally dwarfed by how we determine volume, to begin with. In a typical 550-gallon IBC 1/10 of an inch is equivalent to 0.87 gallons so out 60 gallons would be ±0.435 gallons on most site glass systems (Why are you proofing 60 gallons in a 550-gallon tank? No, idea this is mainly supposed to be illustrative) so temperature is only 14.6% of the total error in the measurement. The best proofing tanks are sized for the proofing that you’re doing so you’re not getting an error (0.435 gallons on a 5-gallon dilution) where it would kill you but instead where it’s not so bad (0.435 gallons on a 500-gallon dilution). Next, they need to be tall and skinny the more skewed this ratio is the more accurate your sight glass will be which would eventually (1 ft x 65 ft) make temperature into your largest error.
These are all just sources of compounding error so you can be wrong on your temp for your proof correction, then wrong on your volume of spirit to be diluted and then wrong on the volume of water you added they can add up (or they could also cancel where you’re high on everything or low on everything so it balances out but that is unlikely).
I’ve found the solution to these problems is generally technology. While not the cheapest method (that’s just being very precise in your measurements and doing things like only diluting with water that is at room temperature into a spirit at room temperature so that your errors there will cancel out) I’ve found it will speed up the dilution process and ensure that you waste less time on proofing that can be spent elsewhere and there are economic benefits to having your proof spot on as well. The first step in the technological process is getting scales or load cells on your proofing vessels and for weighting the water you’re adding to your spirit. Once you’ve done this you’ve traded you volume inaccuracies for a weight inaccuracy and the three temperature errors for a single error. The weight error requires that you typically have two different scales for your dilution water vs your proofing tank this is because trade rated scales have a weight range at which they maintain their accuracy (similar to the volume/ sight glass error above) and a scale that is accurate to 0.2% (Legal for trade Scale – NIST rated) for 1-5,000 pounds will have an error of 2 lbs and if you use it to measure your dilution waste (250 lbs) you’ll not have an error or 0.8%. I typically recommend a small scale for barrel scale measurements 500 lbs and then a large scale for your proofing (5,000 lbs.) to remove the resolution error. Once you have the weight you will no longer need to correct for shrinkage to 1 weight of water with 1 weight of ethanol (50% ABW) combined with 2 weights of water will drop your ABW in half, you will still need to correct ABW to ABV (ABW x 1.25 = ABV) for your bottle proof. This method also removes the waiting period for temperature correction since you no longer care about temperature except for proof so once you fully mix your water and spirit you’ll be ready to get your second check in and move on to your next step.
The only thing preventing this method from being foolproof (besides nothing being foolproof) is the error on the hydrometer and associated thermometer. The easiest (and most expensive) way to fix this is with an Anton-Parr-5000M (Mettler Toledo also has similar devices) which is approved by the TTB for tax purposes. These devices will correct for temperature internally and give you a proof without the errors mentioned above. Their largest source of error is maintenance and the fact they are a highly calibrated piece of lab equipment that needs to be treated as such. When these are combined with load cells the major sources of errors are almost complexly eliminated and you are left with sampling error are your primary source (basically taking a sample from the top of the tank when your spirit isn’t well mixed might not represent the average contents)
There are some other sources of error that I see often enough to mention. The first one is the evaporation from proofing tanks. In environments that see high evaporation (high temperature and low humidity) there can be a large loss from tanks that are left open and as we know ethanol and water do not always evaporate at the same rate so if you proofed perfectly and then walked away on an open tank and came back three days later you could be over or under proof depending on what was evaporating. The easiest way to solve this is to only proof in closed tanks and since then the vapor doesn’t have anywhere to go, also ensuring that your closed tanks are full, putting 5 gallons into a 500-gallon tank will still leave plenty of space for evaporation even in a sealed tank.
For people using non-R/O water in their dilution (I’m looking at you, barrel aged water people) that water will not have the same density and weight a pure water. This will affect your dilutions and needs to be corrected for. While having solids from a barrel or minerals won’t affect you’re ABW in total it will change your alcohol by weight of water and will really be screwed up when you look at the volume difference between 5 pounds of water and 5 pounds a Calcium Carbonate. If your source is constant you can just adjust the weight reading each time by a percentage but if you’re using different sources you need to check their densities and non-water compositions and then treat the remaining portion as obscuration.
Speaking of which if you’re over 400 ppm you need to be correcting for the obscuration of your spirit before these methods will work for you (I think the DMA can handle some obscuration but check with them for levels and types). The easiest way to do this is to follow the TTB steps. Which breaks down into (basically I’m shortcutting here) pull a 1-liter sample and put it in your lab still and distill until the still head temp reaches 100 C (corrected for your elevation). Once you’ve collected all of the liquid add distilled water to the collected distillate until it is back to 1L. Do all of your proof measurements on the new sample.
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