You’ve made your first batch from your new still and the first thing on your mind is, How potent can it be ? You grab your Spirt /Proof hydrometer and drop it in the jar…. It reads 90% abv and you burst out laughing giving out high fives to everyone in the room! At least that was what happened to me, my first time. But hold on just a second that moonshine you just measured is probably well above the 20 degrees Celsius your hydrometer is calibrated for. What does this mean? Your actual % abv could be more like 80% – 85% ! To get the actual % abv you need to correct for temperature! What !!! I know I know .. Don’t worry it’s easy to do and will allow you to get a more accurate % abv when you dilute your spirits down to 40% abv assuming you want to do that!
A Proof Hydrometer ( Alcoholometer ) measure the % abv of your spirit and is generally calibrated to 20°C meaning when you measure your spirit it should be at 20°C and should not contain any additives such as glucose, flavourings or vegetable glycerin. If your spirit is above 20°C it will be less dense ( lighter) then at 20°C and the hydrometer will sink lower this will cause a higher reading on your hydrometer. If on the other hand your spirit is at a lower temperature then 20°C it will become denser and thus will float higher and give a lower reading.
Alcohol Hydrometer Temperature Correction Table
Let’s use the table below to convert observed %abv to actual %abv in the example below, If you’ve purchased a Hydrometer from your local brew shop or online they generally come with this correction table.
You have just run your reflux still and collected the spirit using your hydrometer you take a observed reading of 93 % abv and note the actual temperature of the spirit is 30°C . Let’s use the temperature correction table below to get the actual % abv of your spirit .
Step 1 : At the top of the table in the first column you’ll see numbers 5 -30 . This represents the temperature of the spirit. So we know it is currently 30°C. Go over to the 30th number in this row, all the way to the end.
Step 2 : Now we know the hydrometer measured 93 % abv with your finger run down the 30th row until you come across 93.3 this is the closest reading to our measurement.
Step 3 : Now follow the row over to the left that the 93.3 number is listed in until you get to the 20°C row it’s in grey to make it easy. You should see 91 This means are actual proof is 91 % abv
As an experiment let your moonshine cool to 20°C and see how accurate your temperature correction was. You can also place it in the freezer for a while and then re take the measurement with your hydrometer to get an idea of just how much temperature can affect the reading.
It’s a really good idea to familiarise yourself with the table above and understand the concept behind temperature correction but it’s not imperative to always use the tables. There are two alternative methods for the hydrometer temperature correction tables explained below.
Estimating Alcohol Hydrometer Temperature Correction – Simple and fairly accurate
To estimate the actual % abv without a temperature correction table you just need to remember one general rule
- For every 1 °C over 20°C subtract off 0.33% from your hydrometer reading
Let’s test this method of estimation with our previous example.
- Actual Temperature of spirit is 30°C
- Observed Hydrometer reading s 93%
- Actual Hydrometer reading = 93 % – (0.33*10°C = 90.7 % abv
If you recall from using the table above are results were 91 % abv . Where as the estimate method was 90.7% abv not a big difference .
If you’re not a fan of the tables and the estimation technique is a little crude for your liking there are many free alcohol hydrometer temperature correction calculators available online. I’ve posted links to several I’ve found useful in the past. I’ve also heard rumours of there being an app you can now download for your phone! If anyone’s used such and app or has another useful online calculator please share a link to it in the comments.
Online Alcohol Hydrometer Temperature Correction Calculators
- HomeDistller.org Temp Correction Calculator – This one allows you to select Celsius, Fahrenheit or use the HD Table. It’s free and easy to use if you have internet handy.
- Offline Temperature Correction Excel Calculator – Will be handy to anyone who doesn’t always have access to internet when distilling. This excel sheet uses TTB table 1 to find the true proof
- True Proof App – I found this app which is pretty handy for quick temperature corrections and doing dilution calculations. It can do calculations in °F or °C;and volume in US gallon, Imperial gallon or litres.
If you know of another temperature correction calculator or distilling app that you’ve found useful please post them in the comments below.