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Measuring CO2 without very expensive equipment has always been difficult: The method of choice for DCO2 is an Anton Paar Carbo QC analyser, but at something like $10k it’s a little out of reach for most. The Zahm and Nagel comes nowhere near the AP for accuracy and precision but they still see fit to charge around $2k. The method we used at the big blue involved a specially made vessel, a vacuum pump, a mercury manometer, concentrated caustic soda and sulphuric acid. Not for the faint of heart.
Fortunately for us there’s an easy method that comes close to the performance of the Zahm and Nagel at less than 1/50th the price. The idea isn’t new, it’s based on the Du Jardin Salleron Carbodoseur but I’ve added a couple of tweaks that make it more practical for brewers.
The cheap ass version of this uses a 100 ml measuring cylinder with a 24 / 29 conical fitting (AKA a mixing cylinder), available for about $20 , a 24 / 29 stopper with a through fitting (known as a thermometer adapter), available for about $10 , a piece of 6mm glass tube and a thermometer (which you should already have).
A slightly more convenient version is the same cylinder with a ready made plastic stopper with tube and valve integrated, available from Alla or the original from Dujardin Salleron, but like everything they make expect to pay over the top .
To make this work for beer, we add water. For every beer I’ve tried 50 : 50 cold water and beer works a treat but super carbonated beers may require further dilution (see below). The water needs to be cold (preferably less than 5 oC) and substantially free of CO2. The easy way to achieve this is to fill hot water into a bottle and refrigerate it. The method works best if the beer is also cold.
Fill the cylinder to the 50 ml mark with the water, put the cap on, close the tube (with your thumb if using the cheap ass version) and shake it baby. Open the tube (or take your thumb off) and check whether the level has changed. If it has, there is dissolved CO2 in your water and you need to get water with none: try boiling it first.
If the level didn’t change, tip that lot of water out and refill the cylinder to exactly 50 ml with water from the same source. Now fill the cylinder to exactly 100 ml with your beer, pouring very carefully so as to minimise CO2 loss. Measure the temperature of the beer / water mix carefully and note the result. If it is more than 12 oC or less than 0 oC start again.
But the cap on, close the tube and shake. Open the tube and let the beer / water mix spurt out. Repeat until no more comes out. Since it is really important to minimise gas leaking, it is best to start with small steps and vent often.Take the cap off and add a drop of antifoam (or vegetable oil if you are a real tightass) and place the cylinder in the fridge until the foam settles (no more than 3 minutes).
Once settled, read the remaining volume of the mix. Remember to do this with the cap / tube removed.
If the volume is more than 94 ml go back to the start and omit the water addition (just use beer). If the volume is less than 40 ml go back to the start and reduce the beer / water ratio.
If the volume is between 94 ml and 40 ml note the result and apply the following algorithm:
DCO2 = sqrt (98 – Vol) x e^(1652 / (T + 273.15) – 6.75) * dilution factor (= 2 for a 50 / 50 mix).
If you don’t feel like doing the maths, I've attached a spreadsheet with a calculator and a table:
The DCO2 calculation returns a result in g/l CO2: if you prefer to work in volumes leave out the dilution factor and you’ll be near enough.
To validate this I tried a few beers including some VB cans. Unless the leopard has changed its spots VB will be ~5.3 g/l CO2. My measured values were between 5.0 and 5.1 g/l which is reasonable assuming a small gas loss on opening the can.
*The algorithm is based on the volume loss being proportional to the square of the dissolved CO2 content but tweaked to fit the empirical curve derived by the OEM (which is why it starts at 98 not 100). The exponential term is derived from simple Arrhenius kinetics and takes account of the change of CO2 solubility with temperature. The dilution factor is obvious.