This article is part of a series, if you have not read the previous articles please check them out first: Draft Beer 101 & Draft Beer 102.
So how do I pour a beer the way the brewer intended?
We established in the first article of this series that different beers need to have different levels of Co2 to create the feel and flavor that the brewer is looking for.
In the second article, we focused on the idea that in order to get the beer to pour properly, we need to match the amount of pressure applied through the draft system, to the amount of pressure that exists in the beer… Those levels of Co2 chosen by the brewer.
So we need to find out what the pressure is of the Co2 in the beer, and match that to the pressure of the Co2 we’re applying with the draft system. So, how do we find out what the pressure is inside the keg?
Like with many things in life, there are two ways to find out. We can do the math, or we can physically check.
Doing the Math!
Pros
- It’s extremely easy with the help of a computer or smartphone.
- It’s not messy
- You don’t need to be near the actual keg to figure out the needed pressure.
Cons
- If you have incorrect info going in, you will have incorrect results.
The pressure of the Co2 in beer is a simple function of the amount of Co2 in the liquid, as expressed by Co2 grams per liter, or Co2 volumes, and the temperature of the beer. The warmer the beer is, the higher the pressure will be, and the higher the Co2 is, the greater the effect of each increased degree will be.
What’s the temperature of the beer?
The best way to test this is to just use a liquid thermometer on another beer in the same conditions. It’s optimal to test the beer in the cooler, and at the tap, to see if there’s a difference in temp. If so, always use the higher number. We need to know the warmest temperature the beer will reach, since if at any point in the system the pressure in the beer overwhelms the pressure being applied, the beer will break out in foam.
How much Co2 is in the beer?
The best way to find this out is to ask the brewer. Brewers aim for a very specific pressure level in their beers, and they have techniques get it there either through yeast activity or through force carbonation. B.United and OEC Brewing keep that information on their websites, and also on a form attached to the keg during delivery. Some other breweries keep that information online as well. In any case you always have the option of reaching out to the supplier if you don’t know!
American breweries measure Co2 by volumes. A volume is equal to 1 liter of Co2, in one liter of beer, at 0° Celsius, at standard atmospheric pressure. Kegs filled with beer are normally 2-4 Co2 volumes, though both higher and lower readings are possible. The average beer is around 2.5 Co2 volumes.
European breweries use grams per liter, the weight of the Co2 present in a liter of liquid. A liter of Co2 at 0° Celsius at standard atmospheric pressure is approximately 2 grams when measured by weight instead of volume. Kegs filled with beer are normally 4-8 Co2 volumes, with the average beer being around 5.
1 Co2 volume is approximately equal to 2 Co2 grams per liter, so it’s an easy conversion.
Once you have both these data points, you can figure out the pressure in the beer!
Here are three ways to do it!
To use this chart (click above), simply find the temperature on the vertical axis, find the Co2 volumes in the body of the chart, and check the horizontal axis for the appropriate pressure. This chart has a somewhat narrow range, but it’s something that can be laminated and hung in a keg cooler for quick reference! In addition, you have to make adjustments if you are using a mixed gas (blender box) system (more on that later)…
Mcdantim Web Applet – Suggested method!
The Mcdantim Web Applet uses a few more data points than using the Zahm Nagel chart, which can give you a little more accuracy and it’s extremely easy to use. Go to Mcdantim Web Applet and enter the temperature and Co2 volumes. In addition you can enter the ABV and approximate altitude of where you are– both can affect the psi recommendation. Also, extremely importantly you need to know the Co2 content of the gas you’re applying to the beer. We will get into this more into a following article, but to keep it simple– the Zahm Nagle Chart is normative, it lets you know how much pressure is in the keg, the McDantim web app is prescriptive– it lets you know how much pressure to use on the keg.
The reason these numbers may be different is different blends of Co2 and Nitrogen can be used in beer dispense. If you’re using pure Co2, you can use 100%. Some bars have nitrogen gas blenders, that dispense usually 60% Co2 gas, but they will be marked on the machine. Sometimes pre-blended tanks of 30% Co2 and 70% nitrogen are sold. These are only intended for use with specially nitrogenated beers
Once you enter all this info you should have a good working approximation of the psi needed to pour the keg
McDantim Draft Calculators – Excel Sheet Download
You can use this excel spreadsheet from Mcdantim as well! It’s the same math used in the web app, but you can take a closer look at the underlying math and use it without internet access. The Easy PSIG tab is the one to use in this case.
Next time we will look into physically checking on the pressure in the keg and much more…