Gas Dynamics

Pumping Up the Volumes

Let’s start today with a hardware discussion. When discussing pushing beer from a keg into your glass, the gas regulator is an important element and brewers should know how they work and how to maintain them. These work horses are all around us and if you are like most normal people (meaning not an obsessed homebrewer) you might not think twice about the gas regulators in your life. The regulator in your gas stove. Water heater. Furnace. Grill. Kegerator. But of course, we are not like most people as we are homebrewers and we are afflicted with the need to figure stuff out and maybe occasionally fix stuff. How the heck does this regulator contraption work anyway? Let’s find out. We will also discuss carbon dioxide gas (CO₂), nitrogen gas (N₂), and the cool stuff that happens when you combine the two (beer gas). Pour yourself a cold one. Think about the fine job your regulator is doing. Let’s dive in.

A properly functioning gas regulator has one job. That is to regulate the flow of gas from within the gas cylinder. How this is accomplished is not at all complex either. The basic explanation is a gas regulator controls the flow of gas from the supply to the receiver via a mechanically activated valve. Gas from the cylinder enters a chamber through the high-pressure inlet. The gas is unregulated at this stage meaning the pressure within the gas cylinder flows as freely as the diameter of the inlet size will allow. These gas molecules are at delivery pressure (high) and collect within the sealed body of the regulator (bonnet). Here the high-pressure gas exerts pressure on the diaphragm. The diaphragm in turn is held in position with a calibrated spring assembly. If the force of the spring is equal to the force of the inlet gas, the gas in the bonnet is stabilized and nothing much happens. If the gas pressure overcomes the resistance of the spring pressure, the diaphragm opens a crack within the bonnet, allowing the gas molecules to pass through and on to the low-pressure outlet. Gas is free to flow into a keg or into atmosphere. If the gas is going to the keg, eventually the closed system will equilibrate with the spring force within the bonnet and the diaphragm will close, stopping the flow of gas. This description is simplified, but not by much. 

Not All Gas Regulators are Created Equally

There are many varieties of gas regulators. Of course, we are not discussing the regulator on a natural gas pipeline here, so no real need to elaborate. Draft system regulators do come in a variety of price points, however, and there are some design choices made between the lower and higher price varieties. Be aware that the differences are not great between the mid- and high-end range. A middle of the road regulator from a name brand company is all that is required. No name junk is just that. Avoid it. High-priced commercial units work great, but so do mid-range units. You’ll get more shiny metal bits and a solid dial to turn, along with some improved internal guts. But not enough to warrant the price tag. Expect to pay $50 for a decent regulator and up to $100 for the “Pro” variety (I run a draft install company, and therefore am a professional, and we do not buy “Pro” regulators. The cost/benefit just doesn’t work out). There are many ways to spend too much money in this hobby of ours. Get a name brand regulator and you’re good in my opinion. Use that saved money on fancy glassware or maybe fancy yeast.

A trade secret is that you will not be using a rebuild kit if your regulator fails. Go ahead and try. We have. If you want to spend a few hours pulling your hair out trying to get all the bits and small parts seated just right, numerous times, go for it. Save your money and ask me how I know. 

You cannot accidentally thread a CO₂ regulator onto an N₂ tank, nor can you attach a nitrogen regulator to a CO₂ tank. The male-to-female threads are opposite. A CO₂ tank threads onto a male-threaded CO₂ regulator and vice versa. I mention this because you may see thread adapters that are available to adapt tanks and regulators. My word of caution is to avoid conversion kits. Nitrogen bonnets are steel, but do not dial in CO₂ pressure accurately. The bonnets of a CO₂ regulator are plastic and will not tolerate the high pressure of a nitrogen cylinder. 

Regulators Are A One-Way Valve

The gas will only flow in one direction. Despite this fact, many times brewers think by lowering the applied pressure they can lower the carbonation of the keg beer. Or maybe they don’t think that at all? If you apply too much pressure to your keg, there is only one way to correct the problem. Turn off the gas, either at the valve or by turning the round handle on the gas cylinder completely off (counterclockwise). Pull the Cornelius keg or Sanke coupler pressure relief valve (PRV) until enough gas is removed. Then reapply gas pressure to the correct volume. If not carbonated to the desired amount, you’ll need to get the excess gas out of solution. Shake the keg, pull the PRV and wait 20 minutes. Do this procedure until you feel like you have achieved beer flatness. Now work back up to the correct applied pressure. Getting the pressure perfectly dialed requires a bit of skill and patience.

Recall that the diaphragm opens when the spring pressure is less than the tank pressure. As the two forces get closer to equal, the gap will get smaller and smaller until it closes entirely. The needle on the gauge will rise quickly, then slowly creep as it approaches your set point. The tendency is to overshoot the desired pressure because we don’t see the needle immediately landing where we want it to. Give it a few minutes to creep into place and if you are still low, then dial in a tiny increase in pressure. If you do overshoot your target, go back to venting gas. Unfortunately, when venting keg gas you are also venting beer aroma. This is a particular concern with a brew that you tirelessly slaved over, trying to get every bit of hop aroma you could muster.

Gas Cylinders Are High Pressure 

There are some legitimate safety concerns with gas cylinders. Respect that a tank of CO₂ gas is at around 860 PSI (5,930 kPa) and a tank of nitrogen is closer to 2,200 PSI (15,170 kPa) at ambient temperature. Neither gas is flammable. In fact, atmospheric air is about 78% nitrogen and CO₂ is an effective fire suppressant. However, if a cylinder of compressed gas falls and the regulator snaps off, you are dealing with a rocket of gas molecules. This is not theoretical either, as there are documented injuries. Fire code typically requires chaining the cylinder securely to a wall. Consider doing the same in your home. Nothing will get your attention faster than the sound of high-pressure gas blowing out. Not fun!

Gauges Lie To Your Face

The gauges on your regulator cost about $5 each. They work as well as any $5 piece of mechanical equipment could be expected to work, which is just good enough. A relatively cheap upgrade here is to replace the gauges with higher quality units. For a few bucks you can thread any ¼” NPT gas gauge to your regulator. A worthwhile addition is to install a digital gauge that is much more accurate, particularly at the relatively low pressure we use in homebrewing. 

Another important area to mention is the difference between gauge pressure and absolute pressure. Atmospheric pressure changes based on elevation and brewers need to understand that gas expands at different levels based on that discrepancy. 5 grams of carbon dioxide expands to 3.58 L at 8,000 ft. (2,438 m), compared to only 2.58 L at sea level. This is why beer served on an airplane may seem overly carbonated. When carbonated beer changes elevation, this absolute pressure delta may wreck havoc on the carbonation level.

When it comes to gauge accuracy CO₂ regulators are normally described as Grade B, 3-2-3. The numbers represent the +/- percent accuracy across the dial with the first number representing the first third, the second the middle, and the final number the last third. A 30-PSI gauge in the 0 to 10 PSI range and the 20 to 30 PSI range is therefore accurate within +/- 0.3 PSI (0.002 MPa). That is pretty good, to be sure. The 30-PSI gauge is the sweet spot. You have enough range to force carbonate at high pressure, but the dial is reasonable for dispense pressure. A gauge that goes to 120 PSI or more is going to be awfully hard to precisely dial in to your dispense pressure. You may see these regulators; they are meant for soda or seltzer. The regulator body will be made of steel and not plastic though, so if you get one and replace the gauge, perfect. It’s still hard to dial in 12 PSI with a regulator designed to dispense at 80 PSI. The slightest turn of the dial results in a big jump in delivery pressure because the spring is quite stiff.

What about that other gauge on your regulator? You have a low-pressure gauge that is telling you the amount of pressure being supplied to your keg (the right-hand gauge). You also have a high-pressure gauge, which is telling you the pressure in the tank. First, if the needle is not in the red, you still have CO₂ gas. Second, if the needle is in the red or below, your tank is about to run out or already empty. The high-pressure gauge only really tells you what you already know . . . you are out of gas. Why? Because the carbon dioxide in the tank is in liquid form since CO₂ gas liquefies at a relatively low pressure-temperature intersection. Not unlike your propane tank, as long as there is liquid CO₂ available, there is gaseous CO₂ available. If there is liquid CO₂ in the tank, the pressure on the high gauge will be steady, only varying with changes in temperature. Near zero indicates the liquid has boiled off and your draft system is literally running on fumes. Weight is a better method of estimating tank levels with CO₂ so long as you know the tare weight of the tank. 

The exception to this is a tank of nitrogen, which is not liquid when under pressure. That high-pressure gauge is indeed a good indicator of remaining gas molecules within the tank. The needle moves towards zero as the tank empties. 

Knowing Your Volumes

If you are a brewer that adheres to style guideline, an important consideration is getting the volumes correct for the style. Anywhere from 2.2 to 3.0 v/v is typical. Volumes are an important part of brewing but not part of the rabbit hole of today’s discussion. When speaking to commercial brewers, I mention that hitting your target carbonation level is important and very few among us can realistically notice a difference of a tenth of a point or two. Personally, I have many targets to hit in my homebrewing and dead accurate volumes is not super high on the list. Point being, consistency is probably more important than precision. Knowing what we know about gauge accuracy, an exercise in precise volumes might be futile anyway.

Do You Need Nitrogen Gas?

Most of us (probably) do not need nitrogen gas and I am (almost) sure of it, too. Do you have a long-draw system, with trunk line and remote faucets? If not, you do not need nitrogen. Do you want to serve nitrogenated beers like an Irish stout? If not, you do not need nitrogen. But for those that answer yes to either of those scenarios you will need a nitrogen tank, regulator for it, and a stout faucet if you plan to pour nitrogenated beer. But word of caution: If you are using anything other than 100% carbon dioxide, you are making your life more difficult than needed. It’s the reality of it.

As noted, nitrogen is useful in two applications. It is essential for nitrogenated stouts (aka Guinness style) or any nitrogenated style for that matter. Nitrogen is also needed for many long-draw systems where increased pressure is required to balance the system. The sort of draft beer system where the kegs are 30 feet (9.1 m) or more from the faucets or in the basement and faucets are upstairs. The nitrogen is used to overcome system resistance and gravity without over carbonating the beer. If you are interested in the anatomy of a long-draw system, it can be found here: byo.com/article/long-draw-draft-system/

Once again, do you need nitrogen? Of course you do! The magic of nitrogen allows you to dispense magic nitro stouts and other fun styles. The magic of nitrogen allows you to design and install a long-draw system. The magic of nitrogen affords you the opportunity to experiment with all sorts of fancy whiz bang contraptions you did not even know you needed. Nitrogen is great and because you are a homebrewer who loves to tinker with new things, you’re going to agree. 

Nitrogen gas exhibits some important qualities that differ from carbon dioxide. Importantly, nitrogen is about 80 times less soluble in water (beer) than CO₂. This means that if you need to apply 30 PSI of pressure to your beer, the nitrogen will (mostly) not dissolve into the liquid (and the little that gets dissolved does wonderful things to the texture and foam of the beer). The reason for this poor solubility is that the oxygen ends of a CO₂ molecule have a slight negative charge. Water molecules are attracted to these polar areas, allowing CO₂ to dissolve in water. Carbon dioxide is the outlier among gases in this quality. The beauty of brewing is that these chemical reactions do not occur because of brewing but are beneficial nonetheless. 

Fun fact #1! Carbon dioxide gas, when dissolved in beer, becomes carbonic acid. I believe CO₂ is the underappreciated fifth ingredient in beer. Water, malt, and hops are famously in the German beer purity law (the Reinheitsgebot we know and love). Later, yeast was included. Is it time to add carbon dioxide? The carbonic acid is an essential counter to malt sweetness and enhances hop bitterness as well. Flat beer is simply sweet and often unpleasant. Nitrogen, being inert, does not offer any acidity. The sweetness of the malt shines through. Hop bitterness does not have any crutch to help do the job. I think many of us have had that morning small pint, poured the night before. The beer that has gone flat. We discover that a), warm beer is miserable and b), flat beer is oddly sweet. I digress.

Let’s hypothesize that your draft system needs 30 PSI of pressure to overcome gravity, resistance, or both. 30 PSI of CO₂ is going to give you 4.25 volumes at 38 °F (3 °C). Way too much. Let’s substitute nitrogen gas for part of that carbon dioxide. With 70% carbon dioxide and 30% nitrogen (aka 70/30 blend), the keg is only receiving a portion of CO₂ molecules, and as a result is also achieving 2.96 volumes at 38 °F (3.3 °C). The 30 PSI of pressure still exists though and, with nitrogen being insoluble, we achieve quite a bit of push to overcome our system’s design constraints. We can have our beer and eat it too. Some common blends are 70/30, 60/40 and 25/75, with the first number being the percent CO₂ present in the blend. These three blends are basically all the blends we will ever encounter, though we can also get special order blends for lightly carbonated white wines, for example. With blend gas, we can dial in system resistance to match our applied pressure, while still maintaining the appropriate volumes. The 25/75 blend can be found packaged at your gas supplier. This blend was popularized by Guinness and it works well for that application. 

As an aside, sometimes we encounter 25/75 in commercial systems that do not need it. The 25/75 blend will pour great but all the other beers (other than than those meant to be nitrogenated) lose carbonation from day one. Guinness (and most all nitro stouts for that matter), is carbonated to about 1.2 volumes, which we know is less than half a typical ale. All other beers on 25/75 blend lose carbonation until they are equilibrated to around 1.2 volumes, more or less, at standard serving settings. If your gas cylinder is 25/75 with anything other than your nitro dispensed beer, you are also wasting money as it is expensive too. 

How Do I Get Magic Gas Blends In My Brewery?

After you have determined that you do in fact want nitrogen gas and blended gas, you have a few options. For nitrogenated beer (Guinness style) you can buy pre-mixed bottles of 25/75 (aka Guinness gas or simply beer gas). Any gas supplier will have this blend readily available but be sure to double check you are getting the specified blend.

If you are designing a long-draw system and 60/40 works, things get a bit more complex and a bit more expensive. First, do what you can with your design to get to 70/30, as this blend is usually a better choice that will hit the sweet spot of carbonation and push better than 60/40, but this is dependent on the system and its requirements. Next, look into McDantim gas blenders. A gas blender is just that. One line of CO₂ gas and one line of N₂ gas go into the blender. Blended gas (either 60/40, 70/30, or 25/75 depending on model you purchase) comes out. A gas blender is accurate and useful, but the costs can be steep for a homebrewer. Expect to pay between $800 and $1,500 for your blender. Also know that regardless of the brand you choose, all gas blenders are made by McDantim. Buy the best deal you find from a trustworthy vendor. Blenders are expensive, but also bullet proof and dead-on accurate. 

What about pre-mixed bottles of blended gas? Unfortunately, as we get away from the 25/75 mix, things get squirrely. Nitrogen and carbon dioxide do not co-exist peacefully in one tank. Think oil and water. The two different molecules do not occupy space evenly, and the result is uneven delivery of gas. Let’s go back to high school physics for a moment. All gas has both a critical temperature and a critical pressure. The critical temperature of a gas is the point where the gas is liquified under pressure. Above the critical temperature, the gas will not liquify, no matter how much pressure is applied. 

The critical pressure is the point where gas becomes liquid, provided that the critical temperature is not exceeded. Carbon dioxide and nitrogen have different curves. Liquefying CO₂ is not too difficult to achieve, because the critical temperature is 88 °F (31 °C). The critical temperature of nitrogen is -232 °F (-148 °C), and that is not something that can be achieved without an unrealistic investment. Liquifying N₂ is realistically not a thing. If both gases are filled into a cylinder and we want to maintain a specific blend, we cannot exceed the critical pressure of CO₂, because once the CO₂ becomes liquid, the desired ratio is not going to be maintained. Fun fact #2! Because a tank full of nitrogen is entirely gas and not liquid, there are significantly less nitrogen molecules in the tank, and the tank runs out pretty quickly. But as stated earlier, the high-pressure gauge is a good indicator of remaining nitrogen in the tank.

I hope this helps explain some regulator and beer gas mysteries. Now back to brewing beer.

Written by Bill Jablonski