Draft Physics

The Problem with Draft Calculators

Most “draft calculators” you find online are toys. They assume you’re a homebrewer pushing 100% CO2 into a keg of room-temp water and hoping for the best. In a high-volume, professional environment—especially when you’re running Nitro programs or 70/30 Beer Gas—that kind of “guess and check” methodology is how you end up with flat highballs or foam-cannon Negronis.

If you want to build a stable draft program, you have to stop thinking about “Regulator Pressure” and start thinking about Partial Pressure.

The Henry’s Law Bottleneck

At its core, carbonation is just Henry’s Law: the amount of gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid.

But it’s not a linear line. It’s a polynomial curve defined by temperature. As your temp goes up, CO2 solubility drops off a cliff. This is why a draft system that’s “perfect” at 34°F becomes a disaster at 42°F.

Our model uses the standard brewing polynomial to map this curve, but that’s only half the battle.

Dalton’s Law: The Secret Sauce

The biggest mistake operators make is forgetting Dalton’s Law of Partial Pressures. Total pressure in your headspace is the sum of all gases present.

If you’re running 100% CO2, your regulator pressure is your carbonation pressure. Easy. But the second you hook up a 75/25 Nitro blend or 70/30 Beer Gas, the math changes. Nitrogen provides the “push” (mechanical pressure), but it contributes zero to the carbonation.

To find the truth, we calculate the Effective CO2 PSI:

P_effective = ((P_gauge + 14.7) × CO2%) − 14.7

We have to account for the 14.7 psi of atmospheric pressure (Absolute Pressure vs. Gauge Pressure). If you’re running Beer Gas at 30 psi, you don’t have 30 psi of carbonation power. You have about 10 psi. If your recipe needs 2.5 volumes of CO2, 10 psi at 38°F isn’t enough to hold it in solution.

The “Stripping” Effect

Why Nitro Pours Feel Different

This is where the model gets surgical. When you run a Nitro program (25% CO2 / 75% N2) at low pressures, the Effective CO2 PSI can actually go negative.

Mathematically, this represents a “CO2 Stripping” effect. Because the partial pressure of CO2 in your gas blend is lower than the partial pressure of the CO2 already dissolved in your liquid, the nitrogen will actually pull the carbonation out of the drink to reach equilibrium.

This is why a Nitro Espresso Martini has that creamy, low-fizz mouthfeel—not just because of the nitrogen bubbles, but because we’ve physically managed the CO2 exit-velocity.

Data-Driven Service

In this system, we don’t “dial it in” by wasting 4 gallons of product. We calibrate the gas blend and the regulator to the specific ABV and Brix of the batch.

If your Effective CO2 PSI matches your Target Volumes at your Service Temp, the system is in equilibrium. It won’t over-carbonate, it won’t go flat, and it won’t foam.