# Why does the keg temperature rise after pouring a beer?

I have a refrigerated pony keg. Inside is pressurized CO2 and beer. Connected to the bottom of this keg is a weight and temperature sensor. According to these sensors when a beer is poured from this keg, the temperature (at least at the bottom of the keg) rises by several degrees Fahrenheit (2-3 degrees). I assumed that in a contained system like this that a pouring event would mean a decrease in the pressure within the keg tank and lead to a temperature decrease.

Is there a reason that this device would measure a temperature increase in this case? Is my device broken?

• Ok, thought of a potential answer. The temperature sensor is on the bottom of the keg. So when beer is poured out the weight of the keg is reduced. The temperature sensor is now under less pressure with the keg itself, thus reducing the heat transfer of the cold surface and causing an observed increase in temperature. Think that's it maybe Commented Feb 17, 2022 at 0:53
• Do you observe the pressure change on the CO2 regulator gauge? If so, how long does it last after you stop pouring? Does the temperature change happen during the pour or after? How long does the temperature change last? Does it immediately return to normal when you stop pouring? Does slowly return to normal after you stop pouring? Commented Feb 17, 2022 at 1:20
• it changes right at the start of the pour, hits its highest point a bit later and then over a few minutes the temperature returns to baseline within the refrigerated container. Commented Feb 17, 2022 at 1:23

Beer is not an ideal gas, but I hope we can still get an intuitive feel for what's going on by looking to the ideal gas law.

$$P V = n k T$$

The volume of the keg is fixed. The CO2 regulator introduces more gas to the keg to maintain a specified pressure. There is likely some delay on pressure equalization, but I'm assuming it happens relatively quickly.

If the whole left-hand side of the equation stays constant during the pouring, then as the number of beer particles, $$n$$, goes down, the temperature must go up to compensate.

This is complicated by the introduction of new C02 particles, but this seems like a satisfactory zeroth order explanation for now.

If this is what's happening, then the temperature will be returned to normal over the course of a few minutes by the refrigerator. If the temperature change is transient, changing only during the actual pour, then I'm wrong.

• I think I misunderstood the volume aspect of this equation at first, since it represents the volume of the container. This makes sense. Commented Feb 17, 2022 at 1:21
• I would recommend waiting a day before accepting an answer. Questions without accepted answers get more attention, and you may get a second opinion if you wait Commented Feb 17, 2022 at 1:25