Knowledge Base: Gas Expansion and Adiabatic Cooling

One of the questions we often see asked online is, “does the cooling effect of the gas expansion also cool down the espresso?”.

It’s a good question, and a clever one, and it is also easily answered. The short answer is no. The long answer is why this is so.

First, the gas passes through a long narrow constriction on its way from the regulator to the water bowl. The shape of the TWIST acts as a giant heat sink, soaking in ambient temperature to warm the gas as it travels along its path. The flukes of the TWIST help this by acting like a giant radiator. If you hold your finger over the gas outlet on the handle and pull the trigger, it will feel just slightly cold, but not freezing. This is because the metal in the handle has already raised the temperature to close to ambient. If you hold the TWIST for a little while, the heat from your own hand will also contribute to this process.

Second, the gas passes up a long narrow pipe drilled up the side of the water bowl. The water bowl is pre-heated by the hot water it contains, and the area the pipe occupies is surrounded by a large mass of metal–more so than any other part of the water bowl. Any change of temperature in that pipe will have a negligible impact on the water because it is all soaked up by the pre-heated metal.

Eventually, that would also serve to pull some temperature from the water, but the metal also has a much higher thermal density than the gas. Therefore the temperature of the gas changes much faster than the temperature of the metal–about 200 times as fast.

In fact, there is a total mass of about 400g in the water bowl from the metal and water that is boiling hot after a pre-heat, and only about 2g of actual gas to heat up. Therefore even if the gas was still freezing, it would rise to near boiling at the cost of half a degree of temperature from the water bowl and the water inside. However the gas is already much warmer than that, so it takes even less of a toll. The final temperature change in the water is immeasurably small because it is much less than is given off through simple radiation.

Finally, during use the gas flows into a small space at the top of a water bowl. It is already well above freezing, yet it only contacts the top surface of a comparatively large mass of water–it doesn’t travel through the water. The gas mixes with the steam from the boiling water in the small space at the top of the bowl, as well as radiated heat from the metal surrounds. In the end, gas that is already close to room temperature doesn’t stand a chance. It is dealing with an opponent that is boiling hot and about 200 times the thermal mass. It’s like trying to cool the Sahara with a refrigerator.