Water Bath

A water bath in sous vide cooking is a container of water held at a precise temperature by an immersion circulator. The circulator heats the water and pumps it in a continuous loop, maintaining temperature within ±0.1°F across the entire bath. The sealed food item is submerged in this bath until it reaches thermal equilibrium with the water.

Why water, not air: Water is roughly 25 times more thermally conductive than air (0.6 W/m·K vs 0.025 W/m·K). This means water transfers heat to the steak's surface much more efficiently than an oven at the same temperature. A steak reaches equilibrium faster in a 131°F water bath than in a 131°F oven — though the oven scenario would produce well-done steak long before equilibrium at 131°F, since the heating element produces air temperatures far above the setpoint.

Water bath as thermal reservoir: A large volume of water has enormous thermal mass. Dropping a cold steak into a 12-quart bath at 131°F barely changes the water temperature — the circulator compensates within seconds. This temperature stability is what makes sous vide so reliable: the steak literally cannot overcook past the bath temperature.

Container options: Any insulated container works — a large pot, a cooler, or a purpose-built sous vide container. Larger volumes maintain temperature more easily. A 12-quart container is sufficient for 2–4 steaks.

Practical tip: Cover the bath to reduce evaporation and heat loss. Ping pong balls floating on the surface work surprisingly well (they trap a layer of insulating air). Plastic wrap also works.