Sous vide cookers, also known as immersion circulators, are kitchen appliances that allow water temperature to be precisely controlled over a long period of time -- hours or even days. While the term may sound fancy, sous vide simply means the food is cooked in a plastic bag, ideally sealed with a vacuum sealer (though a plastic zipper bag with most of the air removed from it can work nearly as well).
When you cook sous vide, the immersion circulator is set for the desired doneness -- for a medium-rare steak, that would be about 135 degrees Fahrenheit. The machine heats the water and maintains it at that temperature so that you can cook the steak for as much time as it takes to reach the same temperature as the water all the way through, eliminating the risk of overcooking.
While the science is simple, there is a wide variety of differences in quality and features of sous vide machines. At the Drexel Food Lab at Drexel University, we use immersion circulators for applications including and beyond sous vide, such as gently reheating delicate foods, quickly chilling wine or food (not possible with all circulators), poaching perfectly cooked eggs in the shell, or holding warm sauces like hollandaise that are vulnerable to breaking when heated on a stovetop. For this article, I tested five sous vide machines and talked to chefs about what they look for in an immersion circulator and which models they like best.