Sous-vide without a pump using convection


I'm currently planning on creating a sous-vide setup using an induction cooktop and a PID controller. I will be an Arduino micro-controller setup similar to this control of induction cooktop

Most sous-vide setups use circulators that circulate and mix the water in the bath to keep uniform temperatures throughout the bath. I was wondering if a pump is required if I'll be using a pot on top of an induction cooktop. Since the heat source is at the bottom, won't the water rise naturally through convection and keep a constant temperature throughout the bath?

Best Answer

What you will be dealing with is called stratification. Given a reasonable volume of water the difference can be quite remarkable. A one metre height of water can stratify water from 20°C to 95°C as long as the water is not disturbed and heated gently, even if heated from the bottom

The simple solution is to regularly stir the water, say once every five minutes. This would be OK for items only requiring an hour in sous-vide, but for much longer this can get rather tedious

If you are targeting temperatures in the 40°C to 65°C range a small aquarium pump would suffice to stir the water. Arrange the intake tube nearly floating on top, and the output tube weighted on to the bottom. At a pinch an aquarium bubbler would help significantly too. Both these devices will cool the water somewhat, but a stove top heater should be able to keep up

Example: Using a small bubbler in a large, well-insulated chest (Rubbermaid cooler, Esky, chilly bin etc.) with around 10 l of water at 60°C, keeps the temperature within 1°C from top to bottom. Heat loss is around 1°C per 30 minutes. By adding about 0.5 l of 95°C water every 30 minutes it keeps the temperature constant over a few hours

To get precise temperature control within ±0.5°C which some sous-vide recipes recommend, you will need a PID controller. For general home use with temperature control or ±3°C you could get away with a stove top heater, with accurate power control and a thermometer control system. PID is not that hard, so you might as well add that to your controller too