I have been learning about and intermittently experimenting with Sous Vide cooking for about 3 years now. I'm still trying to find the boundaries of the technique for my style of cooking. One of the keys to a successful experience with it, is to have a good vacuum sealer for your product. This ensures that there is no air trapped in your bag, which would make your bags float, reducing the evenness of heat transfer into the food, as well as creating an insulating layer on one or more sides of the food.
However, with some softer foods, pulling a strong vacuum will cause the food to become misshapen or even damaged. So, I was thinking today, what if I could use a rigid-bodied container, such as a mason jar? Experiment time!
I took two 1-pint mason jars, filled them with an equal amount of water, and pulled a -9Bar vacuum on one, and no vacuum on the other. Afterwards, I placed them both in a 2gal cambro and filmed the process. I was surprised to discover that I couldn't see any difference between the two!
After some contemplation about the physics involved, I think I've figured out what's going on. Whether something floats or sinks is a function of the density of the item (as compared to the density of the water it displaces). With a standard plastic bag, pulling a vacuum keeps the weight the same while reducing the volume, thus increasing density significantly. With a rigid-bodied container, it has the opposite effect! It reduces weight (the weight of the air that is pumped out) while keeping the volume the same, DECREASING density.
Once again, reality is under no obligation to be intuitive.