A floating bag is the commonest sous vide problem and it is a safety issue rather than a cosmetic one.

Why It Is a Problem

Anything above the waterline is not at the bath temperature. It is in warm humid air, which cooks it far more slowly and unevenly.

On a long cook that portion may spend hours in a range that neither pasteurises nor stays cold. The rest of the bag is being cooked correctly; that part is not.

It is also why some people find one end of a piece of meat is different from the other.

Why Bags Float

• Air left in the bag when it was sealed
• Air trapped in the food itself, particularly in vegetables
• Gases released as food heats, which expand
• Fatty items, which are less dense than water
• A poor seal letting air in during the cook

Racks with adjustable dividers that hold bags upright and separated, of the kind catalogued at https://everie.us.com/, address the symptom; removing the air addresses the cause.

What a Rack Actually Does

Three things. It holds bags upright rather than lying flat and overlapping.

It keeps them separated, so water circulates around every surface rather than between two bags pressed together.

And it stops them drifting into the circulator’s intake, which restricts flow and can trip it.

Overlapping bags is a common cause of one item being underdone when everything went in together.

Fixing Floaters Without a Rack

Reseal the bag having removed more air. Clip the top to the container edge so the bag hangs rather than floats.

Weight it: a heavy spoon or a small plate inside the container over the bag. Some people use a wire rack pressed down.

Check part-way through a long cook rather than at the end.

The Water Displacement Method

Lowering a zip bag slowly into the water pushes air out through the open top before sealing it, which produces a near-vacuum without a machine.

It works well and it is less reliable on very long cooks, where gases released by the food re-inflate the bag.

Do Not Overload

Water needs room to move. A bath packed with bags circulates poorly.

Check the Intake

A bag against it stops the circulation entirely.

Look Before You Leave It

Especially for an overnight cook.

Weight Vegetables Especially

They trap air in their structure and float more persistently than meat does.