Two things cause bags to fill with air during low temp cooking.
The expansion of air that already exists in a product (typical in vegetable cookery), and gas that results in bacterial growth (a problem with some cuts of meat).
For vegetable cookery weighting helps, but this is almost always an issue unless you compress a really hard vacuum with a commercial machine.
For most home cooks, the best solution is to use zip lock bags, monitor, and then release the air and reseal as it builds up.
For meat cookery, try pre-searing for 20 - 30 seconds per side...or dipping in boiling water to kill off any surface bacteria prior to bagging and cooking low temp.
Also the sous vide supreme comes with a rack to aid in keeping items submerged.
That can help too.
cooking.stackexchange.com/users/17143/moscafj
Have you tried placing some kind of weight on them?
maybe plates, cooling rack, colander or some sorts
cooking.stackexchange.com/users/25507/Nicolas
I've had this problem and after trying various things, I've found that this works pretty well:
I make sure the SVS is filled to the max line
I cut my own bags from a roll, and make them rather longer than is really necessary - about the internal height if the SVS When vacuuming, make sure the contents are kept at one end of the bag
Then, I place the bags vertically in the SVS, using the stand to hold them in place.
By doing this, as the bag starts to float, the top of the bag his the inside of the lid of the SVS and stops the bag rising further - keeping the contents below the water level
cooking.stackexchange.com/users/28655/dopiaza
Was cooking kebobs wih veggies
and they kept floating.
Tried several methods including trying to wedge a chopstick in the clip to keep bag submerged and using whiskey stones.
Best result was using a set of thongs to both keep bag secure and to keep the food submerged.
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In a few Chefsteps videos, they place a metal butter knife in the bottom of the sous vide bag.
This may not be ideal for some cooking situations, but it works.
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My husband came up with a food safe solution for this problem - silicone coated magnets.
He had some manufactured and has used them successfully in his own sous vide cooking.
Available on Amazon .
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The easiest solution is to shove something heavy in the bag with your vegetables before you pack it... like a knife... or weight the bottom of the bags by attaching something heavy to the outside.
From an Anova guide on the subject: When getting all the air out and preventing the bags from moving is not enough, we look to the next level of options to get those floating bags in check.
Weigh the bags down.
We can achieve this using both internal and external weighting systems.
Alternately, you can use these same weights externally, instead clipping their sealed pouches to the bottom of your cooking bag with a small bulldog or alligator clip!
This is especially helpful when you are batch cooking and plan to transfer your packages directly from the water bath to the refrigerator or freezer (with appropriate chilling in between, of course!).
Other options include "burping" the bag (works better with ziploc-style bags) or putting a heavy weight on top of them to keep them down, usually something heavy, with holes that won't itself be a victim of floating.
A vegetable steamer basket (the flower style ones) works well for this.
There are a nice collection of ideas in the article,
so it's a good read if the ones I've chosen to mention don't work for you.
cooking.stackexchange.com/users/33128/Catija
Two hours into the cooking time I take the floating bag out and cut it accross one corner and revac and reseal carefully across the corner.
This works a treat but don't get any liquid in your vac machine.
cooking.stackexchange.com/users/50466/Des Gillespie
I have had a few items float a little in my sous vide supreme machine. Mostly veggies. How do I keep them from floating up and stay submerged?
How to keep bags from floating in a sous vide supreme
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This can come from surface bacteria on meat, which can be reduced by pre-searing for 20-30 seconds per side. Alternatively, you can release air as it builds up and reseal the bag, or weigh the bag down with something heavy like a metal butter knife. Weighing bags down is considered the easiest way of preventing them floating when cooking sous vide. An alternative method is removing air from the bag and resealing. The sous vide supreme comes with its own device for solving the floating problem. Finally silicone magnets may be a help.
Removing air from the bags when they start floating can solve the floating problem when cooking sous vide. Most people recommend a heavy weight either in or on the bag. The sous vide supreme comes with a special rack for keeping bags submerged. Silicone magnets can help.