tessak said:
you just toss a handful of ping pong balls in and they hold the bags down for you
Let me take the safety one first, because it's the one that matters. Any part of the food sitting above the water line is not at bath temperature, so for that part the pasteurisation clock has not been running. Pasteurisation is a time and temperature pair, and the clock only starts once the core actually reaches the bath temperature. A strip poking into the air is nowhere near that. So the honest position is: the submerged portion cooked as intended, the exposed portion did not, and how much that matters comes down to how long it was exposed and what the food is.
For a solid cut of intact beef like your ribeye, the risk lives on the surface, not the interior, so a short exposure that you caught and pushed back under, in a bath already up to temperature, is a very different situation from a poultry piece or a rolled or ground cut where surface bacteria have been folded inside. If any of it is genuinely uncertain and the cut is one where the inside matters, or anyone at the table is pregnant, immunocompromised, very young or elderly, the safe move is to treat it as undercooked and finish it to a standard temperature, or not serve it. That is a call for you and, if you have particular health concerns, your own professional, not for a thread.
Now the causes, because fixing those makes the safety question go away. Two things float bags. First, trapped air: brisketmike has it right, residual air plus gas and steam coming out of the food expands as the bag warms toward the low 50s Celsius (around 130F), so a bag that sank cold rises later. The reliable fix is the water displacement method, lower the open bag slowly into the water so the pressure pushes the air out, then seal it at the surface. Second, low density food: fish and a lot of vegetables float even perfectly sealed, and those simply need weighting or a rack. The bags and vacuum sealing notes walk through the displacement method properly.
On the ping pong balls: they are not a weight, they are an insulating cover. They float ON the surface to cut evaporation on long cooks, they do nothing to hold a bag down, so what you are seeing is exactly what they do. That distinction matters more than it sounds, because the two problems get confused constantly.
Which brings me to the other thing hiding in your big cook. On anything past about 4 to 6 hours, evaporation drops the water level, and a level that starts a comfortable inch over the food can fall until the food breaks the surface, and now you have a floating problem you never created. A lid, cling film pressed onto the surface, or yes the ping pong balls, keeps the level up. The water bath tips cover keeping a long cook topped up. For weighting the actual bags, use a rack, a clip on the rim, or dedicated weights, and check the level once when you pass by rather than standing guard.
