How vacuum sealers work: chamber and external machines explained

Picture the two machines side by side. An external sealer has a nozzle or a channel in the lid that presses onto the open end of the bag, and a pump on the other side of that seal pulls air through it, the same basic idea as sucking a milkshake through a straw. A chamber sealer never touches the bag opening at all: the entire bag, food and open end together, goes inside a box, the lid closes, and the pump removes air from the box itself. As the pressure around the bag drops, the air inside the bag simply expands out through the still open mouth of the bag on its own, because there is nowhere else for it to go. The bag is not being sucked at from one end, it is being released into a lower pressure space on every side.

• External: a pump connected to a nozzle or channel pulls air through the open bag end
• Chamber: a pump removes air from a sealed box, and the bag equalises with the box on its own
• Both machines finish the same way, with a heated bar welding the film shut while the vacuum is still holding

Buy the wrong film and an external sealer looks broken even though nothing is wrong with the machine. Embossed bags carry a grid of raised channels running the length of the film, and those channels are the only path air has to travel from deep inside the bag to the nozzle. Put a smooth chamber bag like Avid Armor's chamber pouches into an external sealer such as the FoodSaver Compact and the film collapses flat against itself right at the nozzle within the first second of the cycle, sealing off the far two thirds of the bag before it has evacuated at all. The machine still runs its cycle and still welds a seam, so the bag looks sealed, and the food inside is sitting in ordinary air. This is the single most common first week mistake with a new sealer, and it runs in both directions: chamber film is cheaper precisely because it skips the embossing step, which is exactly why it cannot work in a machine that pulls from one end.

Every bag needs a stretch of empty film between the food and the eventual seal line, and the two machine types need different amounts of it. The vacuum bag size calculator builds this in automatically, but the numbers are worth knowing on their own.

Embossing costs money: the channel pattern is pressed into the film during manufacturing, and that extra step is priced into every roll of external roll stock. Chamber pouches like Avid Armor's chamber pouches skip it entirely because a chamber machine does not need a channel to reach, which is why smooth chamber film runs roughly half the cost per square inch of embossed film at the same thickness. That saving only shows up if you actually own a chamber machine such as the VEVOR 10.2 inch chamber sealer; buying smooth bags for an external sealer saves nothing, because they will not evacuate in the first place.

Only the chamber machine handles it cleanly. Because a chamber sealer never pulls air through the bag opening, liquid inside the bag is never being dragged anywhere, which is why a chamber sealer can seal stock, brine or a marinating steak without flooding a seal bar. An external sealer pulls everything toward the nozzle, air and liquid together, so the honest workaround with a machine like the compact Anova Sealer is to freeze the liquid into a solid block first and seal it once it is no longer liquid. Chilled liquids also boil under vacuum at a lower temperature than warm ones, which is its own topic with its own arithmetic: see the full guide to vacuum sealing liquids before trying to seal warm stock in any chamber machine.

Buy an external sealer first if most of what you seal is dry: portioned meat, cheese, vegetables, or bags going straight into a sous vide bath. The FoodSaver with automatic bag detection is a reasonable default here, since it senses moist versus dry settings rather than making you guess. Move up to a chamber machine only once liquids, brines or high volume sealing become a regular part of the kitchen, at which point the cheaper film and the ability to seal soup without a mess start paying for the higher purchase price.