The Sous Vide Water Displacement Method (Archimedes principle)
This explains how to remove air from a Ziploc freezer bag (or other heavy duty Zippered bag).
Self sealing plastic bags can be effective for use in sous vide, especially for beginners who are not sure if they want to invest in channel or chamber vacuum devices. We want to remove as much air as possible from the bag so that heat from the bath is conducted evenly to the contents–air interferes with the efficient conduction of heat.
Not all self sealing plastic bags are suitable for sous vide processing. They must be capable of withstanding sous vide temperatures and they must be capable of being securely sealed. The best bags for this purpose are Ziploc Freezer bags.
In order to implement the water displacement method, you place your food in the bag and zip it until there is only 1″-2″ left open at the corner. Put your finger in the opening and pinch the bag to hold it up. Slowly lower the package into cold water. You will see the water slowly force the air out of the package. Remove your finger from the opening and seal the rest of the bag just before the opening meets the water line. Assuming you have some form of protein in the bag, the whole package should sink. A small amount of water or air in the bag is harmless, but you can repeat the process if you feel there is too much. With a little practice, this technique becomes very easy.
If you are not confident that your seal is water tight, you can hang the top over the edge of the vessel and use the lid to prevent it from being exposed to the water. Remember that the food itself must be under the water line at all times.
Getting all the air out of Ziploc bags is easy. You just place your food in the bag, including any liquids or marinades, and seal all but one corner of the bag. Place it in the water bath, being sure everything below the zip-line is covered by water. You can see how all the air is forced out of the pouch.
Then seal the rest of the bag. I try to seal the food before the water has heated up but if the water is hot you can use a wooden spoon to hold the bag under. I almost always use the gallon size Ziplocs, I find the extra room at the top makes them easier to seal.
Channel and chamber vacuum devices are readily available. Channel vacuums have become more reasonably priced over the years, but chamber vacuums are still several hundred dollars and not absolutely necessary for sous vide processing. This article explains the easiest way to seal certain types of conventional self sealing plastic bags.