Let's face it - diving equipment costs money. There are lots of areas where you need the best of the best; you would never try to make your own BCD, for example, and making even a simple mask and snorkel that work right is a task that is well beyond most average divers' capacity. Luckily, there are some things you can make for yourself, and if you're into salvaging anything heavy from below the surface, then you can save quite a bit of money by making your own lift bags.
What Not to Do
Before we get started, it's very important that you remember the rules of gas expansion and pressure at depth; remember that air expands as it ascends, so making lift bags from weak material is only going to result in failure. Overfilling any lift bag is also a recipe for failure, and so is trying to lift a heavy item with a lift bag that is improperly sized.
Also on the list of what not to do, and something you should already be aware of, is the fact that you should not use your personal air supply to fill lift bags. It is not worth the risk.
Making A Lift Bag That Works
As mentioned, the air inside lift bags expands on its way up. Commercial lift bags have over-pressure relief valves that prevent overexpansion and prevent accidents; while they work very well, they can be costly, particularly when you're first starting out. So, how do you get around the problem of potential overexpansion? The answer is surprisingly easy; it involves leaving a hole at the bottom of the lift bag for excess air to escape.
There are a few different ways to make lift bags. One of the easiest and least cumbersome ways to do this is to use old inner tubes. If you're planning to send small items to the surface, bicycle inner tubes will do the trick; for larger items, inner tubes from bigger tires will work. Before you dive, prepare the inner tubes by removing their valves - just cut them out with a sharp knife. Roll the inner tubes up tightly and secure them with string or keep them in a mesh bag before deployment to prevent them from taking on water. Ensure you carry extra cord and caribiners or other fasteners with you so you have something to work with, and be sure it is strong enough to handle whatever it is you plan to lift. Before filling the tubes with air, secure them to the object in question. Then, fill them up about half way. In some cases, you might need to use more than one to lift an item; be creative and you'll come up with some surprising solutions.
Another way to make inexpensive lift "bags" is to use five gallon buckets with lids, or for big items, use 55 gallon drums, either steel or plastic. Make sure they're full of air up top, and be sure they are sealed securely. Whatever anchor you use to drop them is going to have to stay down below. Let some air out when you're ready to send up your items, and leave the hatches in the lids open for air expansion.
Another option is to sink buckets or drums without lids, and add air slowly once your items have been secured. As you displace the water in the bucket or drum with air (leaving it upside down so air is trapped against the ceiling) you'll get your lift. The potential for a wave to knock one of these over once they reach the surface is real, so be sure you have a catcher topside to haul in whatever it is you are lifting.
When working with larger lifts like these, be sure you and your dive buddy discuss a plan and signaling ahead of time. Even smaller lift bags can be hard to manage on your own! With practice and ingenuity, you can save money and bring up your lobsters, scallops, or legal artifacts. Have fun, and be safe!