[SamSteele]

There are generally three different models.

Manual inflation: You have to pull the cord to inflate. Getting it wet won't cause it to inflate. I'm not a fan because they won't save you if you get knocked out before hitting the water.

Automatic inflation: You can manually inflate them by pulling the cord, or there is a small wafer that dissolves when it gets wet, triggering the inflation of the vest. This can be from falling in the lake or, in some cases, from heavy rain. Replacing the cartridges after firing requires replacing the wafer too, and these recharge kits are around $30.

Hydrostatic inflation: These can be manually inflated by pulling the cord as well, and they also have a replaceable wafer inside the trigger mechanism however they require a certain amount of hydrostatic pressure to dissolve and activate the inflation. This pressure comes from going below the water surface a small distance. The advantage is that they are far less susceptible to firing just by being made wet from rain. The disadvantage is they are the most expensive to buy outright and the recharge kits are more costly as well.

I hope that helps.

SS