There are a few things you should know about the use of water pans:
1) the more water in the pan the more it will reduce smoke chamber temps and help to stabilize temps;
2) the less water in the pan, the higher the chamber temp (without increasing the flame);
3) smoke chamber humidity will increase as water levels decrease due to higher water temp and more rapid steaming;
4) the water pans position in relation to the heat source will effect the boil-off rate and humidty: closer = faster boil-off/higher humidity;
5) depending on the recipe for a poarticlar sausage, if it calls for a low temp/cold smoke, this typically is a drying stage, so low humidity would be desired (empty water pan);
6) a water pan filled with washed sand (play sand), then covered with foil to catch drippings can be used for more stabilized smoke chamber temps while not adding humidity;
7) the water pan should always be installed during use of the smoker to baffle and redirect the heat for true indirect cooking....this is a must for vertical smokers. Without the pan, drippings could catch fire on the gas burner;
As Bearcarver mentioned, depending on what you are smoking, you may opt for a wet or dry pan. This may also be a decision based on your current relative humidity. Example: if I'm smoking pork ribs, and I would normally use a medium/high humidity during the first 3-4 hours, but it's stormy weather with high humidity, I may not add water until after the first 2 hours, but only add what will evaporate in a shorter period of time. Then, during the final stage of cooking the ribs, if I want to form a slight bark on the surface of the ribs, I want a dry water pan for reduced humidy to set the the bark. Same goes for chicken: if I'm looking for a crispy skin, I want a low humidity smoke chamber.
Hope this helps...good luck with the sausage smoke!