Good drenching equipment is essential to make sure that your stock is getting the right dose for worming. Underdosing can result in parasites surviving and developing a resistance to the drench.
A drenching system usually consists of a backpack or container to hold the drench, a hose, a handpiece, a nozzle and a barrel, which holds one dose at a time. The handpiece, barrel and nozzle together make what is called. If it is to be used as a backpack it should fasten securely to the farmer's back. Stock can get jumpy during drenching and a loose backpack could get knocked off and trampled.
The hose should seal tightly to the container and the gun without any leaks. Drench is very expensive and you don't want any going on the ground.
The handpiece should spring back into place cleanly when depressed. The barrel should be opaque, and measurements should be clearly visible on the side. The nozzle should deliver a short squirt of liquid when the handpiece depressed, and not dribble too much. No liquid should drip out of the nozzle if the handpiece is not touched.
Check the accuracy of the barrel: fill the drenching system with water and squirt it into a measuring container. If the measurement is slightly off, this isn't the end of the world. Just remember how much it is off by and adjust you drenching measurements to match.
Check the seals in the gun: place a finger over the end of the nozzle and gently attempt to depress the handpiece. If the handpiece depresses, liquid may be going back through the seals.
Always flush the instruments out with water after use.
Apply lubricating oil (not cooking oil!) to rubber pats, eg o-rings and rubber washers (you may need to take the instrument apart to do this).
If you have concerns about your drench gun, a rural supply store should be able to organise for it to be serviced. Replacement parts and new drenching systems can also be purchased at rural supply stores.