The three piece airlocks are a great choice for monitoring and stopping contaminants entering your fermenter.

Simple set up, just fill with pre-boiled water, or sanitiser mixture, and insert into your pre drilled hole/bung on your fermenter. The airlock creates a closed fermentation from inward contaminents. It will allow CO2 to blow off but not let any O2 or bugs into your fermenter.
 
The ideal scenario is to fill your airlock with ethanol such as our Super Kill Ethyl Sanitiser. This will prevent bugs from growing in the liquid that you fill the airlock with and also if this gets sucked into you beer accidentally it is harmless and will not effect beer flavour.  It will just very slightly increase ABV.

Note if you are fermenting in a fridge/freezer, remember to check for moisture loss in the airlock. you will need to either fill with Ethanol Spray or Salty water to stop it from freezing or evaporating through to the chill plate of your fridge/freezer.