Rowly - Don't go to very much trouble to use an external connector when reducing from larger to smaller airline. Your situation does not require it since you are only dealing with 100' of 3/8". Pottsy has described Koenders old sediment bowl. New version as Freeze Control Device is a sediment bowl with a simple mechanism to inject methyl or ethyl alcohol into the airline when ice blockage occurs. They are beneficial if running the windmill in northern winters. The problem of running the mill during winter is you hope the check valve on the diffuser ALWAYS works, which it doesn't esp as the unit ages. Leaky check valves allows water to feed back intothe airline to the waters edge. Guess what happens then. Water in hose freezes stopping airflow, windmill continues to crank, pressure continues to increase with no outlet. Bham! Something breaks; at weakest point. I also highly recommend (which Koenders does not make available) an adjustable pressure relief valve on sediment bowl or freeze control if running mill during winter.
Pottsy - Rather ordering a diffuser from USA go to the nearest sewage or water treatment plant and ask the manager to see his largest Canadian supplier book. They will have them. To see what they look like see:
www.stoneycreekequip.com
