Interesting question! I installed my sprinkler system in 1985 and, following directions of the designer, I installed several automatic drains. It wasn't until that fall that it occurred to me that I had no way of know if they were working. Now, I had socked the lines 2 feet deep, but sometimes it does get cold enough to freeze that deep, so I started having my system blown out. I do it myself now, but I still don't know if the automatics are working or not. Since I have my own air compressor, blowing the lines is not a big deal or expense, but if I had to hire it done it would probably cost $50 or more every year. Guess my compressor has paid it way pretty well.