I built an air humidifier (for a small test chamber which should hold constant humidity) that is basically an impinger filled with water. To increase the humidity inside the chamber I pipe air through the impinger and to lower the humidity, I supply it directly from the pump into the chamber.
Due to constructive restrictions, the distance between the impinger and the chamber has to be quite big and therefore it takes quite a while until humid air is pushed into the chamber after switching the valve.
Right now, I use a PID controller (Brett Beauregard's PID library), but I am not quite sure, if a PID controller fits my problem. In case I experience a slight overshoot (let's say, the desired humidity is 60 % and we measure 62 %), the output value falls to 0. Then the measured humidity starts falling and after passing the 60 % line, the output is set to > 0. As mentioned earlier, since it takes quite a wile until the humid air reaches the chamber, the measured humidity falls way below 60 % and starts increasing.
To sum up - if the humidifier just starts working after falling below the desired value, I get an inevitable oscillation (period time is roughly 5 min). Do you know a type of controller, which might fit my needs any better? I thought about setting a constant minimum value so that, the humidity might not fall that fast. But since the environment humidity (and temperature) are not constant, this would lead to other problems...
Thanks in advance,