pec0054,
What I use is an ordinary digital thermostat. It has a sensor to measure the temperature. Also it has an electrically controlled switch, which is called a "relay". The controller can be set for heating or cooling, as you prefer. In this case it is used for cooling. When the measured temperature is higher than the set temperature, the switch passes current, and when is is lower, it cuts the current. That is how I run the
pump on and off.
GpsFrontier,
The temp. inside the greenhouse is anywhere between 4-11 degrees lower than outside temp. When the outside air is hotter and drier, the cooling efficiency is better. For example, right now outside temp is 36 and inside temp is 28. Yesterday when it hit 40 outside, it was 30 inside. The inside temp. is measured in the middle of the greenhouse, so it represents the average temp. Actually it is a bit cooler near the cooling pad and a bit warmer near the fans, as you can imagine.
Relative humidity inside the greenhouse is usually about 70 percent, which is ok for lettuce.
The coils work but not as good as I want. It lowers the temp. of the NS by about 3 degrees only. The ideal temp. of the NS is 24 degrees but most of the time that does not happen during the day. So basically the pump runs all the time during the day and shuts off only at night. But still, it is helpful because It can keep the temp. below 30 degrees, which is high but not very bad.
I need a longer coil and a more powerful pump to further cool down the NS. But I won't upgrade, It is too much trouble.
PS: All temp. is given in CELCIUS, not FAHRENHEIT.