The next factor you will want to control in your greenhouse or indoor garden. If the humidity is too high, your crop will suffer from rot and mold problems, as well as a tendency to "overheat" due to its inability to transpire moisture into the already saturated atmosphere. Humidistats control humidity in much the same manner as a thermostat controls temperature. A humidistat can be set to operate a ventilation fan once a particular level of moisture in the air is breached. Rarely when growing hydro-ponically in an enclosed area will you run into problems of too little humidity, however, if this is the case, you will either need to lower the temperature or the intensity of light so as not to dehydrate your crops. A level of 60-70% humidity is generally accepted as best for most crops. Too dry an atmosphere will cause excessive water transpiration and leave a high concentration of nutrients in your reservoir - very similar to what too high an ambient temperature or too strong a light intensity would cause. There are also a number of Thermostat/Humidistats on the market which combine both functions into the control of a single ventilation fan. The fan is programmed to operate when either the preset temperature or humidity levels are exceeded.
Unless you plan to keep a constant watch over your garden's temperature and humidity, it is a good idea to invest in automating these environmental controls to do the watching over for you. Some people have even gone to such lengths as to connect temperature and humidity sensors to their computers and integrate environmental controls through software programs and custom charting applications. This is pretty high-tech stuff but I'm sure NASA is all over it, as every last ounce counts when you're on your way to Mars.
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