Measuring Relative Humidity
Marijuana Horticulture
by Jorge Cervantes
Relative humidity control is an integral part of insect and fungus prevention and control. Humidity above 80 percent discourages spider mite but promotes fungus as well as rot and stem rot. Humidity levels below 60 percent reduce the chances of fungus and rot.
Measure relative humidity with a hygrometer. This extremely important instrument will save you and your garden must frustration and fungi. By knowing the exact moisture content in the air, humidity may be adjusted to a safe 40 to 60 percent level that encourages transpiration and discourages fungus growth.
There are two common types of hygrometers. The spring type is accurate within 5 to 10 percent. This hygrometer is inexpensive and adequate for most hobby growers whose main concern is to keep the humidity near 55 to 60 percent. The second type, a psychrometer, is more expensive but very accurate. A psychrometer that measures relative humidity with a wet and dry bulb is an excellent way to keep an accurate vigil on relative humidity. Today there are many exceptionally accurate high tech gadgets, plus they are equipped with memory!
A humidistat is similar to a thermostat, but regulates humidity instead of temperature. Humidistats are wonderful and make controlling the environment very easy. Humidistats costs less than $100 and are worth their weight in resin glands. A humidistat and thermostat can be wired “in line” to control a vent fan. Each can operate the fan independently. As son as the humidity (or temperature) exceed the acceptable range, the fan turns on to vent the humid (or hot) air outdoors.
The HID lamp and ballast radiate heat, which lowers humidity. Heat from the HID system and a vent fan on a thermostat / humidistat are all the humidity control necessary for most grow rooms. Other dry heat sources, such as hot air vented from a furnace or wood stove, dry the air and lower the humidity. be careful. Do not let piped in, warm, dry air blow directly on foliage. It will rapidly dehydrate plants.
Increase humidity by misting the air with water or setting out a bucket of water to evaporate into the air. A humidifier is convenient and relatively inexpensive. Humidifiers evaporate water into the air to increase humidity. Just set the dial to a specific humidity, and presto! the humidity changes to the desired level as son as the humidifier is able to evaporate enough water into the air. A humidifier is not necessary unless there is an extreme problem with the grow room drying out. Problems seldom occur that can be remedied by a humidifier. All too often, there is to much humidity in the air as a result of irrigation and transpiration. If a ventilation system is unable to remove enough air to lower humidity, a dehumidifier could be just the ticket!
A dehumidifier can be used anytime to help guard against fungus. Just set the dial at the desired percent humidity and presto, perfect humidity. Dehumidifiers are more complex, use more electricity than humidifier, and cost more, but to growers with extreme humidity problems not yet cured by a vent fan, they are worth the added expense.
Air conditioners draw moisture from the air and lower the humidity. The moisture condenses into water that is collected in a container or expelled through a tube outdoors. The condensed water carries the fragrance of cannabis. Sniffer dogs can easily smell the fragrance of cannabis in the tainted water expelled outdoors.