Relative humidity inside a greenhouse can be accurately measured using a sling psychrometer. Other devices include humidity pocket meters, humidity pens, hydrothermograph and humidistat, which measures relative humidity within a 4 percent precision level.
Humidity is generally measured in terms of relative humidity, or RH, which refers to the quantity of moisture in the atmosphere with respect to the maximum amount of water vapor the air can contain at a particular temperature. RH forms an inverse relationship with temperature, where increasing temperatures lead to decreasing RH.
Plants measure humidity in terms of vapor pressure deficit, or VPD. Low VPD indicates a high RH, usually greater than 90 percent. High VPD denotes a low RH, typically below 50 percent. High and low levels of VPD may cause deprivation of essential nutrients required for the plants to grow. Favorable levels of VPD fall within the range of 55 and 95 percent RH.
To ensure optimal conditions inside a greenhouse, certain instruments are used to measure and regulate RH levels. A sling psychrometer is equipped with two thermometers. Dampening the wick contained in one thermometer and spinning both thermometers for a minute will provide the wet and dry bulb temperatures. The difference between the two temperatures is compared to a chart to determine the RH. Modern electronic devices are designed using a wick made from nylon, cellulose acetate butyrate or polystyrene.