Relative humidity is very important for many agricultural environments, such as fruit and vegetable storage facilities, greenhouses, and tobacco curing and handling facilities.
Several scientific instruments measure temperature and relative humidity. The more accurate the instrument, the more it will cost. Devices that are economical for agricultural use in a barn, greenhouse, or a semi-protected environment often do not hold their accuracy over very long periods (six months to a year or longer) if they are used carelessly.
Several instruments that are readily available and affordable for general farm use are described here. Often called hygrometers or hygro-thermometers, some are for visual indication only, while others also have recording capabilities. Calibrations or comparisons with known conditions can be done periodically to determine an instrument’s relative accuracy. Some calibration procedures are presented in this article.
Duncan, George A.; Gates, Rich; and Montross, Michael D., "Measuring Relative Humidity in Agricultural Environments" (2005). Agricultural Engineering Extension Publications. 4.