For the measurement of relative humidity several types of sensors are widely available. They are Psychometers, Aspirat, Capacitive and Resistive Sensors. The basic requirements for Humidity Sensors used to maintain the climate in the mushroom growing production, are:
- measuring relative humidity range – from 10 to 100%
- accuracy – 1% RH
- stability to condensation
Capacitive and Resistive Humidity Sensors have got the prevalence lately. These sensors are supplied with built-in converters into 0 to 10 V, 4 to 20 mA, RS485 standard signals. It allows integrating them easily into any measurement system. Another condition for a stable operation of these sensors is a dust filter. The sensors with RS485 output, as a rule, have a built-in temperature sensor. Capacitive and Resistive Sensors are often used in the office ventilation, where the relative humidity ranges from 30 to 70%, with impossible dew-fall point. Organizations which are involved in designing and mounting of such office climate systems, usually copy the same technical solutions for humidity measurement in agro technologies. However, their usage in closed warehouses, greenhouses, mushroom growing and other farms is not recommended because in high humidity conditions these sensors have an indication error up to 6% and fail when they are influenced by condensate. Best of all, these types of sensors are used for measuring relative humidity and temperature of the outside air by placing the sensor in the shade under a canopy.
In conditions of high air humidity it is recommended to measure with the help of Psychrometer consisting of two matched platinum resistance thermometers called wet and dry bulbs. The wet bulb is connected to a bottle with distilled water through a fabric wick. Psychrometer should be placed in a horizontal position so that the cold air on the wet thermometer does not hit the dry bulb thermometer and introduce additional error. Dry thermometer can also be used for measuring air temperature in a growing room. Psychrometer should be mounted at the point in space where airspeed is constant and equal to about 2 m/sec near the wet thermometer. In this case, the sensor will provide high accuracy and stable indications in high humidity conditions.
Aspiration Psychrometer is used at the point in space with variable or weak air flow. It consists of a psychrometer and electric fan which creates a normalized air flow near the wet thermometer. Today the aspiration psychrometer, thanks to its design, is the most accurate (accuracy – 1%) sensor for the measurement of high air relative humidity.
