For measuring of air relative humidity several types of hygrometers are widely available. They are
- Capacitive and Resistive RH Transmitters
- Aspiration Psychrometers
Capacitive and Resistive Humidity Transmitters have got the prevalence lately. These sensors are supplied with built-in converters into 0 ... 10 V, 4 ... 20 mA, RS485 standard unified 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 transmitters 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. The point is 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 Psychrometers 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 room. Psychrometric sensor 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 psychrometric sensor and an 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.