Humidity plus Temperature Measurement and Control

Wet & Dry Bulb Humidistat: The Digital Humidification Unit (DHU) is a system that maintains a set Humidity value in an enclosed environment. It consists of an Electronic Control Unit and a Humidity Sensor.  The Wet & Dry Bulb Humidity Sensor (PT 100) continuously monitors the humidity in its local environment and converts the measured value into a digital electronic signal that is sent to the Control Unit for interpretation.

The Control Unit receives the digital data from the Sensor and calculates the Humidity and Ambient Temperature which are then shown on numerical displays. The Humidity value is sampled at regular intervals and compared against the desired setting. If the measured value is below the setting, a solenoid valve is switched on to activate the atomiser sprays and raise the Humidity. When the measured Humidity value exceeds the setting, the solenoid is turned off. In this way an average Humidity value is maintained in the environment. The Temperature reading is also compared against defined threshold values and used to control external heating or cooling devices in a similar manner if required.

The control unit performs two main functions:-

• to interact with the user by presenting information about the humidity and ambient temperature and provide a method to set the various operation parameters
• to collect data from the sensor and calculate humidity and ambient temperature. Compare the calculated values with threshhold levels and make decisions about when to turn the appropriate external controls on and off 

The sensor consists of two identical Platinum resistance thermometers, one wet, and the other dry. The wet thermometer has its sensitive end wrapped in a tight fitting muslin cloth material (wicking) that is soaked in distilled water from a small reservoir. When the thermometers are ventilated the wet thermometer temperature will be lower than the dry thermometer temperature due to evaporation. The temperature difference is used to determine the Humidity. A fan is used to ensure a constant stream of air is passed over the thermometers. The temperature reading of the dry thermometer is a measure of the ambient temperature.

The electrical resistance of the Platinum thermometers is directly related to temperature and the sensor electronics converts this resistance into a voltage. An Analogue to Digital converter samples the voltages and returns the corresponding digital values to the control unit when requested. 

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