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  • Created: Jun 01, 2017

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Humans are very sensitive to humidity. Oftentimes, humans can detect the possibility of rain by just feeling humid. The rate at which water sweat evaporates depends on how much water is already in the air. On dry days, sweat evaporates quickly, which means it also carries away heat faster. On humid days, when the air is already saturated with water, sweat evaporates more slowly. This explains why it feels so much hotter in high humidity. In science, humidity is the presence of water in air. Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100% relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creatin­g the possibility of rain. This reference design features a humidity sensor from Sensirion. Humidity sensors use capacitive measurement, which relies on electrical capacitance. Electrical capacity is the ability of two nearby electrical conductors to create an electrical field between them. The sensor is composed of two metal plates and contains a non-conductive polymer film between them. This film collects moisture from the air, which causes the voltage between the two plates to change. These changes in voltage are converted into digital readings showing the level of moisture in the air.

This reference design uses PIC16F877A to monitor humidity and temperature using Sensirion’s SHT31 humidity and temperature sensor. The SHT31 is built on a new CMOSens sensor chip that is at the heart of Sensirion’s new humidity and temperature platform. In this design, the PIC16F877A receives data from the SHT31 sensor through I2C interface. It then displays the humidity and temperature reading on the LCD module. The ALERT pin is connected to the interrupt pin of the PIC16F877A for alert conditions as configured by the user. This pin goes to logic HIGH, when alert conditions are met. Both the SCL and SDA lines are connected to the external pull-up resistors to pull the signal high. The 5V power supply is decoupled with a 100nF capacitor that shall be placed as close to the sensor as possible. The sensor shows best performance when operated within recommended normal temperature and humidity range of 5°C – 60°C and 20%-80% Relative Humidity (RH), respectively. Long-term exposure to conditions outside normal range, especially at high humidity, may temporarily offset the RH signal (e.g. +3%RH after 60h kept at >80%RH). After returning into the normal temperature and humidity range the sensor will slowly come back to calibration state by itself. Prolonged exposure to extreme conditions may accelerate aging.

The presence moisture in air influences various physical, chemical, and biological processes. Because of this, humidity and temperature are among the most important parameters to be monitored in manufacturing industries, medical, and domestic applications. Humidity monitoring also helps to prevent heat exhaustion and heat stroke among athletes or workers.