How to control humidity in a constant temperature and humidity test chamber is essential for achieving accurate and reliable test conditions. Humidification and dehumidification are necessary processes that ensure the environment inside the chamber meets the required specifications. This paper explores various methods used in such chambers, analyzing their advantages, disadvantages, and ideal application scenarios.
Humidification essentially involves increasing the partial pressure of water vapor in the air. One of the earliest methods was spraying water onto the walls of the test chamber. By controlling the temperature of the water, the saturation vapor pressure at the water surface could be managed. The water on the chamber walls created a large surface area from which water vapor diffused into the air, gradually increasing the relative humidity. This technique was widely used in the 1950s, but its effectiveness was limited by the rudimentary control systems of the time, such as mercury-electric contact conductivity meters. These systems had poor response times and large hysteresis, making it difficult to achieve rapid or precise humidity adjustments. Additionally, the method often led to water droplets falling onto the test samples, causing contamination. Drainage requirements also made this approach less practical over time. As a result, it was soon replaced by steam humidification and shallow pan humidification techniques.
Despite its limitations, the early wall-spraying method still has some benefits. Once the system stabilizes, the humidity fluctuations are minimal, making it more suitable for constant damp heat testing. Moreover, the process doesn’t add extra heat to the system, and if the spray water is cooled below the desired test temperature, it can even help dehumidify the chamber. This makes it a versatile option in certain applications.
Humidity can be expressed in several ways, but in testing equipment, relative humidity is the most commonly used measure. Relative humidity is defined as the ratio of the actual water vapor pressure in the air to the saturated vapor pressure at the same temperature, expressed as a percentage. The key property of water vapor saturation pressure is that it depends solely on temperature, not on atmospheric pressure. Through extensive experimentation and data collection, scientists have established a clear relationship between temperature and vapor pressure. This relationship is described by the Goff-Gratch equation, which is widely used in meteorology for creating humidity tables and charts.
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