Relative Humidity L40/RH
Humidity measurements in thermal analysis
When conducting experiments in thermal analysis, the surrounding atmosphere, including its humidity, can significantly impact the behavior of the sample and the reactions that take place. Understanding the role of humidity is crucial for various applications, such as assessing its influence on building materials, determining the storage time of pharmaceuticals and foods, or evaluating its impact on the mechanical properties of polymers. However, it’s essential to distinguish between two key concepts: water vapor and relative humidity.
Water vapor refers to the amount of water present in the air as a gas. It’s typically measured in units such as grams per cubic meter (g/m³) or parts per million (ppm) and directly indicates the concentration of water molecules in the air.
Relative humidity, on the other hand, is a dimensionless ratio expressed as a percentage. It describes the amount of water vapor in the air compared to the maximum amount of water vapor that the air can hold at a specific temperature. Relative humidity varies with temperature, and it’s a measure of how close the air is to being saturated with moisture. For example, air with a relative humidity of 50% contains half the amount of water vapor required for it to become saturated at that temperature.
In thermal analysis experiments, both the absolute water vapor content and the relative humidity can be critical factors affecting sample behavior and reactions. Researchers should consider these factors carefully, depending on the specific goals and requirements of their experiments, and select the appropriate environmental conditions to achieve accurate and reproducible results.
Difference between relative humidity and water vapor
Relative Humidity Generators are primarily employed in experiments conducted at or near room temperature, while applications involving water vapor are used in higher-temperature settings. When water is heated beyond its boiling point, it transitions from a liquid to a gaseous state known as water vapor or steam. The introduction of this steam into a reaction chamber or instrument is referred to as a water vapor application. In contrast, any gas can hold a specific amount of water at a given temperature, a property known as humidity. To illustrate, consider air, which invariably contains a certain level of moisture, even below the boiling point of water. This moisture level is quantified as either humidity or relative humidity.
Humidity measurements
The humidity generator typically operates within a temperature range spanning from room temperature up to 80°C. It can precisely control relative humidity levels, ranging from as low as 0.2% to as high as 98%. This versatile capability finds applications in various thermal analytical devices such as Dilatometers, Differential Scanning Calorimeters, and Simultaneous Thermal Analyzers. It is especially useful for analyzing a wide array of substances, including food, pharmaceuticals, building materials, and biological processes.
It’s worth noting that the same quantity of water vapor in the air, measured in grams of H2O per kilogram of air, can correspond to different relative humidity values. This variation depends on the temperature because the atmosphere’s capacity to hold moisture undergoes temperature-dependent changes. This maximum water-holding capacity is strongly influenced by temperature and fluctuates from a fraction of a gram per cubic meter (at temperatures below 0°C) to approximately 600 grams per cubic meter at 100°C.
Relative humidity
Relative humidity is the most commonly used measure for quantifying humidity. It is a straightforward concept: it represents the amount of water present in the air relative to the maximum amount of water vapor that the air can hold at a specific temperature, expressed as a percentage. For instance, when the relative humidity is 50%, it signifies that the air contains half of the water vapor it could hold at that particular temperature.
Relative humidity typically falls within the range of 0.1% to 100%, allowing water to exist in the form of water vapor. When the relative humidity reaches 100% and the surrounding air cools down, it surpasses the dew point. The dew point marks the maximum amount of water that air can retain at a given temperature, and as a result, water condenses from the air, forming liquid water. At this point, an equilibrium exists between liquid water and water vapor at that specific temperature.
On the other hand, when the temperature rises above the boiling point of water (which is 100°C under standard sea level conditions), water in the air can only exist in the form of water vapor.
In the context of Earth’s living conditions, relative humidity plays a crucial role. It helps to visually represent the very narrow range of water vapor concentration in which mammals like humans feel comfortable.
These considerations give rise to two primary application scenarios for thermal analytical applications. The first scenario involves a temperature sweep application where a specific humidity level is set at room temperature, and then the sample and its environment are either heated or cooled to a predefined temperature.
In this case, the quantity of water within the measurement chamber remains constant, but the relative humidity undergoes changes in relation to the temperature
On the other hand, the second scenario entails isothermal measurements, which allow for the establishment of defined and constant humidity levels ranging from 0.2% to 98% relative humidity. Cold air, particularly below room temperature, has limited capacity for holding water vapor, and this capacity diminishes with decreasing temperatures. Air below 0°C cannot contain water vapor.
When the relative humidity exceeds the dew point (e.g., during cooling), water vapor condenses and takes on a liquid form. If the ambient temperature is below 0°C, the condensed water will freeze. This process requires additional hardware equipment, such as a heated transfer line for sample temperatures above room temperature.
A humidity generator is used to create an atmosphere rich in water vapor by passing a gas through warm water to saturate it. Subsequently, the gas is adjusted to 100% relative humidity by introducing dry air to achieve a specified relative humidity level using a dew point sensor. Custom configurations can be tailored to carrier gas and compositions, involving additional Mass Flow Controllers (MFCs) or external dew point sensors.
You are interested in a Humidity generator?
You need more information?
Contact our application experts!
Applications
Here’s a typical application example that illustrates a measurement in relative humidity, exploring the impact of moisture on the thermal expansion of two different brick materials:
In this application example, the effect of moisture and humidity on brick materials is examined. The left curve displays the isotherms of two types of bricks at 20°C and 60°C, as well as the moisture content absorbed by the samples. On the right side, you can observe the moisture-dependent Coefficient of Thermal Expansion (CTE). It becomes evident that the degree of humidity significantly influences the thermal expansion behavior.
P.sin; J. Lukovicova; G. Pavlendova; M. Kubliha; S. Uncik; Experimental Performance of HygroThermal Deformation of Contemporary and Historical Ceramic Bricks, International Journal of Mater
Downloads
