Two Pressure Humidity Generators
Humidity is one of the key parameters that need to be controlled in various sectors, including microelectromechanical system manufacturing, silicon wafer production, electronics fabrication, complete cell testing, solar cell manufacturing, and many more. Various industries use humidity generators to manage humidity to prevent moisture damage to sensitive processes and equipment. There are various types of humidity generation systems. The two most common types are mixed flow systems and two pressure systems. While the 2-pressure system has the advantage of precise humidity control without the need for sensors, the mixed flow humidity generator system has the advantage of simplicity, portability, and lower running cost.
What is involved in relative humidity generation and control with good precision?
Humidity generation and control with precision are accomplished by the application of diverse scientific principles and technologies to control temperature, pressure, and moisture distribution, as well as optimized chamber designs and advanced control systems, well-designed sensors, and well-planned device architecture. This system ensures that the air stream used as dry air input is efficiently dried. In addition, many Geosystems engage mixed-flow systems or two pressure humidity generators to generate humidity.
High precision and accuracy sensors, modern PID control systems, and appropriate feedback and control systems ensure precise humidity monitoring and humidity control. Humidity calibration chambers are used in various institutions to maintain a steady temperature and ascertain precise humidity through pressure variation. Geo calibration’s two pressure humidity generators handle this by incorporating a temperature control system surrounding the saturator and along the channel through which humidity is supplied to this chamber. This chamber is also designed with temperature control systems.
Working principles of the two-pressure humidity generation system.
This system proves to be the best in terms of accuracy. The humidity generation in the two-pressure humidity generator is based on the well-established Dalton’s Law of Partial Pressure. Wherein the total pressure within a system is taken to be the sum of all the partial pressures of the gases within the system, as each gas exerts pressure on the system as it would if it was the only gas present in the system. The water vapor present in a given enclosed volume of gases can be expressed as a fraction of the total pressure of the enclosed volume. Furthermore, the ideal and non-ideal gas laws relate the number of moles present in the gas per unit volume with the pressure and temperature of the gases.
To achieve the gas law in a humidity generator, a gas stream from a compressed gas source is saturated with liquid or solid-state water under elevated pressure. The temperature is maintained in the system through a thermocouple and temperature control system. Figure 1 is a schematic diagram of a conventional 2-pressure system.
Figure 1. Diagram illustrating the conventional 2-pressure system.
Based on Dalton’s Law, at the set temperature, the partial pressure of each gas present in the given volume remains the same and adds up to the total pressure. Thus, the relative humidity is calculated as a ratio of the pressure in the saturator to the pressure in the chamber (equation 2). According to the gas laws, the total number of gas molecules remains the same in both the saturator and the chamber. The relationship between RH% and the pressure for a 2-pressure humidity generator is shown in equation 1.
Here F (Pc, Tc) and F (Ps, Ts) represent the enhancement factor at the specified temperature and pressure within the test chamber and saturator respectively. The term ew(Ts) and ew Tc represents the saturation vapor pressure at the saturator and the test chamber temperature respectively. Ps and Pc represent the saturator and the test chamber pressure.
The enhancement factor F is included to account for the non-ideal gas behavior whereby water vapor molecules in a gaseous mixture do interact. At the operating conditions, the enhancement factor is approximately 1. It is represented by equation 2 below
Where Xw represents the mole fraction of water vapor in the saturated mixture, P is the total pressure above the surface of the condensed phase and es represents the pure phase saturation vapor pressure of water.
At constant temperature and an enhancement factor of 1, the equation is simplified to.
Where Pc is the vapor pressure in the chamber Ps is the vapor pressure in the saturator.
The mixing ratio of moist and dry air in the chamber can be determined by the equation
Where Mw and Mg refer to the molecular weight of the water vapor in the system, and of the carrier gas respectively. The term ew(Ts) represents the saturation vapor pressure of the pure liquid water over a plane surface at the saturator set temperature. Ps represents the saturator pressure while f represents the enhancement factor as defined in the previous section.
The metering of moist and dry air can also be achieved by using a volume ratio.
Different RH% can be achieved by setting the right corresponding pressure and temperature. The maximum pressure achievable in the saturator directly determines the minimum RH% that can be reached. As the two-pressure system is based on the thermodynamic principles of water vapor and gases, the 2-pressure humidity generation system requires accurate measurement and control of temperature, pressure, and flow in order to set precise humidity within the chamber.
As humidity is analytically derived from the direct relationship between the pressure exerted by water vapor and the carrier gas, so this method is more accurate. Over the years, Geo Calibration has collaborated with several research organizations to design two-pressure humidity generators that can be customized to meet a wide range of industry and R&D needs.
To learn more about the hybrid two-pressure humidity generator for your specific research or industrial need, contact our engineers at Geo Calibration. At Geo Calibration Inc. We design, manufacture, sell and distribute a wide range of temperature and humidity instruments for research and development, pharma, industrial manufacturing, and the calibration service sectors. So, if you need any assistance regarding this equipment, you can contact us at (631) 471–6157 and (631) 943–1999 or drop us a mail at — email@example.com today!