HPS

Humidity Profile Simulation

1. Lyman alpha Resonance Fluorescence Hygrometer

Air and wall temperature of the chamber can be regulated independently within certain limits. The relative humidity inside the chamber during a simulation run is defined by the ratio of the actual water vapor pressure and saturated water vapor pressure with respect to the air temperature inside the chamber. The design of the humidity simulation is such that the wall temperature determines the dew (frost) point temperature of the air inside the chamber. This means that the water vapor partial pressure can be lowered by cooling the walls to a temperature lower than the air temperature. Differences between air and wall temperatures up to 25°C can be reached in the chamber, as long as the air pressure is above 100 hPa.

The water vapor pressure in the chamber can achieve its maximum value corresponding to the water vapor saturation pressure determined by the temperature of the walls, the coldest parts inside the chamber. The relative humidity of the chamber air is therefore defined through

where I_o the lamp intensity, [O₂] and [H₂O] the concentrations of oxygen and water, and s _O2 and s_H2O the corresponding absorption cross sections. C is a constant, which accounts for the different beam geometry, reflectivity of the MgF₂ mirror and sensitivity of NO cell B relative to NO cell A.

If factor C is known, the water vapor concentration can be determined by

Under conditions of very low water vapor concentration ([H₂O] » 0), the factor C is obtained from

if saturation with respect to the wall temperature is reached. E is the water vapor saturation pressure in percent, T_Wall and T_Air the absolute temperatures of the chamber air and the walls respectively.

For E(T_Air) we always use the water vapor saturation pressure over a plane surface of liquid water, since relative humidity with respect to liquid water is the usual measure in the scientific community. E(T) is derived using the Goff and Gratch (1946) formulation as recommended by the World Meteorological Organization [WMO-Report No.8, 1983] and adapted to the International Temperature Scale 1990 (ITS-90) [Sonntag, 1994].

At a constant air temperature different relative humidities can be set by regulating the wall temperature of the chamber. Figure 1 gives an example of a humidity simulation run at different temperature and humidity levels inside the chamber.

Figure 1:  Measurement of RH (by Lyman-alpha fluorescence hygrometer), air temperature, wall temperature, and pressure as function of simulation time during a typical run of the environmental simulation chamber for the calibration of humidity sensors in the MOZAIC project.

In the lower panel the actual air and wall temperatures as a function of the simulation time are shown. The upper panel displays the relative humidity expected from the actual air and wall temperatures and the actual relative humidity measured with the Lyman-alpha hygrometer. At an air temperature of -30°C there is good agreement between actual and expected relative humidity, while at temperatures of -20°C and -40°C the actual humidity is much lower than the expected humidity.

Comparing the actual wall temperature with the actual dew point temperature measured by the dew point hygrometer, it is obvious that the prescribed humidity is higher than what is actually found in chamber air. This water deficiency is most likely caused by inhomogenities of temperatures and adsorption on the surfaces of the walls and the fins of the heat exhanger inside the chamber. This is overcome by the addition of controlled amounts of water vapor to the air inside the chamber during a simulation run. The necessary addition of water vapor is achieved by injection of moistened air into the chamber, regulated by a software PID-controller which uses the actual wall temperature as set point and compares with the actual dew point temperature measured by the dew point hygrometer.

The water vapor saturation pressures with respect to a plane surface of liquid water or ice is respectively:

For liquid water the constants are:

a = -6096.9385, b = 21.2409642, c = -2.711193E-2, d = 1.673952E-5, and e = 2.433502

For ice the constants are:

a = -6024.5282, b = 29.32707, c = 1.0613868E-2, d = -1.3198825E-5, and e = -0.49382577

References

Goff, J.A., and Gratch, S., Low-pressure properties of water from -160 to 212 F, Trans. Amer. Soc. Heat. Vent. Eng., 52, 95-122, 1946.

Sonntag, D., Advancements in the field of hygrometry, Meteorol. Zeitschrift, N.F. 3, 51-66, 1994.

WMO-Report No.8, Measurement of atmospheric humidity, Guide to meteorological instruments and methods of observation., 5^th ed., World Meteorological Organization, Geneva, 5.1-5.19, 1983.