Harnessing Humidity with Chilled Mirror Hygrometry

Figure 1. A schematic representation of the CMH sensor. Thermoelectric cooler chills the mirror surface to the frost point. The optical bridge detects the frost layer. The mirror temperature is modulated to the point where the frost layer neither grows nor diminishes, which is the equilibrium condition.

by Gerald Schultz, PhD, Sunwise Turn Consulting LLC

Why Measure Humidity

Of all the substances that exist on the planet, water is one of the most anomalous in its behavior, which leads to an outsized impact in general industry, agriculture, and medicine. It is water’s ability to form hydrogen bonds within the context of its low molecular mass and planar symmetry that forms the basis of its impact.¹ Ice floats and reflects light, steel corrodes, concrete decrepitates, drugs lose potency, grains rot, life swims in it and is infiltrated by it. Three-fourths of the planet is covered by water, and it comprises a significant component of the Earth’s atmosphere, all of which will affect the rate of warming of the planet. For the above reasons, and more, the measurement and control of water vapor, humidity, is widely practiced.

Measurement and Control of Humidity

Technologies that measure water vapor concentration are based on electrical, mass, and spectroscopic changes. Each technology has its advantages and disadvantages with regards to concentration range of measurement, accuracy, precision, and cost. Of all the technologies available, chilled mirror hygrometry (CMH) is unique owing to its high accuracy and repeatability over a wide dynamic range of measurement. CMH is recognized as a fundamental standard and as such is used in national standards laboratories throughout the world, as well as industrial metrology laboratories.

One can determine water content of a system by measuring headspace humidity because water vapor is in equilibrium with its separate phases. Applying CMH to the measurement of the drying efficiency of desiccants is an ideal system to demonstrate the utility of CMH. We chose Drierite, which is the soluble crystalline anhydrite of CaSO₄, and silica gel, microporous amorphous silicon oxide for this study owing to their very different modes of getting water vapor.² Efficiency is the ability of a desiccant to trap water vapor down to a certain residual headspace concentration, the residuum, while capacity is the amount of water vapor that is trapped within the desiccant at a given equilibrium water vapor concentration.

Figure 2. Schematic representation of test system.

Operating Principle of Chilled Mirror Hygrometry

Chilled mirror hygrometry (CMH) measures the equilibrium water vapor concentration of a carrier gas by determining the dew (above zero) or frost point (below zero).

The temperature at which dew and frost forms corresponds to the equilibrium vapor pressure of water. The measurement is made by chilling a planar copper surface that has been coated with a thin layer of an inert reflective metal e.g. gold, platinum, chromium. The dew and frost points are defined as that temperature where a thin layer of condensate forms, which neither grows nor diminishes in thickness over time. A non-changing condensed layer is in equilibrium with the headspace gas sample. A thermoelectric cooler (TEC) is used to chill the mirror. The condensed water layer is detected optically by a reflected ray of light. The thickness of the condensate is minimized so as to avoid a temperature gradient, which would lead to errors in the dew or frost points. The reflectivity of the surface is automatically monitored by means of a sensitive optical bridge, an LED light source and a photo-sensor.

Figure 1 is a schematic representation of a chilled mirror sensor that sends information to its electronic control unit. Accuracies can be +/- 0.2^oC mirror temperature over a range of water vapor concentrations of hundreds of thousands of ppmv to 0.1 ppmv corresponding to dew and frost points from 60^oC to -90^oC. No other humidity sensing technology has this dynamic range. Chilled mirror technology has the following boundary conditions: the condensation point of the carrier gas must be lower than that of water vapor, and the carrier gas and water are immiscible. If water condenses into miscible phases, then the dew point is altered by Raoult’s Law. However, this situation still gives useful information in industrial processes where the Raoult’s Law error can be factored into a repeatable process.

Test Set-up

Figure 3a. (left) Stainless steel drying tower, inlet humid carrier gas (40%RH in air), and X3 chilled mirror sensor are shown. Stainless steel line with compression fittings is used between the effluent of the desiccant tower and the sensor.

Figure 3b. (right) Teraterm used for data acquisition at 5 second intervals. Dew Master chilled mirror control module is shown.

Drying efficiency is measured by passing ambient air at 40%RH and 25^oC through a specially designed stainless steel drying tower filled with desiccant.³ The desiccated air is then sent through a chilled mirror hygrometer and the dew or frost point is measured. See Figures 2, 3a and 3b. The tower is sealed with Teflon O-rings, and the sample is flowed at 2.5 L/min. The drying tower is designed to minimize leakage of moisture from the environment into the system. Stainless steel sample tubing with Swagelok compression fittings is employed to isolate the entire system from ambient humidity.

The chilled mirror hygrometer system is comprised of an X3 three stage sensor and a Dew Master control module manufactured by Edgetech Instruments Inc. The dew and frost points are largely independent of sample temperature.

Because of the very high drying efficiency of CaSO₄ (Drierite), and the cooling limitation of the TEC, an auxiliary liquid chiller is needed to sufficiently cool the CMH sensor to the frost point. At a chiller temperature of 8^oC, the mirror of the CMH sensor can achieve a temperature of -81^oC.

Data and Analysis

The efficiencies for CaSO4, Drierite and Baker Silica Gel are shown in Figures 4 and 5. Pronounced ringing in Figure 5 is due to over potential of the chiller. The time to reach equilibrium is shorter in the case of silica gel because it has a higher frost point i.e higher residuum.

Drying Efficiencies of Calcium Sulfate and Silica Gel

Table 1 compares the drying efficiencies of Calcium Sulfate (Drierite), and Silica Gel (Baker). The relative efficiencies compare favorably with literature values by gravimetric analysis using phosphorous pentoxide,⁴ and thermogravimetric analysis,⁵ although there are differences in the absolute values. This is probably due to systematic errors e.g. incomplete regeneration and humidity permeation from the ambient. CMH has an advantage in that it is a direct means of measuring desiccant efficiency by headspace analysis. Drierite has greater capacity than silica gel at lower relative humidity (%RH).

Figure 4. (left) Efficiency of Drierite measured by CMH. Spike at 5 minutes is due to ABC cycle whereby light intensity is referenced on a clean dry mirror. Maximum cooling potential of the mirror is -81^oC. 

Figure 5. (right) Efficiency of silica gel measured by CMH. Ringing in measured frost point for the first few minutes is due to excess cooling potential of the chiller.

TABLE 1 EFFICIENCY OF DESICCANTS

Conclusion

CMH is an ideal system for headspace analysis. Drierite/CaSO₄ was found to have a greater drying efficiency than silica gel by a factor of more than 100 times. This trend is in agreement with literature values. Drierite has a greater capacity than silica gel at lower relative humidities, which is consistent with Drierite/CaSO₄ being driven by chemical adsorption, rather than physical absorption in the case of silica gel.

References

1. Pimmental, The Hydrogen Bond, W. H. Freeman and Company, 1960.
2. Balkose et al, Applied Surface Science, V134, p39-46 (1998). Tang, Gao, Zhao, ACS Omega, Vol 4, 7636 – 7642 (2019).
3. Hammond Co.
4. J.H. Bower, NBS Bureau of Standards Research Paper, RP649, V12, (1934); Hammond, and Withrow, Industrial and Engineering Chemistry, V25, p653 (1933).
5. Preturlan et al, Industrial and Engineering Chemistry Research, V58, p9596 (2019).

Gerald Schultz, PhD, is Founding Member of Sunwise Turn Consulting, LLC, a science and technology consultancy in sensors and materials science. Schultz has been developing sensors and applications for the measurement and control of humidity for major corporations and innovation companies for more than 35 years. He can be reached by email at [email protected].