A portable hygrometer has been devised to implement a pre-existing technique for detecting water trapped in partially enclosed volumes that may be difficult to reach and cannot be examined directly. The technique is based on the fact that eventually the air in such a volume becomes saturated or nearly so. The technique is straightforward: One measures the relative humidity and temperature of both the ambient air and a sample of air from the enclosed volume. If the relative humidity of the sample is significantly greater than that of the ambient air and/or if the sample is at or close to the dew point, then it can be concluded that water is trapped in the volume. Of course, the success of this technique depends on the existence of an access hole through which one can withdraw some air from the enclosed volume.

The portable hygrometer (see figure) includes (1) a commercially available small electronic temperature-and-humidity sensor of the “humidity stick” type, (2) a flexible plastic sampling tube with a suction cup at its inlet, and (3) a commercially available sampling pump, the air-intake manifold of which is modified for coupling to both the outlet of the sampling tube and the sensory tip of the humidity stick. The total cost of these and ancillary components was about $1,300 in 2003.

This Portable Hygrometer was assembled from commercially available components and materials.

At the beginning of operation, the inlet end of the sampling hose is positioned to collect ambient air and the humidity stick and the sampling pump are turned on. After allowing about 20 seconds for the humidity stick to equilibrate with the sampled ambient air, the temperature and humidity readings of the humidity stick are recorded. Next, the suction cup is placed over the access hole to withdraw air from the enclosed volume. If water drops are observed in the sampling tube, then there is no need for further sampling, and the sampling pump is stopped immediately to avoid drawing liquid water into the humidity stick and pump. If water drops are not observed in the sampling tube, then the relative- humidity reading is monitored until it reaches a maximum (usually after about 20 seconds), at which time the relativehumidity and temperature readings are recorded.

The suction cup is removed from the access hole and after about 30 seconds for equilibration, the temperature and humidity readings for ambient air are taken again. The suction cup is again placed over the access hole and the air from the enclosed volume sampled again to obtain second temperature and humidity readings to confirm the first readings. Because some ambient (presumably drier) air could have entered the enclosed volume between the first and second humidity readings, the second enclosed-air humidity reading could be lower than the first one.

This work was done by Robert C. Youngquist of Kennedy Space Center and Jan Surma and Steve Parks of ASRC Aerospace. For further information, contact the Kennedy Innovative Partnerships Office at (321) 867-1463. KSC-12593


NASA Tech Briefs Magazine

This article first appeared in the December, 2005 issue of NASA Tech Briefs Magazine.

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