Sears et.al. are very careful scientists. I'm sure they took into account the low amounts of water vapor in the Martian atmosphere in designing their experiments. This particular chamber for example was designed to also be used for simulating conditions in space to determine rates of production organic chemicals in interstellar clouds. Obviously you don't want the amount of water vapor in such simulated diffuse clouds to be the same as on a water world such as Earth.
Also this experiment just confirms what other scientists have found when they actually did an *experiment*, rather than rely on calculations that gave highly varying results.
In short ALL the experiments that have been done to see if liquid water can exist on Mars show that it can.
This fact means something to those who know that science is dependent on experimentally verifying the conclusions of your theories.

"Experiments under Martian pressures and temperature cycling, and with a variety of wind velocities, performed in the laboratory of one of us (C.S.), tend to support the conclusion that thin layers of liquid water are generally formed each morning at tropical latitudes on Mars when diffusion is limited. A variety of experiments (23) indicates that many terrestrial microorgansms can grow, even if liquid water is available for only 15 minutes
each day, in otherwise subzero conditions; or in soil
microenviroments where liquid water is available as a
consequence of freeze-thaw cycling."
"Contamination of Mars"
by Carl Sagan, Elliott C. Levinthal, Joshua Lederberg,
Science, 15 March 1968, vol. 159, no.3820, p. 1191-1196
http://www.profiles.nlm.nih.gov/BB/A/B/J/H/_/bbabjh.pdf

Martian Outflow Channel Formation by Water Erosion Under
Nonequilibrium Conditions.
Abstracts of the Lunar and Planetary Science Conference, volume 20, page 666, (1989).
"An experimental study was carried out in a 2.4m by 4.9m cylindrical vacuum chamber in order to investigate the possibility that martian outflow channels were formed by the melting of ground ice. Frozen soils that were subjected to an increased geothermal gradient at
temperatures about 273K and at an average pressure of 8.38 mbar, melted and formed water-filled depressions and outflow channels. The depressions formed as a subsurface ice layer thawed. Water initially appeared at the surface either during gradual collapse of the surface material or as geysers. The mode of appearance of water at the surface
was a function primarily of the strength of the ice and the
temperature of the water vapor."
http://adsabs.harvard.edu/cgi-bin/bib_query?1989LPI....20..666M

MARTIAN GULLIES AND THE STABILITY OF WATER IN THE MARTIAN ENVIRONMENT.
J.Thompson1, D.W.G. Sears2, P.H. Benoit2, and M.S. Kareev2. (1) Arkansas-Oklahoma Center for Space and Planetary Sciences, Dept. Physics, Univ. Arkansas, Fayetteville, Arkansas 72701. (2) Arkansas-Oklahoma Center for Space and Planetary Sciences, Dept. Chemistry and Biochemistry, Univ. Arkansas, Fayetteville, Arkansas 72701
Lunar and Planetary Science XXXIV (2003) 1035.pdf
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1035.pdf

Bob Clark