At my school we have decided to try and construct a water barometer, which will work the same way a mercury barometer does, but will be 10+ meters tall. Once set up, the "vacuum" part of the barometer (above water level) will be filled with water vapor, and we can correct for the vapor pressure readily enough. However, of more concern are other gasses, particularly oxygen, outgassing into the "vacuum" at the top of the barometer. We will address this at first by using degassed water, but eventually, since the bottom of the water column must be open to the air, the water will dissolve oxygen (and, presumably, other gasses), some of which will sooner or later make it up into the "vacuum". My question is this: is there any way to calculate the rate at which oxygen will make its way from the air outside, up the water column, and into the "vacuum"? I have no sense as to whether it would take minutes, days, or years for enough oxygen to be transferred to affect our atmospheric pressure measurements. Any ideas for retarding this process? Thanks!