## Evaporation of a mostly sealed erlenmeyer flask

I am trying to determine whether the evaporation rate of a 3L erlenmeyer flask would be negligible given the following circumstances. The flask would have a rubber stopper in it, with a hole approximately 1mm in radius greater than the steel drip line feeding into it (leaving an open ring that has an outer diameter of ~8mm and an inner diameter of ~6mm). I would like to take realtime mass measurements as the drip line does it's thing. The process would span several days, so evaporation has to be taken into account. The process is drawn roughly here:

Thanks a lot!

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 Mentor I would expect ~100% relative humidity inside, with a water density depending on the temperature of the environment. If you add a volume of X, the same volume of (humid) air leaves the flask and takes away some mass. Apart from that, it is probably hard to evaluate, as the air movement inside+outside is important. Can you reduce the gap somehow? If you do not have to measure the weight continuously, maybe you can close it most of the time. Can you determine it experimentally? Just fill it with your fluid, wait one day and compare the mass?
 Ahh, yes, I was thinking of the evaporation rate in a static system. But since we'll be adding water to the flask (at a rate less than 1mL/min), each drop will push out a corresponding amount of humid vapor. Good point mfb. I worked on minimizing the gap yesterday, once I've completed that, I will experimentally determine the difference between static and dynamic evaporation. Furthermore, I'll have to find the function that relates the volumetric flow rate in and the evaporation rate. Thanks!