Hello, Now that I have an actual problem (in summer work) I thought I'd take the step from occasional forum lurking to actually post. A customer have recently had some problems with moisture in a tank of insulating SF6 gas, protecting some 300kV electronics. Allow me to outline the construction. The tank is cylinder shaped, 1m[tex]^3[/tex] in volume, generally filled with sulphur hexaflouride at ca 1 bar. The high voltage feedthrough from the transformer is placed in the center and the electronics compartment just above - since it needs to do some work on the HV. Now, this isn't the problematic part. In the bottom of the tank there's a small cooling unit. A fan pushes gas into a short tube, where the gas meets a heat exchanger, fed from the outside with water of temperature T, and then proceeds out through a hose pointing at the electronics compartment. In most (non-tropic) installations T=22[tex]^\circ [/tex]C and then the following procedure works well for gas exchange (for maintenance of the electronics or just because discharges builds up contamination): Pump to 1-10mbar Nitrogen, 1 bar Pump to 1-10 mbar Nitrogen, 1 bar Pump to 1-10mbar SF6, 1 bar That this procedure works well is no surprise - the amount of vapor should be effectively reduced both by the pumping and by the gas replacement. Problems do however occur when the cooling water has lower temperatures and condensation occurs. For example, one installation has been "overclocked" by setting T=5[tex]^\circ[/tex]C, which obviously meant quite a bit of condensed water after a few years of operation. My question is now, for a given amount of condensed water (probably a fraction of which is chemisorbed), how long would I have to pump at a given pressure? I would be grateful for known rules of thumb, or simple models. The physics person in me tried to solve this using Langmuir evaporation, but the results are unreasonably optimistic compared to what apparently did not work. Thanks in advance for your ideas. EDIT: Going only by Langmuir is of course simplified, but I don't know how to tackle the pressure gradients etc. Somehow I feel that experience is more valuable in this case than exact answers, especially given that I don't think we want to dwell too long on this. That's also why I put in the engineering section, hopefully that reasoning is sound.