Unless you can get a totally sealed system, (and even if you do) some water loss is going to happen. The trick is, to have the system monitored for the water content, and the basic testing (R.A. - C.I.C.T. - and viscosity) to ensure the properties of th fluid will not allow degradation of your system components. Once you see the water "loss" you can get a good estimate of how often you should add (de-inonized) water to recharge the system and keep it operating at peak performance. Note: If you add more water to the system you should add whichever vapour-phase corrosion inhibitor your suppliers uses to keep the levels proper and avoid rusting oxidation etc.
I hope this helps a little.