is there a way to calculate evaporation through tubing?

I have my first loop running very well, and finally leak free after a radiator barb need some RTV. I'd say I have maybe 4' of 1/2" ID silicone tubing. I know I will have to be topping off my res more often than with other kinds of tubing. I'm using a mcres-micro from swiftech, which doesn't hold too much water. Even in this first week it seems to have gone down a couple mm. Anyway, what kind of loss can I expect over a month? (ounces, mL, whatever) Is there a way to caluclate this with a formula?

and Is there a chance this could be due to a leak that is slight enough to not form a droplet anywhere? I have used a high intensity flashlight to look for any sign of wetness, but I haven't found any. It's been running for about a week non-stop without any noticeable leak.

Thanks!

You might have tiny little bubbles that bleed out for a few days which might drop the water level a little like what you are describing. But, at some point within a few days you should stabilize. You might have a little loss over a long period of time, but it really shouldn't be too much. I used to check my reservoir daily, then weekly, but now I might glance at it once a month. I haven't topped off my coolant in at least 3 months.

is that with silicone tubing, too? huh, I got the impression I would have to top it off like every month.

I had thin walled silicone for a while, I had to top that thing off all the time. I am guessing you are using thick walled so you wont have too much trouble. Dont worry about it too much, just comes with the tubbing your using, and that tubbing is the best IMHO.

yeah, i don't mind some loss at all. I just want to know what loss is attributable to silicone tubing, or if I might be losing so much that I may have an unseen leak.

Had to look to see what I had. It's PVC vinyl tubing.