I am just wondering if the fluid is always 30ppt, even when say used 30-40, times or if some has evaporated over time? Does the solution get stronger? Or will it always be 30ppt? My bottle is almost 1/2 empty, I always ensure that it is tightly closed after each use, I have to calibrate every time i turn it on because it never holds its calibration, usually 1-5 ppt off. Question make sense?