I am working for a glassware company. Recently I found that while calibrating volumetric flask concave meniscus of the water forms when we first time fill the flask with distilled water @ 27 deg C & we put a mark. When again we check the flask next day a prominent concave meniscus doesn't form. Also the correct amount of water level comes at some other place slightly away from the mark that we put the day before. Can anybody answer what the problem could be.