My refractometer has trouble holding its calibration. It'll shift by a couple ppt if I accidentally bump it against something in my cramp room, or even after putting it down on the table for a moment. Arghh. I've used lab quality refractometers back when I used to work in a lab and I don't remember them being so fickle. Is my refractometer a POS that needs to be replaced or is this pretty typical quality for aquariums?