Hi all. I purchased the electronic pH tester at the same time I got the TH-100 test kit. The pH tester keeps losing the calibration, and needs to be recalibrated weekly. Each week it will be at least 1.0 off, either high or low. Is this normal? I use the tester daily, rinse it with distilled water after each use as instructed, and keep it stored inside the house - so dry, warm, and out of sunlight.