This is a known problem with the hannah calcium meter. First, verify that you are adding .1ml of sample and not 1ml of sample. The syringe include is really too large for the resolution needed to accurately add .1ml, so some insulin syringes from the drug store will help considerably, just watch out for the needle. Hanna claims they have tried to source a smaller syringe, but can find nothing under 1ml without a needle and they can't include a syringe with needle in the kit. The other problem is with our RODI water. Hanna believes that it may have more mineral content than we believe and this may be skewing the results. This can be verified by using lab grade DI water. I have not been able to find lab grade DI locally to me and most distilled water still lists a small mineral content, but I was able to find some that shows no mineral content. I rinse everything in the distilled water before I start the test, use the distilled water for the test and also use the insulin syringe and my results now appear to be accurate.