What product do you dose?

[gbroadbridge]

What do you mean by hanna lab grade rodi?

A lot of the problems folks experience with the Hanna Ca checker are due to their RODI water used to create the blank has some impurities.

If you have a perfect RODI system it's not a problem, but it only takes the tiniest amount of impurity to result in a incorrect Ca reading.

Easiest solution is to use a certified lab grade RO/DI which you can obtain from many chemistry suppliers including Hanna ( HI70436 for example)