Like many others on the forum, I have the Extech Exstik 100.
It needs to be checked for accuracy and potentially re-calibrated before each make. It also needs to be kept clean. Because the milk is fatty, I swish it in soapy water after each testing of the milk, then rinse and wipe dry and recheck in a bit of 7.0 solution. Periodically I do a deep cleaning. If you search on the Exstik 100 here on the forum you will find care and cleaning suggestions.
I have also found the company to be responsive when I have contacted them directly.
