The history is this: I updated my old refractomer due to rust. I bought a Apex Salinity probe because I wanted real time logging for auto water change monitoring. So I have 3 pieces of test equipment. I calibrated my Apex salinity monitor with provided 53ms solution. (Including floating solution in tank) I calibrated my old marine depot refractometer with distilled water. I calibrated my new D-D refractometer with "Aqua Craft" 35ppt solution. Then I tested them all with my tank water. The Apex reports my salinity very high. Refractomers both say 1.026. Old one is hard to read, but new one within .0005. Apex is reporting 38.7 = 1.029. Ouch!! Apex ranges from 38.3 to 38.9 : 1.0289 to 1.0293, which is great, so it is not drifting. My questions: 1) Which should I trust more? Refractometer or Electronic? 2) Any recommended certified test solutions? 3) I do not have a temp probe on the salinity Apex module. Should I?