I been testing both NO3 and PO4 with Hannas every other day while I’m tuning some things. I also have noticed an occasional dramatic dip on both on the same testing day. At first I though my tuning experiment kicked in double time only to see the test 2 days later back to the expected curve. This could only mean testing error. What I found was interesting was BOTH had testing errors on the same day.
Theory is cross contamination of residual mixing reagent. I try to rinse many time of course. Then always do 2 fills of the vials in tank water before using the 3rd fill for the actual test. Maybe somewhere in my work flow some undissolved residuals stuck around. One should try and keep the glass separated but bit harder if you’re doing very frequent and a lot of testing. Just a theory for now.
