I find if you leave nutrients over night the salts start to fall out collecting on the bottom.Went down to the garage this AM and for giggles I turned on the pen, and it worked!! I'm now a little skeptical of it's accuracy, but I know the PH of my plain tap water and it was right on, so I tested the water and they got a drink. When the new one comes, I'll calibrate the new one and make sure the readings match between the two. I did let the water/nutes sit over night, and it looks like I got a little precipitate at the bottom of the jug.
This adversely effects the pH as well.
I started aerating the nutrients an hour before using them.
I had 700 litre capacity, so this became something of a concern.
So I aerated it an hour prior and set the pH just prior to feeding each time.
As I burned through pH down I decided to pre mix less nutrients and not keep any nutrients longer than a couple days.
This seemed to improve the necessity to keep phing.
But in coco pH was vital.
Take care.
Stay safe
Bill284