If we don't consider the graphs at the moment, but just the values the thing produces, we could use one meter to hang for a few days in one system, a few days in the next and so on. I have a feeling this meter is very slow - why wouldn't it - because water changes are very slow as well. You have to soak it for 24 hours before using it, so I think it will give the right values only after 24 hours, so it would have to be in one system for a few days is my guess.