hi Martin
We have been working on two version of a phmeter with @pablocremades @Laraja and others from reGOSH. One mobile and another more simple lab bench version.
CoSensores have a few other devices to test water already working. Look here
You can look at Hacking Ecology for in field continuous monitoring
There is also this old thread with lots of information on water monitoring from @Markos and a great list of sensors from public lab from @gretchen I think
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