yes that would work when the weir is at or near the normal range and would be more accurate and easier to implement than other options.
However the river in question can, and often does, rise a metre or more over the weir when there is prolonged heavy rain.
That would completely submerge the unit and I doubt it would survive.
The ‘propper’ solution to this problem is a submerged pressure transmitter but I cant afford the £250-£300 that they cost.
Probes, provided that they aren’t fed with DC which would electrolyse them, is probably the way to go I think, based on comments and suggestions.
That said I ran a couple of simulations and attempting to do it without any external amplification / isolation I think is a plan destined to yield an unstable result.
I have decided to go with the attached circuit …
The probe current is symmetrical due to the capacitive decoupling.
The 100K on the right is the input impedance of the I/O pin.
The 50K and the switch represent the water.
The output is about 4.3V with about a 100mV ripple when the probe is wet and a few mV, after a short decay time, when the probe is dry.
A second comparator would get me to 5V and 0V respectively but I don’t think it is necessary given the input characteristics of the Arduino
I would appreciate a critique if anyone thinks I have missed anything.