[MCA]

I can only relate my experience. I had a handleld Hanna and gave up on it. I use a Pinpoint unit that I can clean and calibrate with 400mV solution. I keep the probe also wet by mounting it in the skimmer.

With a calibrated meter that has time to settle down, you may see ORP changing some during the day. If you are getting serious changes, and the meter is OK, then consider that you may be seeing the pH rise and fall. The lower the pH, the higher the ORP...all other factors being equal. ORP is a measurement of the potential (or ease) of moving electons through the water. That is why we measure in mV. Why do car batteries us acids as the solution around the plates? It is easier to move electorns in acidic solutions compared to base solutions.

So why the rise and fall of pH? Two things may be happening. First is low KH in the water. The second is the plants (including algae on the walls and free swimming). Please give off O2 as a result of photosynthesis during the day. But at night they respire CO2...which goes into solution as carbonic aicd...which can put down the pH...and increase ORP.

So with a calibrated ORP meter, check the ORP, pH, and KH in the morning (when ph should be lowest and ORP higherst) and at dusk (when pH should be highest and ORP lowest). See if you see a pattern.