Hi - I've been reading this Forum over the last few weeks and have got a lot food for thought from it.
However I have not been able to get my head around the behaviour of my pool water regards ORP and pH.
Firstly here is the current details etc:
pH is auto controlled to 7.4-7.6 via 32% HCl
CYA:26, TA:80-100, CH:200-250, Salt:4500, Temp:28-32degC(82-90degF),
Pool is a 55kL plaster finish, 24g/h SWC, pool blanket is on almost always except 2-3 h/day for swimming on good days.
The pool water looks clear - however I have had some "black spot" algae starting to show over the last few weeks.
I got excited a few weeks back and bought a Hanna Combo pH/ORP/Temp (I like the green colour!) and have been doing some testing to see if it is feasible to control my SWC on ORP.
After digesting the impact of the CYA on the effective HOCl (and realising that I needed higher FC levels than the pool shop tells me!) I started experimenting with the ORP vs FC (I took a pool sample at low FC and added diluted Cl and HCl progressively to map out ORP vs FC at constant pH). This went really well at first but I am now noticing that the ORP of samples is sometimes depressed significantly.
For example, with a FC of around 6-8ppm (DPD) in the pool around midday I got an ORP reading of 435mV at 30degC and 7.47pH. What is strange is that testing this sample on the bench inside shows that the ORP and pH progressively rise over a number of hours at about +50mV/h and 0.08pH/h. Eventually after about 6 hours the readings stabilised at 680mV and 7.95pH. Temperature of the sample dropped in the first hour to 27degC and stayed about there for the rest of the duration.
I initially blamed the Hanna instrument and sent it back but it has checked out OK and I get good stable readings of other things (like tap water and pH buffers) in 10 minutes max. For these readings I tended to leave the probe in the water for the duration so the readings should have been OK throughout.
Given the pH seems to rise along with the ORP - I am suspecting that this is a real chemistry going on rather than an instrumentation issue. However, I am struggling to understand what that chemistry may be.
Would love to hear from anyone who can pick what is going on!
PS While I am here - my pool blanket instructions say that the chlorine level should not be higher than 3ppm in order to not degrade the plastic. However the FC-CYA charts indicate that it would not be unusual with an SWC and 60-80ppm CYA to require > 3ppm as a matter of course. How should I interpret the manufacturer's statement? Should it be the equivalent of 3ppm FC at the associated CYA level?