High ORP but low chlorine : OK?

[nono240]

Hi there! First of all, please apologize for any approximative English, I'm not a native speaker...

I'm owning 2 residential pools (~10000 gallons non heated, and ~2000 gallons heated @ 92°F), both using unstabilized chlorine, with PH and ORP regulation (I know). I'm using a Pool Lab 2.0 photometer for the readings (Taylor kits are not easily available in Europe).

I'm usually stopping the ORP regulation during the winter, while keeping the PH regulation at 7.2. If algae shows up, I'm just manually adding calcium hypochlorite once in a while.

I did both opening last month, including probe cleaning and calibrations. The ORP was quite low as expected (250-350mV), with zero chlorine and a lightly cloudy water. Little to no algae (mostly in the corners).

PH is 7.2, and CYA is under 10ppm (both confirmed using photometers reading). I'd rather keep CYA low for ORP regulation.

Did the opening shock treatment using Calcium Hypochlorite, the ORP went off the roof as expected and the water turned crystal clear in 48 hours. So far so good, I turned ON the ORP regulation @ 800mV and didn't bother to check the chlorine levels (yet).

Fast forward to yesterday : water is still crystal clear, both pools at ORP ~800mV and pH at 7.2 as expected. I decided to check the chlorine levels in order to fine-tune my ORP if needed : both pools are readings 0.4-0.5ppm only which seems quite low!

Last summer, a 800mV ORP reading would give me anywhere between 2-3ppm instead

I know ORP readings doesn't directly relate to chlorine levels, and an high ORP means high sanitization power.

Since the water is crystal clear (even the heated pool @ 92°F), does it mean that such low chlorine levels are OK as long as the ORP is high?

Why such a difference in ppm versus ORP readings during last summer?

I'm not adding ANYTHING to the water excepted unstabilized chlorine and PH minus.

I've read everywhere on the forum that 1.5-2ppm should be the target, that's what I targeted last year and it worked well.

Shall I instead target ORP and care less about chlorine levels?

Anyway, the maximum ORP point I can set for regulation is 800mV so I'm kinda stuck.


(all ORP and PH probes are verified and calibrated once a week, and I'm using high quality DPD1 photometer tablets from the manufacturer for the readings)

 

[ajw22]

Welcome to TFP.

Chlorine is what sanitizes the pool. You must target and achieve the needed chlorine level regardless of what the ORP is. ORP numbers do nothing for your pool water if the chlorine level is not correct.

Your problems with ORP are why we don't recommend those systems.

 

[nono240]

Welcome to TFP.

Chlorine is what sanitizes the pool. You must target and achieve the needed chlorine level regardless of what the ORP is. ORP numbers do nothing for your pool water if the chlorine level is not correct.

Your problems with ORP are why we don't recommend those systems.

Thanks for taking time to reply. There's so many contradictory articles about it, and the fact that my water is crystal clear despite the high temperature in the smallest pool (92°F) sounded like a confirmation.

Most articles, if not all, I stumbled upon seems to agree that sanitation is in fact the result of an oxydation process. I'm not a biologist, but if a bacteria membrane breaks at 800mV, why would I need more chlorine levels?

https://files.knowyourh2o.com/pdfs/orpdisinfection.pdf

 

[ajw22]

Alternative sanitizers and "chemical free" pools--The Truth!

Pool owners frequently seek out "Chemical Free" alternatives to pool water sanitization. Many claims in favor of these systems and methods have been made (usually by the manufacturers of such systems) often without adequate scientific proof of their efficacy. Let's take a look at them and see...

www.troublefreepool.com

If you want sanitized water, as defined by the EPA, you should know that there are only three EPA approved primary sanitizers -- chlorine, bromine, and biguanide (aka Baqua, SoftSwim, Revacil, etc.). These are your only choices - period. Everything else is a supplemental sanitizer that MUST be used in conjunction with one of these three if you want sanitized water! Why are these the only three approved sanitizers? These are the only sanitizers with fast kill times that also leave a residual in the water for ongoing protection.

 

[nono240]

Okay, nevermind : both probes were just somewhat dead. Bought a new one, and confirmed both were dead... ORP/chlorine levels are now consistent relative to last year observations.

Sorry for the noise.

 

[Newdude]

Glad you have it where you want it to be.