Chlorine - PPM high but ORP very low

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azstar
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Chlorine - PPM high but ORP very low

Postby azstar » Thu 30 May, 2019 03:19

Hi All,

I'm in australia, we use PPM to measure chlorine, and my pool is currently sitting at 5ppm.. however I have 2x different orp testers that both give me very low readings (115mV)

Pool is clear and looks good, I am really trying to understand why the ORP would read so low when its meant to be around the 700 mark...

PH is 7.5 currently, I do have CYA in pool but at last test about 40ppm... I have a salt chlorinator that produces the chlorine...

What am I missing? why is my ORP so low and it my pool about to turn on me?

Have googled and i know CYA has a effect but shouldn't be this much right?


Teapot
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Re: Chlorine - PPM high but ORP very low

Postby Teapot » Thu 30 May, 2019 06:28

CYA throws ORP readings off. Is the ppm reading accurate?
azstar
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Re: Chlorine - PPM high but ORP very low

Postby azstar » Thu 30 May, 2019 23:45

Hi, thanks for your help.

I just went to a pool shop and current values are:

Free chlorine: 5.4, Total chorline: 5.6

PH: 7.5
CYA Stabiliser: 33
TA: 64
Calcium: 114
Phosphate: 0.2
Salt: 4100

ORP now reading in the 200's but still very low

Thanks
aaron
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Re: Chlorine - PPM high but ORP very low

Postby Teapot » Fri 31 May, 2019 01:10

Ok Aaron, two possibles,.
1. when did you last clean the ORP probe?
2. When did you last change the ORP probe?

You can buy ORP test standards liquids. I would give the probe a clean with clothes washing detergent and a soft brush. Test using a standard and somewhere there should be calibration adjustments.
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Re: Chlorine - PPM high but ORP very low

Postby azstar » Fri 31 May, 2019 07:48

Hi,

Have two orp detectors.

One is brand new ‘ico’, opened yesterday. And the other is a blue pool mate tester which I haven’t used in a while but gives similar results.
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Re: Chlorine - PPM high but ORP very low

Postby Teapot » Fri 31 May, 2019 10:55

Hence why I recommend a test standard, something is not right.
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Re: Chlorine - PPM high but ORP very low

Postby david_oz » Fri 26 Mar, 2021 23:22

Hi, I know this is an old post, but I was wondering if you solved the mystery. I have a very similar issue. My new very expensinve controller uses the orp reading to control the chlorinator. I have to override it to stop the ppm going through the roof.

Cheers David
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Re: Chlorine - PPM high but ORP very low

Postby Teapot » Sat 27 Mar, 2021 04:34

Same question, CYA messes with the orp. What are your levels?
Have you a test standard to check your equipment?
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Re: Chlorine - PPM high but ORP very low

Postby Teapot » Sat 27 Mar, 2021 04:35

david_oz wrote:. I have to override it to stop the ppm going through the roof.

Cheers David

I also do this on mine BTW.
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Re: Chlorine - PPM high but ORP very low

Postby david_oz » Mon 29 Mar, 2021 08:40

Hi Teapot, I understand pH and CYA can affect the ORP reading but that doesn't seem to fit my pool's chemistry. There must be some other factor.

My concern is that the ORP reading indicates how effective the free Chlorine is at oxidising/sanitising. Does the low ORP reading or the high PPM suggest the water is unsafe to swim in?

Code: Select all

Total Alkalinity      99 (low)
pH                    7.2 (good)
Free Chlorine         8.6 (very high)
Total Chlorine        9.1 (very high)
Calcium Hardness      245 (slightly high)
CYA                   20 (slightly low)
Phosphates            0.1 (good)
Salt                  ?(High)

ORP = 450

I increased CYA and Alkalinity per shop recommendations, but it had little effect on ORP. I did a partial 25% drain to dilute the salt, but the ORP reading fell.

Our mains water is quite hard. With winter coming up, I'm going to top up the pool with rain water.

The installer tested the probe with a calibration reagent and it correctly measured 700mV. The manufacturer won't return my emails, so I'm a bit lost. I'm approaching the point where I want to rip the controller out and return it.

Cheers
David
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Re: Chlorine - PPM high but ORP very low

Postby Teapot » Mon 29 Mar, 2021 12:56

I know how you feel about ripping out the controller but dont get too hung up on things.
If your salt is correct and your CYA at 50-70ppm depending on what your chlorinator manufacturer recommends then with a good test kit not dip strips. Just set your CYA to free chlorine 7.5% eg 50ppm CYA requires 3.75ppm free chlorine to sanitise. 50x7.5% = 3.73. And note the orp reading and try to keep somewhere near that. You need to check with a good test kit and forget the ORP,
Sadly ORP is only really accurate for indoor pools without CYA interference. Thats why the later equipment uses amperometric testing.
Depending how technical you are and with details of the probe at varios ORP levels you can check the millivolt output but all this is a long way from having fun with your pool. Just stick to the basics, I dont waste to much time on these details anymore and as said before re calibrate the equipment to keep it on track.
I would not have said your TA was low, if your pH keeps rising lower the TA a bit and ignore most of what a pool shop says. Also the pH doesnt really matter either when CYA is present, so long as its not to low say 7.2 min. It can go up to pH8 and still be fine provided the free chlorine to CYA ratio is maintained. What is your pools construction, Liner, tiles etc?
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Re: Chlorine - PPM high but ORP very low

Postby david_oz » Tue 30 Mar, 2021 10:38

I think what I'm most upset about is spending a truck load of money on a controller to avoid frequent water testing but finding it completely ineffective.

I'll try maintaining the CYA and free chlorine ratio like you suggested and make the ORP reading the setpoint. I still don't understand if all things being equal why my ORP is low, but it sounds like I just need to live with it.

Sticking with the basics is good advice. I remember as a kid, we only ever tested for chlorine and occasionally pH.... and that was good enough.

I have a fibreglass pool.

Cheers
David
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Re: Chlorine - PPM high but ORP very low

Postby Teapot » Tue 30 Mar, 2021 12:11

Its the effect of the CYA on the ORP probe, annoying true, but I work around it now.
With a fibreglass pool you can ignore the industry stated TA figure, my vinyl pool I run about 40-45ppm and haven't added any pH minus in 2 years, the pH remains pretty much rock steady despite 8 adults larking around. The pool top up water is TA 180- 220 ish so I am careful not to loose water, getting the TA down was a bit of a pfaff but I did it. Just be aware that chlorine pucks are acidic and if you run low TA like me the pH will move much easier as there is less buffer solution.

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