ORP value crash

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S

saarinensa

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Oct 3, 2015
9
Pyhtää, Finland
HI,

I am really hoping that somebody experienced with water chemistry could advice me what happens when ORP values are crashing after increasing inhibitor to the pool.

I am having 6 000 litr pool / spa with automated ORP control (Spaboy) and Onzen SWG. The water to the pool is coming from the drill well so according to operators manual I am adding every other week Arctic Best Defense stain and scale inhibitor (methylene-phosphonic acid).

The problem is that every time after adding the methylene-phosphonic acid the ORP values are crashing at the pool and this situation takes days to come back normal. ORP value is even going below zero and somehow the software stops sanitizing at that point. I have tried to test free chlorine amount but because of the really low natural (0,2-0,5) level of free chlorine (system is designed that way) it is almost impossible to tell if there is sufficient amount of chlorine or not. Eventually the water turns cloudy but phenomenom is something that is also described on inhibitors label.

What I would like to know is what really happens to pool water when you add methylene-phosphonic acid on it? Does acid "eat" all the free chlorine away or is it just mixing the ORP-meter readings?
 
It may be that the inhibitor is directly interfering with the ORP probe fouling it. That's one of many problems with using ORP sensors. They are finicky and affected by things that have nothing to do with disinfection or active chlorine levels themselves.

You should see if you can find yourself a FAS-DPD chlorine test kit so that you can test down to 0.2 ppm with 0.2 ppm resolution. Can you get the Palintest FAS-DPD kit in Finland? Or the Taylor K-2006 or K-1515?

I belive the methylene-phosphonic acid being used is actually aminotris(methylene phosphonic acid) or ATMP and it probably gets oxidized by chlorine over time, but I don't know the rate. You could easily experiment by putting small proportional amounts of the product and chlorine in a clean container and see how quickly the chlorine drops. You can use a higher Free Chlorine (FC) level readable by your test kit so you can see the rate of change. Obviously have a control container without the ATMP but with chlorine for comparison. Keep both out of sunlight.
 
Thank you for the answer chem geek. I already have Pooltester DPD kit which tablet reagents should measure free chlorine amounts at 0.2 ppm resolution. Unfortunately for me the color changes in low chlorine amounts are so minor that it is hard to make the difference.

I have to try that test you mentioned with chlorine water and inhibitor. Hope it will sort things out for me. :)
 
Maybe the chemical in the inhibitor is combining with the chlorine to form a product like dichloromethane or perhaps the product contains a chemical such as Sulfamic acid that can lock up chlorine.

Have you been testing for combined chlorine?
 
Yeah... I have also tested for combined chloramine but according to TF Test Kit there are "none". :(

One issue is the factory setting for ORP control which is 550. I simply can not change those settings manually and somehow the manufacturer also have issues to increase the value to requested 650 via internet. Arctic Spas are trying to figure out why it is like that. :(
 
Hi Olof,

I used prefilter when I filled up the pool but didn´t measure the effects to the water. For the Best Defense part I am even not sure if need to use that one because water in our well is extremely soft.

Anyway, today I made slight experiment how the water reacts to the inhibitor (thanks once again chem geek) and it turned out at after 12 hours the inhibitor water was chlorine free while the other sample still had some chlorine left. This weekend I am going to repeat that test and make the measurements little bit more frequently. ;)
 
Yes, please do that experiment. It does sound like perhaps chlorine does oxidize the inhibitor. We know it does for chemicals like EDTA that are not that dissimilar to ATMP except that the latter has phosphate groups while the former does not.

You normally don't need to use stain and scale inhibitor unless your water is high in calcium or metals (e.g. iron) as might be the case when using well water.
 
Repeated the previous experiment and the chemical reaction was quite rapid.

Started out by filling 5 litr plastic bucket with clean water and increased the FC level to 7.5. After that I took away the reference sample and added 3-4 drops of methylene-phosphonic acid (Arctic Best Defense). When I tried to make FC measurement for 0 minute mark I found out that the sample with inhibitor was already chlorine free while the reference sample still showed FC 7.5.

It seems like the inhibitor eats away all the free chlorine immediately but the question is how long does the effect last? According to pools ORP readings it seems like few days because after that time interval the readings are slowly getting up again when the pool is all the time sanitizing.

Personally I think I´m going to stop using that inhibitor at least on regular bases. According to old test laboratory report (newer tests are taken after the water filter) the Fe amount on drilling well water is 0.118 mg/litr. Ca on the other hand is 4.16 mg/litr and total hardness 0.887 dH.

Edit: According to ORP meter it seems like using the inhibitor has one benefit. I have tried to lock the pH with right TA level but like on all SWG:s the pH has been slowly (or little bit less slowly) creaped up all the time. Borates are something that are little bit harder to get in Finland so I have been satisfied with TA "lock". Anyway, now after few times using the inhibitor it seems like the pH is locked somewhere between 7.25 - 7.30. According to PoolCalculator the CSI is now -0.93 so maybe that is not so good thing after all?
 
Some ph stabilizer products contain phosphate. Are you using a ph stabilizer product? How much inhibitor are you using?
What is the water temperature? How often do you replace the water?
What other chemicals are you adding?
 

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pH stabilizer = negative
Inhibitor amount according to label instruction = every second week 10 ml / 1 000 litr = 60 ml
Water temperature = 32,5 C = 90 F
Water replacement = new spa so water is only 1,5 months old. According to owners manual water replacement ever 6-12 months.
 
How often should I change my spa water?

The replacement interval will largely depend on the bather load. 6 to 12 months is too long for a public spa. Also, a 0.2 to 0.5 ppm fc is pretty low and can be depleted fairly easily. What is the spa surface (acrylic, plaster, fiberglas etc)?

What is your typical bather load per day?

I think that I would skip the inhibitor. Perhaps switch to a regular hedp based sequestrant if necessary.

Can you increase the fc some?

Does the spa contain cya?
 
Spa has a fiberglass reinforced, Lucite® cast acrylic shell. Swimspa is only in a family use so bather load depends quite a lot how often the kids are willing to go for a swim. For an average I would say about 5-7 hours per week with three small bathers.

I have intentionally added some dichlor to increase spas CYA level. Level is right now between 20-30 and I have switched dichlor to normal bleach.

Increasing the FC is a tricky one because Spaboys ORP-meter controls the SWG:s operation and the factory setting for ORP is 550. I can´t change ORP-level manually and somehow the manufacturer is also encountering some problems with adjustment. I really hope they are working on that problem so they could comply my request of ORP level 650.
 
With acrylic, there's no need to saturate the water with calcium carbonate. By keeping the calcium and TA levels low, you should be able to avoid scaling without resorting to sequestrant. I would suggest that you discontinue the use of sequestrant and the ORP controller. The CYA will reduce the ORP reading. I would suggest that you adjust the SWG to maintaine about 3 to 5 ppm fc.
 
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