Switched To Swg

[Pool-newb]

Well, I took the plunge (pun ignored) and purchased an SWclG yesterday. The install was simple and probably the fastest major change I ever made to the pool, finishing it within about 4 hours.

I got the SwimPure Plus with T-CELL15 (for up to 40 000 gals) . Probably way overkill for my small pool but I wanted some reserve and didn't want to have to replace the cell anytime soon.

First thing I found out was that I only needed four 40 lb. bags of salt to get get to 3300 ppm. I now assume that my pool is smaller than 8-8.5k I had calculated. It is rectangular and the calculated measurement of 12*24*4*~7.4 is probably off because the deep end has a slope down to the drain. There may also have been some salt buildup from the use of bleach prior to now. I'm sure glad I checked before adding the last two recommended bags.

I also upped the CYA level to just over 60 ppm.

Because the cell is so over dimensioned I set it to 20% and let the pump run overnight. Nine hours later (this morning at 7) the fc level had actually dropped from 3.5 to 2.0 with a cc hovering around 1.0, and ORP had dropped from >670 to around 300. I added 4 cups of chlorine and set the SWG to 75.

At 8:46 the cl level had jumped to 5.0, cc was hanging at around 1, but the ORP had bottomed out to near zero, fluctuating between about -50 and +150. Most of the higher readings I got with the meters in a plastic measuring cup, the lower readings were with the probe directly in the pool.

From what I have googled I believe this low ORP reading is caused by hydrogen outgassing but I never imagined it could push the ORP so low.

Btw, the pool has been solar covered since I completed adding the salt and vacuumed it fairly clean yesterday evening. There was some green algae stuck to the walls but I'm assuming it was dead (or?) because the water was very clear and I have maintained the cl levels since last wall brushing a week or two ago. I have also not been attempting to remove the cc because I was assuming that it was coming from the sequestrant added to keep the metal stains from reappearing.

I also have two identical pH/ORP/temp (Hanna handheld, one about a year old, the other about 1 week) meters. I was having problems with my original meter that I thought were being caused by having dropped it, but after cleaning and calibration I find that the meters reach "similar" ORP values with the new meter reaching the value faster, and usually being about 25 mv higher. Both meters show nearly identical pH values (± .01), and have about 0.4-0.5°C temperature difference).

Questions - is it really possible for ORP indication to be pushed so low in a pool with an SWclG?

Do I really need to keep the cl high (shock levels?) until the cc goes away?

Can cc be caused by metal sequestrant?

Can algae thrive despite cc levels above 1.5-2 ppm with 30 ppm CYA?

What else did I miss?

[chem geek]

Questions - is it really possible for ORP indication to be pushed so low in a pool with an SWclG?

Do I really need to keep the cl high (shock levels?) until the cc goes away?

Can cc be caused by metal sequestrant?

Can algae thrive despite cc levels above 1.5-2 ppm with 30 ppm CYA?

What else did I miss?

Keeping the Free Chlorine (FC) higher will normally reduce the Combined Chlorine (CC) faster, assuming that the source of the CC isn't getting introduced at a greater rate as well. However, depending on the type of CC (i.e. if it's monochloramine vs. an organic chloramine), a higher FC will result in more irritating nitrogen trichloride. If your pool is outdoors and able to be exposed to sunlight, then open the cover since the UV in sunlight will help to break down CC faster and it is also better to air out the pool in that situation (i.e. let outgassing occur more freely). In a properly managed pool with sufficient Free Chlorine (FC) relative to the Cyanuric Acid (CYA) level, the CC should be minimal and get continually oxidized without the need for shocking. It is only in pools with much higher bather loads and indoor pools that have more difficulty with CC.

High levels of metal sequestrant do sometimes show up as producing an additional chlorine demand and depending on the sequestrant can also show up as CC. For example, EDTA could oxidize to compounds that then combine with chlorine. Other types of metal sequestrants, such as phosphanates (including HEDP) would not form CC.

If the FC is high enough relative to the CYA level, then algae will be killed by chlorine faster than it can grow regardless of the CC that is present. In an SWG pool, the FC should never be lower than 4.5% of the CYA level.

Hydrogen gas bubbles from the SWG can saturate the water with hydrogen and this can affect ORP sensors. I've never heard of it lowering the ORP by as much as you are seeing, but it definitely does affect the sensors. However, the ORP value in this situation does not reflect the sanitation of the water which is almost completely dependent on the hypochorous acid concentration and that is dependent mostly on the FC/CYA ratio and pH.

Read the Pool School including Water Balance for SWGs for more info.

Richard

[Pool-newb]

Thanks again Richard for the response

Keeping the Free Chlorine (FC) higher ...

This is getting very irritating. I had no serious problems last year once I got things balanced, this year it just is not working.

I left the SWG at 20 % overnight and still lost 1.5 ppm in 10 hrs.

During the entire day i have been testing different settings on the controller to get something that will keep up with demand:

- 07:00: cl 2.0, orp ~300 - raised generator to 75%

- 08:46: cl 5.0, ORP ~50-100, lowered generator to 50%

- 13:09: cl 4.75, ORP ~60, left generator at 50%, removed cover but day was only about 10% direct sun, lotsa clouds & shade.

- 16:07: cl 1.25, ORP - low, saw faint stains coming back so added one full dose EDTA, moved controller to 66%

- 18:06: cl 1.00, ORP negative, moved controller to 80

I was under the assumption that the percentage adjustment would adjust the current through the cell. It doesn't. There seems to be some kind of duty timer so that it cycles on and off for longer or shorter periods. While I was adjusting it I noticed the "click"point was around 70-80%, and I could hear the relays and see the current reading rise or drop to 0 while adjusting the level.

Read the Pool School including Water Balance for SWGs for more info

.

Yeah, I'd sort of forgotten what was happening on the SWG section so I reviewed.

When I added the borates a few weeks ago I also added a bit too much muriatic acid so that has left my pH low. I didn't worry too much about it because I figured the bleach addition would raise it enough over time, and I wanted to keep the pH down a bit to prevent staining.

It didn't and because my TA is at 100-110 I am going have to raise pH/lower TA it with aeration.

My CYA was also low at about 30 ppm but I added more last night to bring it up to 50-60 ppm.

High levels of metal sequestrant do sometimes show up as producing an additional chlorine demand and depending on the sequestrant can also show up as CC. For example, EDTA could oxidize to compounds that then combine with chlorine. Other types of metal sequestrants, such as phosphanates (including HEDP) would not form CC.

The stuff I am using is listed on the MSDS as "30-60% EDTA. I've been trying not to add much because I was convinced that was causing the problem with CL demand and the cc increase. I had added maybe ¾ the proper dose last week, trying to keep it low but today, once again, the stains are returning.

Hydrogen gas bubbles from the SWG can saturate the water with hydrogen and this can affect ORP sensors...

I've removed the cover to help "air it out" but the ORP is going lower if anything.

./.

It looks like my only option is just to shock the heck ( = EDTA) out if and ignore the stains until I get the liner - and with it the water - replaced.

Are there any ideas as to what could be causing the fluctuating cl load and is this related to the strange ORP values?

Are these two even related?

Do I just still have algae from when there was no cl?

Does this perhaps have anything to do with the ascorbic acid treatment?

Could there be some other stuff in the water that is causing all of this?

[chem geek]

Are there any ideas as to what could be causing the fluctuating cl load and is this related to the strange ORP values?

Are these two even related?

Do I just still have algae from when there was no cl?

Does this perhaps have anything to do with the ascorbic acid treatment?

Could there be some other stuff in the water that is causing all of this?

Don't know about the fluctuating chlorine load. The only "normal" fluctuation is during the day from mostly from breakdown from sunlight vs. overnight from oxidation of organic (and algae).

You could still have some algae growth, but unlikely given the higher FC and relatively low CYA.

Ascorbic acid will consume chlorine, but this occurs relatively quickly and should be done with after a day or two.

Yes, there could be something in the water consuming chlorine and as I pointed out the EDTA sequestrant could be the culprit here. You could use a phosphonate based sequestrant instead to help avoid that problem (I believe waterbear recommends that over EDTA as well). Given the 1.5 ppm FC overnight loss, which is high, I would bet it's the sequestrant.

[Pool-newb]

I'll look around to see if I can find some phosphonate based based sequestrant. I wanted to avoid adding phosphates to the pool but it's gotta be better than this. I also have to say that the EDTA based sequestrant doesn't seem to be working very well either. I guess you get what you pay for in the chemical world too.

After reading this, this and this (posts from waterbear found searching for HEDP, EDTA and chelation) I'm even more convinced that I at least need to try the HEDP base sequestrant. It seems I was overly cautious avoiding the phosphonate stuff based on not reading completly and not fully digesting what I did read.

The EDTA based sequestrant and the lowered pH was not enough in my case to remove the stains at all once they precipitated. I ahve read in other posts that adding a sequestrant will remove slight and light stains. Maybe that too will improve with an HEDP sequestrant. It is also possible that whatever metal I have doesn't respond as well to EDTA as it does to HEDP. (I'm certain it is metal because ascorbic acid works wonders but the pool store has never been able to find any metals. I assume they found nothing because it was all either sequestered or on the pool walls. But I digress.)

I was aware of the ascorbic acid consuming chlorine but as I re-added the chlorine after the ascorbic acid treatment I was able to see pretty dramatic cl usage then a drop as soon as the ascorbic acid was gone. But the cl usage never dropped to what I had last year, and as soon as it got close the stains retrurned.

Now they're coming back again and I think I've just made them angry. :-)

Sure would be great if there was a flocking or similar agent to actually remove the ions from the water for good.

I can see another ascorbic acid treatment in the headlights. Ugh.

[coppersnowblue]

You need a gold tipped orp sensor in salt water.

[Pool-newb]

Thats a good point but I haven't heard it before. I will google it but in the meantime, any links you can give me would be great!