Lower than expected SWG chlorine output

HowardRoark

Member
Nov 12, 2018
7
Phoenix, AZ
Here's what I am seeing. I've had my pool for a little over a year now. My SWG worked great last year, in other words produced the amount of chlorine I would expect. This year it appears to be producing way less chlorine for the same run time. I didn't keep good track of what percentage I ran it at last year but I do remember when I had it at certain percentages my FC was going up day over day. This year even running at 85% I am losing chlorine day over day, and I have more CYA (40->70). I keep my chemical levels in check and am very diligent keeping the ph between the recommended levels to prevent scaling due to my hard water. I have not had any scaling that I can see on the salt cell (or anywhere else) but just to be sure I soaked it about a month ago in vinegar for an hour. My question is why would my SWG appear to be producing much less chlorine than it should be?

Here's a little background:

-Last year my CYA was around 40-50. This year I raised it to 70 as per the suggestions from TFP.
-I lose about 2 FC per day. I'm in sunny AZ.
-I have my SWG currently set to 85% and according to Pool Math that should produce 2.6 ppm FC. But last night I took a measurement and was at 2.5 FC and this morning I was at 4.5 FC which is only 2 FC produced.

Pump run times: 4 hours @ 3105 rpm + 4 hours @ 1725 (overnight) I've verified that the flow check is good at both speeds.
Salt cell diagnostic readings: 26.41V 5.03A 82F 3100ppm

My current values as of this morning.
FC: 4.5
CC: 0
ph: 7.8 (I usually lower to around 7.4 from 8.0 every week)
TA: 80
CH: 750 (we have hard water in AZ but since I control my chemical levels really well I do not have scaling issues)
CYA: 70
SALT: 3200
Temp: 82 F
CSI: 0.32

I've done some searching on the forum and haven't found much. A few of the related threads I have found but no concrete answers.

 
Last edited:

HowardRoark

Member
Nov 12, 2018
7
Phoenix, AZ
Well, I think I found my problem. I performed a OCLT and when I measured FC last night it was 5.5 and this morning it was 4. So a loss of 1.5 FC. It's odd because my pool is crystal clear but I guess I need to SLAM.
 

Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
18,346
Bedford, TX
Howard,

Most of the time, when a SWCG can't keep up, it is because of very low CYA or an algae bloom.

Good catch.. :thumleft:

Thanks,

Jim R.
 

proavia

LifeTime Supporter
TFP Guide
Feb 6, 2015
1,928
Chandler AZ
Good catch.

You may wish to consider keeping your pH a little lower than 7.8 to keep your CSI in the 0.00 to -0.30 (minus 0.30)range. This will help prevent scale build up in your SWG. Also, let your TA drop to about 60 - from the 80 it is now. This will help slow pH rise. In PoolMath you can set different numbers for TA and pH to see how it affects your CSI.

Chances are you are in for a full drain/refill or an RO treatment this Fall to Spring time period as your CH will be well north of 800 by then do to water evaporation.
 

HowardRoark

Member
Nov 12, 2018
7
Phoenix, AZ
Sounds good. My TA was at 60 a month ago but I guess I didn't do a great job of keeping up with acid demand. I usually try to keep my ph below 7.8 but I think I slipped a little bit recently.
 

HowardRoark

Member
Nov 12, 2018
7
Phoenix, AZ
Now I'm just curious how much bleach (in total) I'm going to need to SLAM at a CYA level of 70. Initial amount says 3 gallons. Really hoping this doesn't take more than a few days to cure.
 

HowardRoark

Member
Nov 12, 2018
7
Phoenix, AZ
Going to post a little update.

-As per Pool Math I added ~3 gallon of bleach Thursday morning to take my FC to 28.
-Tested FC 2 hours later and was down to 24, so added another 0.5 gallon per Pool Math.
-Instead of measuring and wasting test reagents I just added 0.5 gallon twice more which got me to the evening.
-Test FC that evening and again in the morning and the difference was less than 1 FC.
-Didn't add any bleach on Friday. At night I took another reading of FC and was 16 and this morning (Saturday) it was 17. I'm chalking the difference up to the fact that I'm using 5ml to cut down on the amount of reagents I need to use.

So, looks like I'm in the clear now. Going to just allow the FC to just come down over the next few days then I'm going to make sure to keep FC at the recommended target of 5 FC. For those curious, to cut down on the amount of reagent I used I got a syringe to accurately measure 5ml and then just did the normal chlorine drop test but each drop counted as 1 FC instead of the usual 0.5 FC.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
31,280
Laughlin, NV
For those curious, to cut down on the amount of reagent I used I got a syringe to accurately measure 5ml and then just did the normal chlorine drop test but each drop counted as 1 FC instead of the usual 0.5 FC.
That is fine for during the SLAM. It is not advised for an OCLT FC test.