After several years relying completely on a salt water generator for chlorine, no contamination events requiring slam, and reliable measurement using FAS-DPD, i now have a 10 month issue of being unable to measure Cl. Initially i thought it was interference from components of a stain removal product, but after a complete pool volume exchange, the problem persists.
Currently (in process of restoring chemistry after no-drain muriatic acid wash for Ca stain removal last week):
Free Cl: 0.5, CC: 0.6
Alk: 100
pH:7.2
Cya: just added 1 gallon liquid (was thinking of potential slam) =28 calculated
CH:175 (just did whole water exchange), my fill water is 125-150
NaCl: not added
T: 81
CSI: -0.43
Issue began 8/2021, after I used a marketed no-drain Ca stain removal protocol/kit from PoolStainRemovers.com. The kit includes some undisclosed proprietary ingredients but mostly includes sulfamic acid and EDTA. I was told that it would interfere with FAS-DPD Cl measurement for a few weeks, but I have never gotten the DPD assay to work since. In the past, the pool sample would rapidly turn pink when adding 1 scoop powder. Now it appears chalky white and a second scoop is chalky pink, and one drop turns it clear. Also, I have measurable CC (never previously)--0.6 determined by using the 25 ml method where 1 drop = 0.2 ppm.
I have:
1) purchased new Taylor DPD powder and liquids 9/2022, made no difference.
2) tested the reagents using a dilution of my HomeDepot Pool Essentials bleach in tap water. Nominal Cl = 10%. After dilution to 8.8 ppm (calculation using pool math, given below), results are: Free Cl 5 ppm, CC 1 ppm. Should bleach have any CC??? n.b. bleach purchased in 1/2022, production date 1/2022
3) I noticed that the yellow single reagent Cl test is also off. Previously it reacted immediately (turned yellow in seconds after adding the reagent and mixing). After the stain treatment, it reacted in a very delayed way, becoming fully developed after 2-3 min.
4) I wanted to get rid of the presumed "interfering substance" in the pool. I did a second no-drain acid treatment in 4/2022 for my calcium scaling, with muriatic acid only (beneficial result, but another story), and then exchanged the pool water. I was able to monitor the exchange by measuring the CH in the outflow, and this indicated that the exchange worked well (abruptly dropped from 700 to 250 to 175 at end of the exchange period.) This did not change the performance of the DPD test, or of the yellow dye test.
Does this ring any bells? What are my next troubleshooting steps? I think my Cl test reagents are good, they are stored indoors, are within expiration period, and appear to change color normally using the test dilution with tap water. I was considering slam (hints of CC) but how can i do that without being able to monitor Cl? Water is clear, and no signs of algae (just Ca stain!). I'm surprised my 3 mo old bleach measures as half-strength (and has some CC per the Taylor kit)--what to make of that? Should I just ignore it all, add pool NaCl, and restart the salt generator? ;-)
Thanks for your consideration!
Ed
---------------------
Cl dilution: 10%-->1%--> 0.1% (wt to volume using kitchen scale, assumed sg =1, but it is 1.14)
18.5 ml into 0.5 gal tap water = 10 ppm (per poolmath)
Currently (in process of restoring chemistry after no-drain muriatic acid wash for Ca stain removal last week):
Free Cl: 0.5, CC: 0.6
Alk: 100
pH:7.2
Cya: just added 1 gallon liquid (was thinking of potential slam) =28 calculated
CH:175 (just did whole water exchange), my fill water is 125-150
NaCl: not added
T: 81
CSI: -0.43
Issue began 8/2021, after I used a marketed no-drain Ca stain removal protocol/kit from PoolStainRemovers.com. The kit includes some undisclosed proprietary ingredients but mostly includes sulfamic acid and EDTA. I was told that it would interfere with FAS-DPD Cl measurement for a few weeks, but I have never gotten the DPD assay to work since. In the past, the pool sample would rapidly turn pink when adding 1 scoop powder. Now it appears chalky white and a second scoop is chalky pink, and one drop turns it clear. Also, I have measurable CC (never previously)--0.6 determined by using the 25 ml method where 1 drop = 0.2 ppm.
I have:
1) purchased new Taylor DPD powder and liquids 9/2022, made no difference.
2) tested the reagents using a dilution of my HomeDepot Pool Essentials bleach in tap water. Nominal Cl = 10%. After dilution to 8.8 ppm (calculation using pool math, given below), results are: Free Cl 5 ppm, CC 1 ppm. Should bleach have any CC??? n.b. bleach purchased in 1/2022, production date 1/2022
3) I noticed that the yellow single reagent Cl test is also off. Previously it reacted immediately (turned yellow in seconds after adding the reagent and mixing). After the stain treatment, it reacted in a very delayed way, becoming fully developed after 2-3 min.
4) I wanted to get rid of the presumed "interfering substance" in the pool. I did a second no-drain acid treatment in 4/2022 for my calcium scaling, with muriatic acid only (beneficial result, but another story), and then exchanged the pool water. I was able to monitor the exchange by measuring the CH in the outflow, and this indicated that the exchange worked well (abruptly dropped from 700 to 250 to 175 at end of the exchange period.) This did not change the performance of the DPD test, or of the yellow dye test.
Does this ring any bells? What are my next troubleshooting steps? I think my Cl test reagents are good, they are stored indoors, are within expiration period, and appear to change color normally using the test dilution with tap water. I was considering slam (hints of CC) but how can i do that without being able to monitor Cl? Water is clear, and no signs of algae (just Ca stain!). I'm surprised my 3 mo old bleach measures as half-strength (and has some CC per the Taylor kit)--what to make of that? Should I just ignore it all, add pool NaCl, and restart the salt generator? ;-)
Thanks for your consideration!
Ed
---------------------
Cl dilution: 10%-->1%--> 0.1% (wt to volume using kitchen scale, assumed sg =1, but it is 1.14)
18.5 ml into 0.5 gal tap water = 10 ppm (per poolmath)
