FAS-DPD test results + adding Salt question


LifeTime Supporter
May 16, 2009
The Triangle, NC
Hello All~~

Our pool has been happy and balanced so far this summer. This evening I got an unexpected, surprising, and significant drop in FC test. The question is: Can the addition of salt throw off a FAS-DPD chlorine test?

Here's the story:

Our Aquacheck salt strips had been saying we have 3720 of salt; the Pool Pilot was saying 2700. Concerned that our strips had become old (bought one year ago, expire in August of 2010), we went to the pool store today and bought a fresh container of Aquacheck test strips (expire in 2011 sometime). We noticed that Aquacheck has actually amended their numbers on their test since last year, but that's a whole 'nuther thread).

The new batch of salt test strips confirmed what the Pool Pilot was reading--obviously age is a factor. We needed and added 3 40-pound bags of salt. Pool Pilot later registered 3300 of salt--roughly our goal. Test strips confirmed. All was happy in the back yard. Joy all around. :)

This evening, I did the second general testing of the day. I saw a very unusual drop in FC. My question is--can the addition of 120 pounds of coarse solar salt throw off a FAS-DPD chlorine test (for a while)?

Before I post numbers, please let me tell you that in addition to the specs listed in my sig below, we have Chemtrol equipment in line (even though we have an outdoor pool). DH wishes to be able to control/monitor our pool during the times that we must be away for several weeks at a time, so we have extra gadgetry with online control.

Our goal for FC is 2.5 with a range of 2.0 to 3.0
Because of the Chemtrol equipment, we keep our CYA at ~ 25 (I know that this sounds low, but the Chemtrol cannot "see" if CYA is high!)

So for the past four weeks:

FC is usually 2.2 to 2.4 (25ml Taylor testing)
CC usually 0, sometimes .2
pH stable as a rock with CO2 injection at 7.5, lowered only if TA walks up, and I add acid to bring it back to....
TA 80
CH 410
CYA 23 ish
Phos 1000-2500
Borates 50
Salt now 3300
TDS off of Chemtrol: ~8090
LSI ~ 0.0
CSI ~ - 0.4
Pool temp ave. 85 degrees

This evening--I do my usual FC test. Holy moly! It reads a low 1.0! Combined Chloramines are dead zero. I surely did not muck this up. I draw another water sample, and I test again. Darn! 1.0 FC and 0 CC again! No change in any of the variables--sun, weather, bathers .....all is the same since we've been swimming along at these levels since mid-June. A whole month I've been getting 2.0 to 2.4 on the FC, never more than .2 of CC on a 25mL test. Pump runs 24/7; no unusual weather, testing twice a day.

Has the addition of this salt altered the abilities of the FAS-DPD chlorine test? I have backup containers of FAS-DPD powder and the other reagents--all of which are new from Taylor this season.

I have jugs of chlorine to dump in if the test, which I will do with different containers of reagents, comes out low tomorrow--don't wish to have to throw the SWCG into boost/wait for it to be effective.

Thanks for reading this, and thanks for any ideas.

Happy Swimming~~



TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
Salt has no effect on the FC level or the FC test.

I would add a little extra chlorine tonight and the do something about what dmanb2b is about to mention.


TFP Expert
LifeTime Supporter
Apr 4, 2009
your CYA @23 is way too low for a SWG...you need it in the 60-80 range.

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