How is this possible.....

[JoeMX82]

I have a 18 month old IG 23K non SWG pool, pebble tech finish located in Houston TX. (think that is all to note). I test my pool chemicals daily and for quite a while now have basically been able to predict the readings. Well the past three days we got a lot of rain, about 7" so I was expecting that some of my figures would have changed. Test results yesterday (post rainfall)

FC 4.0
TC 4.0
PH 7.6
TA 65 (keeping low due to PH rise)
CH 225 (typically keep closer to 300)
CYA 70

I decided to raise my CH up by adding 12 lbs to the pool and running the pumps for 4 hours. Today, 24 hrs later and no rain, tested the water again and CH levels had not changed, but more surprisingly my FC/TC are reading zero (other test results unchanged). I re-tested from another spot in the pool and no change. My typical chlorine usage has been less than 1 ppm a day and I have no signs of algae or any cloudiness to the water. My question is, could somehow adding CH to the pool have an impact on my FC levels? Any idea what is going on here? BTW - I used the TF100 test kit that is only about 3 months old. Greatly appreciate the help.

 

[MarianParoo]

Adding CH did not cause the FC problem; not adding chlorine did.

When your FC drops to or below the minimum for your CYA level, (5ppm for you), your first priority is to get FC back to recommended levels according to this chart: Pool School - Chlorine / CYA Chart

It sounds like you have an algae bloom starting and it may be time to do a slam, especially now that your FC has hit 0. Are you familiar with the process?

Some may say that you could do an OCLT first. Regardless, you need to get the FC beyond 5ppm ASAP.

 

[JoyfulNoise]

The calcium test can sometimes be tricky. Many people do not do the test correctly, i.e., they end the test at the first hint of blue rather than going all the way to the blue endpoint (when the blue color STOPS changing). Also, depending on the water sample test volume (25mL or 10mL ?), being off by a drop can be a significant amount of CH (for the 10mL water sample, 1 drop of R-0112 = 25ppm of CH). Finally, what kind of calcium increaser did you add? Was it pure calcium chloride (unlikely) or was it calcium chloride dihydrate? The dihydrate form requires almost 25% more of the product by weight to get the same effect as pure calcium chloride. Also, if you use road de-icer as a CH substitute, the quality of those products are often suspect - usually not much calcium chloride and LOTS of magnesium chloride and road salt (not to mention dirt and particulates).

As others have stated, adding calcium increaser of any kind will have NO effect on FC. The two events were simply coincidental.

 

[JoeMX82]

Appreciate the quick response and to clarify a couple of things, I used Regal Calcium Chloride which I believe is pure and tested with the 10ML of water test. I did the test twice thinking that perhaps I screwed it up but same result. I will test again tomorrow and see if anything changes but starting to wonder if I have a bad product here.

The calcium test can sometimes be tricky. Many people do not do the test correctly, i.e., they end the test at the first hint of blue rather than going all the way to the blue endpoint (when the blue color STOPS changing). Also, depending on the water sample test volume (25mL or 10mL ?), being off by a drop can be a significant amount of CH (for the 10mL water sample, 1 drop of R-0112 = 25ppm of CH). Finally, what kind of calcium increaser did you add? Was it pure calcium chloride (unlikely) or was it calcium chloride dihydrate? The dihydrate form requires almost 25% more of the product by weight to get the same effect as pure calcium chloride. Also, if you use road de-icer as a CH substitute, the quality of those products are often suspect - usually not much calcium chloride and LOTS of magnesium chloride and road salt (not to mention dirt and particulates).

As others have stated, adding calcium increaser of any kind will have NO effect on FC. The two events were simply coincidental.

- - - Updated - - -

Thanks for the help and agree with your points. Typically I have kept the FC higher but with all the rain I let it drop a bit low, still would not have thought 24 hours would result in this. I am upping the Chlorine now and if it continues to disappear I will slam the pool. One question, could you tell me what OCLT is?

Adding CH did not cause the FC problem; not adding chlorine did.

When your FC drops to or below the minimum for your CYA level, (5ppm for you), your first priority is to get FC back to recommended levels according to this chart: Pool School - Chlorine / CYA Chart

It sounds like you have an algae bloom starting and it may be time to do a slam, especially now that your FC has hit 0. Are you familiar with the process?

Some may say that you could do an OCLT first. Regardless, you need to get the FC beyond 5ppm ASAP.

 

[pabeader]

It stands for Overnight Chlorine Loss Test. You check your FC level as soon as the sun is off the pool. Then check it again in the morning before the sun hits the pool. It shows chlorine loss not due to sunlight.
It is covered here: Pool School - Perform the Overnight FC Loss Test (OCLT)

 

[tstex]

It stands for Overnight Chlorine Loss Test. You check your FC level as soon as the sun is off the pool. Then check it again in the morning before the sun hits the pool. It shows chlorine loss not due to sunlight.
It is covered here: Pool School - Perform the Overnight FC Loss Test (OCLT)

The idea is that UV breaks down CL, thus if your CL levels do change during post and pre sun, it is due solely to killing Algae or the likelihood thereof. If however, you have a lot of debris in your skimmers, leaves in pool, etc, the organic matter too is attacked by CL and lowers your FC. Ideally, if you remove all visible debris and clean all skimmers, pumps trays, etc, then perform the OCLT test post & prior sun, you'll be very accurate in determining if CL is being consumed by Algae, thus a potential SLAM....

 

[JoeMX82]

Cleaned the pool and started the SLAM process with liquid chlorine - thanks to all!

 

[MarianParoo]

Good for you; glad you went for the SLAM. Best of luck!!! There are plenty of us who would love to help if you have any questions!

- - - Updated - - -

...And it is great that you now know what the OCLT is as that's how you determine when your SLAM is complete.

 

[GaryT58]

The idea is that UV breaks down CL, thus if your CL levels do change during post and pre sun, it is due solely to killing Algae or the likelihood thereof. If however, you have a lot of debris in your skimmers, leaves in pool, etc, the organic matter too is attacked by CL and lowers your FC. Ideally, if you remove all visible debris and clean all skimmers, pumps trays, etc, then perform the OCLT test post & prior sun, you'll be very accurate in determining if CL is being consumed by Algae, thus a potential SLAM....