CH dropped by half overnight

[PanamEric]

Hello all,

I am puzzled by my CH test results of today and am hoping I can get clarifications here.
Two nights ago we had huge amounts of rain. There are overflows builtin to the pool so the excess water drops out. So usually CYA and CH drop after heavy rains.
I figured my CH would have dropped yesterday, as both overflows were pouring out full, but it did not.
(See tests results below)
I ran out of CYA reagent (have some ordered but not in yet).
So yesterday I used the pool calculator to increase my CYA by a bit until the new reagent comes in. I went for 10ppm to be safe which is around what I usually lose after heavy rains.
Tests
before rain CH 230 (after rain 230)
before rain TA 50 (after rain 60)
before rain PH 7.4 (after rain 7.6)
before rain FC 1.5 (after rain 2.5) (my handyman added a bit of calhypo 3 hrs before i tested)
before rain CYA 40 (after rain assumed around 50)

I just tested today and the CH test turns very clear yellowish pinkish with 5 drops of reagent, instead of usual light red. When I added 5 more it turned the usual color and it required 25 drops of titrant to turn blue. I repeated the test with half the water in the test tube and 5 drops of reagent and it required 12 drops of titrant to change.
So I am thinking my CH is around 125ppm (25 div by 2)? Am I counting this right or should it be times 2?
I did shock to 20ppm with calhypo yesterday a few hours after increasing CYA...
Today FC is testing at 3
TA 60
PH 7.6
CYA assumed around 50
and CH inconclusive (either 125 or 500)
the pool calculator tells me that the calhypo I added to shock should have brought my CH up by 12?
I ran the CH test twice with 2 different kits...
Does it make sense that the CH remains unchanged 12 hrs after heavy rains but drops by nearly half 24hrs later?
What's going on??

I am using calhypo because my SWG is out of order awaiting parts...
Thanks
Eric

 

[Richard320]

Not sure what test kit you're using... but with Taylor/TF100 when you do the lower volume CH test, you reduce R-0010 to 10 drops, and the R-0011 to 3 drops, from 20 and 5 respectively. This with water volume going down from 25 to 10 ml. So be sure you're using the right quantities. I've forgotten more than once and started the test using 25 ml and gotten all confused.

I'd also wonder if the water is thoroughly mixed. You may have stratified layers, with old pool water versus fresh rainwater.

Other than that, I'm out of ideas.

 

[PanamEric]

Thank you Richard,

Your answer makes perfect sense. In fact after I read your post I thought of going to check the valves...my handyman often forgets to turn the bottom drain back on after vacuuming and indeed only the skimmer was on.
So I thought you had it...
I then shut off the skimmer and waited a bit to force the bottom water through the jets and collected this water to retest. Unfortunately it did the same thing...turned yellowish instead of red.
I then got interrupted by a phone call and left everything there. When I returned 5 minutes later the sample water was the normal reddish color (although a bit lighter than usual) and I did manage to test my CH at 240 (24drops) which would make perfect sense given the ~12ppm increase caused by calhypo.
What also makes sense is that since this is salt water, the less dense rain water would tend to stay at the top and since I have overflows it pretty much evacuates before it has a chance to mix.
The remaining mystery is why the reagent takes 5 minutes to turn to red??? Is there some sort of chemical explanation that anyone can contribute?
Yesterdays test I got the red as soon as I mixed the tube but it did not rain at all yesterday or last night.
In any case thanks for trying
Btw I am using HTH 6-way test kit. Normal procedure is 5 drops of reagent in 25ml bottle and each drop of titrant is 10ppm.
Thanks
Eric