Can this be right?

[another one]

A few days ago, I started draining some water to lower my CYA level.
See this post:
http://www.troublefreepool.com/draining-water-t24420.html

Anyway, at the start of the drain, here's the readings from the TF100 kit
Note, I did 3 tests of everything before I started draining and TA, CH, CYA were the same for all 3 tests, pH and FC changed because of addition of bleach and acid.

pH - 7.5
FC - 16
CC - 0
TA - 170
CH - 300
CYA - 55

So I drained and refilled 700 gallons (2.5%) and did the pH, FC test - ph - 7.5, FC - 14
Next day, another 700 gallons, pH - 7.4, FC - 14
Next day, another 700 gallones, pH - 7.4, FC 13.5

Then I didn't do anything for 3 days, no draining, no acid, nothing

Yesterday I measured everything:
pH - 7.4
FC - 13
CC - 0
TA - 100
CH - 250
CYA - 40

And today, I measured it again and got pretty much the same readings except pH was 7.5.

So how could draining and adding 7.5% water cause the CYA to drop from 55 to 40,
CH to drop from 300 to 250 and
TA to drop from 170 to 100?

 

[Isaac-1]

Ok, here goes a shot, maybe you drained more than you think you did. In the case of pools with deep ends and shallow ends (particularly those with hopper style deep ends) it is easy to under estimate amount of water removed since the deep end alone can account for much less than 25% of the water volume. If we work with an inflated estimate of 1,000 gallons removed and replaced these numbers start falling within the margin of error of the test. Remember TA will read lower with a lower pH, if your fill water is very acidic and low in TA you would have 2 factors causing it to show as low, the typical color matching pH test is only good to about +/- .1 resolution, so if your initial reading was .1 high 7.6 and your current is .1 low 7.3 then everything may make sense. CYA test is another test that is highly dependent upon perception, so I would say your readings are again within the ball park. FC loss looks in the right ballpark for an indoor pool, based on my personal observations. This only leaves CH, which again (if you assume 10% addition, which seems a larger margin of error for fill, but who knows, of clean low CH water) would put you within a one drop each side testing error.

Ike

p.s. is it possible that your water is not yet fully mixed, if the fill water is warmer than the pool water it could tend to float on top if you are not drawing from both the floor drain and the skimmer.

 

[another one]

Some good possiblities. Another point I should make is the pool water is 62 degrees and the water coming in is 58, so not much of a change, and both drains open (bottom and skimmer) and it has been 4 days since water was added, pump on 8 hours per day. I did put the 8 oz of sample water in a warm, 80+ degree room) for 4 hours before I checked the CYA.

Also, the drain amount was 1-1/2 inches, and the pool is about 500 gallons per inch. But I timed the drain based on timing how long it takes to drain 10 gallons, then do 70 times that time.

The both the TA and CH dropping so much is what has me concerned, but maybe I'll just have to add Calcium Chloride and baking soda to raise them as I get the CYA closer to 30. I figgured I stop for a while until I understood what was happening with the chemestry.

Also, the chlorine is really taking a long time to drop and the cover has been opened the whole time, but the pool is indoors with 8 skylights plus large windows.

 

[waste]

Also, the chlorine is really taking a long time to drop and the cover has been opened the whole time, but the pool is indoors with 8 skylights plus large windows.

The chlorine isn't getting burned off because the windows are opaque to certain UV wavelengths (I suspect it's the ones that cause skin to get sunburn - and, no, I can't recall the specific angstrom lengths )

BTW - with an indoor pool, you probably will want to keep the cya < 20 - a little is good for keeping the chlorine smell down, but you don't need anywhere near 40.

Also indoor pools lack both the UV load and the 'blown in' organic load that often make outdoor pools loose the chlorine so quickly.

 

[another one]

You are right about the CYA, Ted. Too bad I didn't discover this site a few years ago, but, I'm going to get things right!

I'm going to let things settle for a few more days, then drain some more, maybe I won't have to drain as much to get the CYA to 20 or so.