Pool calculator and odd readings

[red-beard]

To reduce/correct the CYA in my pool, I drained about 90% of my water. I have raised the CYA to 30 and that is stable. I added borates to the pool to 30 ppm. I've corrected the TA to 80 with baking soda. ph is about 7.5. Hardness is a bit low at 200, but I'm using Cal Hypo to raise the hardness.

Using Zappit 73, 73% cal hypo for chlorine. I do my additions at night. Pool pump runs 24/7 at about 40 GPM (Tristar VSP at 1150 rpm 71 Watts), except 9PM to midnight 2850 rpm (100 gpm).

I target 8 FC ppm for my nightly dose. I've had mustard algae issues and I keep the pool from dropping below 3.5 FC. The past few days have been ultra sunny. My pool has direct sun from 8AM to 630PM. The nightly reading is 3.8 FC. Pool calculator says to add 18oz by volume to get 8.0 FC. Pumps run high speed for 3 hours after the addition and low speed for 6 hours. It should be well mixed. My FC reading is 10.0 at 6AM. By 6PM, it drops back to 3.8, as I would expect.

A loss of 4.2 is not unexpected. A loss of 6.2 is too much. Is something with the cal hypo affecting my FC test? I'm using FAS-DPD and using 25 ml for the test. I collect water from a spot well downstream from a return and collect samples in a collection bottle about 18 inches down and 12 inches from the pool edge.

The plan is to move back to 12.5% bleach once my hardness reaches 250.

Any clues why the reading seems too high in the morning? Again, nightly it's where I expect it to be.

 

[Richard320]

I blame testing error. 8 FC at .2 resolution is 40 drops! If a quarter f them are too small because you're getting tired of t and start squeezing the bottle so they come off faster, that would do it.

Try switching to 10 ml sample and one scoop of powder and divide drops in half. Or multiply by .5 if you're a digital guy. Fewer drops, fewer chances for errors.

 

[red-beard]

I think you are right, testing error. I was adding about 30 drops fairly quickly to get past 6 ppm.

 

[YippeeSkippy]

30ppm of CYA may be a touch low for your amount of sun. Try raising it to 40.

 

[red-beard]

I am slowly lowering the "target" from 8 ppm to 7.5 ppm for my nightly additions. I do not want to raise the CYA much, or my FC levels to prevent Mustard Algae will be extremely high.