Disappearing chlorine...

[Watermom]

And to add to what Carl said above, if algae is trying to start again, you need to shock the pool up to 20. Then, try and hold it there for a few days until you can go overnight with no more than 1ppm cl loss. Test and dose with bleach at least morning and evening when the sun is off the pool, and if someone can test and dose a time or two during the day, all the better.

[nthach]

Also, I'm noticing some reprocity error with the CYA tests - when I do them, I drop in the CYA/melamine reagent mix until I cannot see the black dot - between 40-50. When me and my friend did it in the sun, he dosed it in the tube until the dot just disappears between 60-70ppm. Could age and vision have anything to do with this?

[waterbear]

So you have a 10 ppm discrepancy. This is within the precision of the test. Split the difference and call it a day. It was from the different lighting conditions since I assume you did not do the test in full sunlight when you got the 40-50 ppm.
BTW, I prefer to do this test in open shade and not full sun. It makes it easier to determine when the dot disappears because there is no glare on the surface of the sample.

[chem geek]

Taylor recommends that you stand outside with your back to the sun so that you shade the tube in front of you looking down into the tube. If you have the sun shining directly on the tube, you'll get a reading that is too low. If you do the test with insufficient lighting, you'll get a reading that is too high. It's a finicky test, especially compared with the titration tests.

[CarlD]

But, even more importantly is to do the test the same way every time. This allows you to track changes and mark progress. You learn over time if you are over- or under-representing the results and compensate.