SLAM and confusion over CYA

[survivalcar]

I literally can't tell what my CYA is because it looks different in every light. I have complained about it in here before but this time my question is related to slamming:

So when testing CYA, the little dot starts to disappear around 60 and in most settings I can still kind of make out the little dot around 50 but barely. Same thing when the level gets to 40. It looks like I can't see the dot and then I carry it inside and I can start to make it out. Why is measuring CYA so inexact?

So my question is, when slamming, let's suppose that the actual CYA is 40 but when reading it, I mistake it for 50. Am I doing any damage by using the slam chlorine level for 50 (24ppm) as opposed to the slam chlorine level for CYA of 40 (16ppm)?

I would assume that overshooting the minimum chlorine level for slamming would be safer than UNDERshooting the minimum chlorine level. Is this correct? That is, at the worst am I just wasting a lot of chlorine, or will the process not work with too much chlorine?

Thanks!

 

[mknauss]

That is, at the worst am I just wasting a lot of chlorine,

Correct.

When you do the CYA test, try this next time.

Once you have your solution ready, back to the sun, etc. Fill the vial to a line, say 80, lower the vial to your waist level and glance for the dot, you see it, add solution to the 70 line, glance, see it, repeat until you no longer see it with a glance. Then use the CYA value one step above the line you read. So if you stopped at 50, use 60 ppm CYA.

The vial is in logarithmic scale. So it is not viable to interpolate between the lines. Just use the whole numbers, such as 50, 40, 30, ....

 

[survivalcar]

Thanks! So I have only done this once before and I remember it was confusing last time as well. So can I tell you how I got here?

When they came to open the pool there was debris in the bottom (leaves etc., we live in New England) but the water was very clear.

We had metal stains and years ago I had success using Jack's The Blue Stuff. Following the instructions, I lowered the pH to 7.0 using MA. Then I added the Blue Stuff and after a couple days it appeared as if it didn't do anything. I thought no harm, no foul and just raised the pH using sodium carbonate.

The next morning the water was cloudy and I started doing the SLAM. So the SLAM began on Saturday afternoon.

I realize that the pH goes wild while slamming but I noticed on Sunday that the pH was over 8, so it was either going crazy because of the SLAM or I overshot the target while raising it, so I put in some MA and measured again and as of yesterday the pH was 7.6.

But this is the third full day of slamming and I would say there is literally no change in the water. Just looks cloudy. I can see the bottom but it's got a powdery look.

Is this normal after 3 days? At what point do I start to reconsider how I'm reading the CYA and start upping the chlorine level so that it matches a lower (?) CYA? Can I add CYA during the SLAM?

 

[mknauss]

Do an Overnight Chlorine Loss Test