CH shouldn't drop without draining water?

[drewh]

Hi all,

In my black algae thread one person mentioned my pool shop CH levels being suspect because they were less in a second test done a few weeks afterwards, they said CH would only fall if you drained the water.

Got my TF-100 test kit and tested CH, was 300. I did this twice to make sure testing was right. Today 4 days later, it's 250 - again tested twice and exactly the same. How could it drop almost 20% with no change to water?

We had maybe 10-15 mins of heavyish rain yesterday but that prob added 1inch of water max. As a % refill / dilution it would be like 2-5% or less.

 

[JasonLion]

Two other ways CH can go down: a leak masked by an autofill system, and calcium scaling (calcium moves from in the water to in the scale).

 

[drewh]

Leak masked by auto fill doesn't apply, but not sure if I do have calcium scaling? don't think so... I have noticed sometimes in the past a few small lumps appearing at the edge on the floor, then that goes away. Could that be calcium scaling forming on the marblesheen surface?

Was just interesting to see it going down by such an amount given it shouldn't reduce by itself. I don't want to test it too often or will run out of agent for the test.

 

[Richard320]

Plug all your test results into poolmath and see what the CSI value is.

If it is near .5, scaling is a real possibility.

 

[drewh]

Thanks Richard, CSI is currently 0.11. After I've slammed the pool & now I have the TF-100 kit will keep it just slightly negative to help reduce calcium buildup on the SWG plates.

Hopefully next time I do the test for CH it'll be the same or higher, not lower again.

 

[gtemkin]

Since there are no non-testing events that account for the change, it's very likely the calcium level didn't change, but something in testing did.

I've used a TF-100 for many years and noticed the 2 color change tests, TA and CH, act in ways that can skew results enough to account for what you're seeing.

For the TA test, as it approaches the green to red change point, the color changes immediately to red when a reagent drop is added, but returns within a couple of seconds of mixing, back to green. For me, that behavior lasts over a period of 2 to 3 drops before it finally "holds" at the color change. I call this a lagging color change.

On the other hand, the CH test tends not to change to blue instantly when a drop is added, it seems to take a few seconds of mixing before it "reacts". And when it does change blue, it tends to stay blue and not regress back to red. I call this a leading color change. Because of this, it's very easy to overshoot the color change point with the CH test. In addition, because the CH test often takes many more drops than any other of the test, there's a tendency to rush the drops and not wait for complete mixing to take place before adding the next drop.

So my general thought is that allowing time for complete mixing is extremely important. You didn't mention if you purchased one, but the Speed-Stir magnetic bar stirrer is really the best way of getting consistent mixing. It appears to get universally positive reviews from everyone who has bought one.

 

[drewh]

I know what you're saying about the TA test in particular. But with the CH test I didn't rush it and put one drop, waited for it to fully resolve swirling, then put the next. I did the test twice and on the same exact drop it changed (so twice and both times 250, and about 4 days prior did it twice and got 300 both times). Also took the sample from the same spot as I normally do.

Will test it again in a few weeks (so I don't run out of reagent) and post what I get.

 

[drewh]

Just an update, tested the CH again now 9 days later and it's 225. So it has gone from 300 to 250 5 days later to 225 9 days later. I did the test twice today again so all results are 100% accurate.

Tested the fill water and it's CH is 75. Maybe that's why? Though I usually only fill maybe 1-1.25 inches in height looking at the side of the pool once every 2-3 weeks.

 

[JasonLion]

Replacing water due to evaporation will raise the CH level. Replacing water due to leaks/splashout/backwashing will lower CH. No calcium is lost, only water, during evaporation, so any calcium in the fill water will raise the CH level further.

 

[drewh]

Ok makes sense re fill water.

I have backwashed twice since the 250 test result so that would probably be it then. I'll test again in two weeks before the next backwash and hopefully will be 225 again. I've tested everything twice so I'm following the right procedure etc.