Question about testing CH

[cheddar85]

Hey everyone,

After refilling the hot tub, I started balancing the water. Due to the water softener, CH was 0. Over a few days, I gradually added enough calcium chloride to get the level to 140. I added about a third of the required amount on the first morning, then tested the level the next. I did this until I got to 140. That was 2 days ago. This morning, I tested it again (just to be sure) and after adding the 5 indicator drops, instead of it turning red like it has been, it went to a very pale pink. So, 2 questions:

Why is this happening now when it's been working fine for the past several days? No water has been replaced.

If I add 5 more drops, it changes like it normally would. Is the test result still valid?

I really need to get the TF-100, hopefully money will allow for that Friday. I'm using the Wal-Mart drop kit right now.

My test results are:
FC - 4
TC - 4
pH - 7.6
Alk - 60
CH - 140?? (not so sure now... )
CYA - 40

*EDIT* After trying the test again just now in better lighting, even though the sample is very very pale, it still takes 14 drops to shift to blue. Is this just a result of a crappy kit?

 

[JasonLion]

Several things can cause variations in the appearance of the colors. High FC levels is one common cause, though there are several others. Inexpensive test kits are not as good at compensating for these interferences, so seeing them is much more common. These variations do not invalidate the test results as long as you can still see the color change (a few of them turn the sample clear and then you can't see the color change any more).

 

[257WbyMag]

I would trust the TF-100 over the other kit. You'll know for sure when you receive the kit. But you also might be dealing with the fading endpoint issue as well.

Here's a link that gives detail to the CH test. You might try following the instructions on dealing with fading endpoint that are listed on the page and see if it helps.

http://www.taylortechnologies.com/Chemi ... ntentID=70

 

[cheddar85]

Thanks for the quick replies guys, I think for now I'll trust the amount that I know I put in the tub until I get the TF-100.

 

[Leslieslsa]

Yep, your chlorine is way to high to rely on an accurate reading right now. Chlorine will actually bleach out the colors on you test kit when they get too high. So either wait it out, or add some chlorine neutralizer.

 

[polyvue]

Yep, your chlorine is way to high to rely on an accurate reading right now. Chlorine will actually bleach out the colors on you test kit when they get too high. So either wait it out, or add some chlorine neutralizer.

Really high chlorine levels can certainly be problematic when testing pH (and can result in a bleached-out chlorine test result with the DPD test method).

I don't think a Free Chlorine (FC) level of 4, however, is high enough to cause interference with any of the tests found in the K-2006 or TF100 (including the Calcium Hardness (CH) test). In tandem with 40 ppm CYA, 4 ppm of FC falls within typical recommended levels per the Chlorine / CYA Chart. Also noteworthy I think, in regard to CH, Taylor details only potential interference from metals such as copper (see the link provided above: http://www.taylortechnologies.com/Chemi ... ntentID=70)

Total Alkalinity (TA) is bit touchier regarding high chlorine (see guardex-test-kit-says-not-to-perform-ta-test-when-fc-is-high-t18002.html) but my own experience shows no difference in the test result for TA – at least up to 10 ppm FC. Taylor suggests adding an extra drop of thiosulfate to the sample when there is a “high level of chlorine present” but doesn’t define what “high level” is.

 

[duraleigh]

As polyvue indicates, 4ppm FC will have no effect on virtually any of the tests we commonly use.

Importantly, as Jason says, as long as you get a color shift that you can identify, it doesn't invalidate the CH test.

Taylor has info on their website detailing interferences with the CH test.

 

[cheddar85]

polyvue and duraleigh:

I haven't gotten the TF-100 yet - hopefully I can order it tomorrow. The kit I'm using now is the Wal-Mart drop kit. I think Jason nailed it though. Last night after we got out of the tub I checked the chlorine again. It was down to 1 so I decided to check the CH again. Not to my surprise, after adding the indicator it went dark pink/red like before.

However, I get the same results from the test no matter how dark or light the sample turns when I add the indicator. Also, if it only turns a really pale pink, if I add 5 more drops of indicator, it darkens to the color I'm used to seeing. The results also stay exactly the same.

Hopefully that's not confusing anyone, I just thought it might help someone in the future with the same problem.

By the way, if you see this again duraleigh, hopefully tomorrow or Sat. I'll be giving you another order to ship. Either the TF-100, or the XL. For the small price difference and how OCD I can be about my water ....probably the XL.