Metal interference with the CH test

[gtm]

I've been doing a good bit of reading this evening about metal interference with the CH test. My experience is somewhat similar to Swimgirl's post here. There are a lot of posts about the metal interference issue, but this is the one that seemed clearest to me on the "direction" of the error one would expect if CU is present.

Here's the history:

I have been using the Taylor kit and then the TFP kit for about 5 years now, meaning since I discovered this forum. I don't know what was done to my poor pool before I took custody.

I always had trouble with the CH test. I read about the "floaties" and the need to swirl and swirl and swirl and did the best I could, but my results were always more variable than I expected, and also higher than the local pool store was telling me. I didn't believe them, BTW.

I bought a stirrer from duraleigh about a year ago and that made things a little better, but the results were still more variable than I thought reasonable. Also, the results were consistently lower than what I was getting before using the stirrer, meaing 275-350 ppm as opposed to 350-400 ppm.

Recently I actually read the TFP kit instructions ( ) and ran across the comment about modifying the CH test when CU might be present. Yes, I read the instructions about a year after receiving the kit ...

When I use the modified procedure, two things happen. First, the results are now rock-solid reproducible. Second, the results are substantially lower, meaning dead on 225 ppm as opposed to 275 to 350 ppm before. Note that the lower reading is pretty close to what the pool store was telling me, FWIW.

So my interpretation is that I must have some metals in the water, and the 225 ppm reading is the correct one. The higher reading I was getting before was an incorrect one.

Does that make sense? That seems consistent with what chem geek says in this thread, but I was just hoping for a reality check before I spent $40 on calcium, and took a chance on maybe adding too much. Again, I found a lot information about possible problems with the test if metals are present, but it was a little difficult to tell whether using the usual testing procedure with metals in the water would tend to make you measure high, as opposed to just making the test results more noisy.

Thanks in advance ... Gary

 

[duraleigh]

Hi, Gary,

That's a great post and makes good sense to me. I have never had metals so cannot relay "hands on" experience but it makes sense what you are doing and a lesson for all plagued with metals.

A bit off subject, but do you know how copper got in your pool? It is pretty unusual to occur in either city water or well water.

Yes, I read the instructions about a year after receiving the kit ...

Been there, done that on many new things I get The only thing more sinful than reading the directions FIRST is to ask for driving directions when you are hopelessly lost and your wife is in the car.

 

[gtm]

Hello Dave,

First, thanks for the reality check. Much appreciated.

Second, your "off subject" question really got me thinking and searching. It was a really good question, in other words!

I have no idea why I should have copper. In the time since I found this forum I have never added anything that would cause that. I have had some perpetual staining issues, but always assumed it was iron. Again, I've never used anything that should add copper.

I went back through 5 years of receiepts from the Pool Fool who was taking care of it before I found you all, and I didn't find anything except a reference that read, "Did metals treatment, $100". But that was 8 years ago!

I also took a water sample to the local Leslie's late this afternoon. The manager there, who I know and trust somewhat, did a strip test and decided that if copper was present, it was 0.2 ppm or below. I looked at the strip myself and would agree with that.

On the other hand, part of the reason I went to the PS to have them check for CU was that I dug out a water quality report from the City, and this says that the 90th percentile for CU testing is 0.55 ppm, which seemed high. They test this at the tap, though, and attribute it mainly to household plumbing. The PS reading would seem to indicate that my tap water is not so high.

I'm not sure what to make of all this, but I've decided to do the CH test as if there was CU present. I'm getting much more reasonable results this way.

Thanks again, Dave. I really appreciate the feedback.

Cheers, Gary