Checking results from different sources

[doncaruana]

Suspecting that my LaMotte kit was off or the pool store was, I decided to get a couple of samples and test them. I went down with a few empty water bottles and filled completely and capped each of them under the water (nothing escaping into the air, etc). I then wrapped them in a black garbage bag (to minimize the sun messing with the chlorine) and off I went.

First, I went to a different pool store. These were the numbers I got:

FC: 2.91
TC: 2.91
TA: 177
pH: 8.0
CH: 108
CYA: 29

Then I headed off to my regular store, which was only about 5 minutes away. This time I get this set:

FC: 2.6
TC: 2.6
TA: 121
pH: 7.8
CH: 92
CYA: 72

Unfortunately, I didn't get a chance to test my kit at that point, but a little later I test with a 3rd bottle I got at the same time with the ColorQ:

FC: 1.5
TC: 2.1
TA: 148
pH: 7.5
CH: 87
CYA: 32

The chlorine numbers I kinda get from the ColorQ. I don't get them between the two stars (2.6 vs 2.9). I really don't get any of the other numbers.

	    #1    #2   ColorQ
FC     2.91  2.6  1.5
TC     2.91  2.6  2.1
TA     177   121  148
pH       8   7.8   7.5
CH     108   92    87
CYA     29   72    32

I consistently get right around 30 for CYA at home. And I'm positive I haven't put enough CYA in to raise the levels as high as 72 (from 0). How could TA and pH be that far off between all the measurements?

Am I not doing something right in transport or something? Is there a better way to make sure the testing is accurate? I had heard that ColorQ was decent at least not horrible. I called LaMotte and they are sending me "known concentration" sample (TA, pH and CH, I think). Is there a decent "over the counter" set of tests I can buy that aren't a zillion dollars? I'm struggling with balancing the water because I can't trust any of the tests I have...

Thanks!
Don

 

[harleysilo]

Are you in Canada or something? http://tftestkits.net/splash-page.html Best $68 bucks i've ever spent.

 

[linen]

What about your colorq are you suspecting? Generally it is best to rely on your own tests, rather than pool stores as I am sure you have read and as you just experienced first hand. We have seen countless number of threads like yours where there is not consensus with different pool store measurements.

I don't think your ph, TA, CH and cya would be affected by being in a bottle for awhile. With that said cya, is the one that the colorq does not do well...and for that matter neither do the pool stores.

And I'm positive I haven't put enough CYA in to raise the levels as high as 72 (from 0).

Since you have started from zero cya, how much have you put in? That will be your best answer, unless you do not know your pool volume, or if you have since had made a major water change.

 

[doncaruana]

I'm in the US, but frankly I was wanting to avoid all the "hold the vial just right and squint" type stuff. It seems like there is still a heck of a lot of interpretation and "inexactness", if you will, in using these tests. I mean, at 3ppm, the FC test has a precision of plus or minus 1.5. That seems pretty broad, unless I'm reading this wrong.

As far as the CYA, I'd have to go measure/weight what's left, but I suspect that 30 number is about right, to be honest.

I'd like a kit that takes the guess work out of it (at least mostly) and so I was hoping to "calibrate" said kit against some "known" sources. But I can't seem to get consistent results. And, yeah, I've read that pool stores aren't necessarily the best either (God forbid you suggest that to them!!).

The one thing I can say is that my results from the ColorQ have been fairly logically consistent with one another.

One question though - what makes alkalinity rise, in normal usage? Anything besides chemicals added?

By the way - the first place told me I needed to raise my calcium hardness in my vinyl liner pool to around 250 or so. I just nodded and let him talk. They said I needed to lower alkalinity while the 2nd (regular store) said I need to raise it.

I pray for the uneducated (or those who don't read this board!)

 

[linen]

I'm in the US, but frankly I was wanting to avoid all the "hold the vial just right and squint" type stuff. It seems like there is still a heck of a lot of interpretation and "inexactness", if you will, in using these tests. I mean, at 3ppm, the FC test has a precision of plus or minus 1.5. That seems pretty broad, unless I'm reading this wrong.

You are reading wrong. I think you are looking at the DPD instructions and not the FAS-DPD FC test instructions. The typical way to do the FAS-DPD means each drop is 0.5 ppm, and you can do the test so that each drop is 0.2 ppm, it just uses more reagent. My understanding is the FAS-DPD FC test is considered more accurate than the colorq.

As far as the CYA, I'd have to go measure/weight what's left, but I suspect that 30 number is about right, to be honest.

We can confirm, if you can tell us how much you have added. Trichlor, dichlor, and of course stabalizer/conditioner (cya) all add cya. How did you add your cya?

 

[harleysilo]

I'm in the US, but frankly I was wanting to avoid all the "hold the vial just right and squint" type stuff. It seems like there is still a heck of a lot of interpretation and "inexactness", if you will, in using these tests. I mean, at 3ppm, the FC test has a precision of plus or minus 1.5. That seems pretty broad, unless I'm reading this wrong.

As far as the CYA, I'd have to go measure/weight what's left, but I suspect that 30 number is about right, to be honest.

I'd like a kit that takes the guess work out of it (at least mostly) and so I was hoping to "calibrate" said kit against some "known" sources. But I can't seem to get consistent results. And, yeah, I've read that pool stores aren't necessarily the best either (God forbid you suggest that to them!!).

The one thing I can say is that my results from the ColorQ have been fairly logically consistent with one another.

One question though - what makes alkalinity rise, in normal usage? Anything besides chemicals added?

By the way - the first place told me I needed to raise my calcium hardness in my vinyl liner pool to around 250 or so. I just nodded and let him talk. They said I needed to lower alkalinity while the 2nd (regular store) said I need to raise it.

I pray for the uneducated (or those who don't read this board!)

You aren't reading it wrong, but for the FAS_DPD Chlorine test the accuracy is The precision of the measurement is plus or minus one drop when up to 10 drops of titrant are used, or plus or minus 10% of the final reading, when more than 10 drops of titrant are used.

You can actually buy known levels of "water" to practice/test your testing abilities on.

The only test that I find annoying is the CYA test, and now that i've done it 10 times i'm confident that i'm doing it right.

 

[CaOCl2]

I mean, at 3ppm, the FC test has a precision of plus or minus 1.5. That seems pretty broad, unless I'm reading this wrong.

10% of 3 ppm is 0.3, so the FC can be anywhere from 2.7 to 3.3.