Test kit problems: them or me?

[loop_pea]

I seem to be asking a lot of questions on here lately, and I have another!

I've been running the pool with what I thought was FC of 4ppm. Anyway, it got to the point where the TC was definitely out of the range of the tester (colorimeter), so I started doing a 50% dilution to take the FC and TC measurements. I was surprised when the numbers came out a lot lower. For example, with no dilution the machine reads 4.5 ppm and at 50% dilution it reads 1.35 ppm (so 2.7 ppm). Big difference.

I'm diluting 50ml pool water with 50 ml deionized water, and I don't think there's error of that magnitude creeping in from the dilution, although there might be sources of error I don't know about.

So which number is right? I'm tending to think that the 50% dilution number is better because the machine is known to be more accurate in that range and the number is more stable (it changes quite a lot with time at higher values of FC). Also, if that's the right number, then I've been unintentionally underchlorinating my pool, which would help to explain why I'm struggling to keep on top of the CC.

The machine was sent back to the manufacturer for recalibration at the end of last season, and has only been used for around 6 weeks this year. I'm wondering is this difference in readings is just a limitation of the method, or could the machine be faulty and need to go back to the manufacturer again? Is there a way to be sure?

 

[chem geek]

It sounds like your colorimeter has a serious non-linearity problem. It doesn't seem at all accurate. It's not you, it's the colorimeter. This is why we recommend the FAS-DPD chlorine test. Send a PM to "teapot" who has FAS-DPD chlorine tests and CYA tests for Europe. You can also contact Dave at TFTestkits, though I suspect it's too expensive to ship to England. Palintest has a FAS-DPD chlorine test but it appears to only be for the U.S.

 

[loop_pea]

The test kit manufacturer kindly sent me a reference kit so that I could check if the colorimeter was working properly.

The reference samples are FC of 0.97 +- 0.08 and 3.90 +- 0.25.

I put them in the machine and it read 0.99 and 4.16. I took the second reading a few times, and it varied from 4.15 to 4.17, with one odd reading of 4.04.

This would suggest that the machine is overreading slightly, but it doesn't seem to be enough to explain the discrepancy.

I'm stumped.

 

[chem geek]

I'm stumped too. If your deionized water wasn't truly deionized and instead has some sort of contaminants in it that chlorine could react with then that would explain your results. Either that or the reference samples aren't as they claim. Both of these explanations seem unlikely.

 

[loop_pea]

I'm on the third batch of DI water - they can't all have been contaminated with something that eats chlorine.

The only thing I can think of is that the DPD reaction is not sufficiently stable at higher levels of FC. Is this possible?

 

[chem geek]

It is absolutely true that the DPD reaction is non-linear and tends to bleach out especially above 5 ppm, but your device is supposed to compensate for this and is apparently reading the reference samples properly. Perhaps there is something else interfering with the results. Clearly, diluting the sample by a factor of 2 should result in half the reading. You might try getting a FAS-DPD kit and see if it has the same problem. If not, then ditch your device.