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?