Diluted CYA Testing - Increased error rate

[max2k]

... It's also fair to mention the amount of error with a 3:1 dilution. ...

Just for my own education- where that error would come from? Let's say I take a cup of pool water, put it in some clean bucket and then add 3 of the same cups of distilled water (to eliminate tap water additives), mix it up and then take 10mL of the mix for testing- wouldn't this dilution bring concentration of everything I measure down x4 times exactly?

Tap water can bring FC error as I read some areas have up to 1 ppm which would multiply by 4 at the end. My own tap water doesn't have any < 0.2 ppm so I used this technique when I was coming down from 300ppm CYA.

 

[needsajet]

I agree with you provided the measuring is done with the same measuring cup and same person. I don't think the error need be significant if it's done carefully, and the volume can be accurately measured (i.e. I would not suggest measuring a few mls without a pipette. I think cups are a better size). But I was told by an expert that we don't recommend anything over 1:1 due to introduction of too much error.

With respect to CYA, there's some good logic (not related to measurement) behind this position. If after 1:1 dilution, it's still 90 ppm or more, you need to drain an enormous amount of your water anyway. Personally, I prefer knowing what I'm up against when spending $ on water, and larger dilutions are common to me from testing liquid feed in agriculture, so it seems fine to me, but I nonetheless accept and support that it's not a TFP recommended practice.

 

[max2k]

I see, I was mostly asking to find out if there's inherent logic flaw in the procedure I missed.

 

[miwitte]

I had the same dilemma last year my CYA was 75 or more using a 4:1 dillution so i was ealily 200+ CYA. I used distilled water and a graduated cylinder to be accurate for the dillution to rule out anything in the tap water and i had same issues. Bottom line your CYA is way too high and there will be significant error in testing. at a 3:1 dillution, seeing CYA of 60, 70 80 means your CYA could be 200 or maybe even higre not sure how dillution works with CAY.

Bottom line you need to drain probably 2/3 of you water or more in my case it was 3/4 of my water to get it to the 50-60 range. This was last year and i have kept the chlorine levels where my new CYA dictates and i have not had a issue since i switched to BBB in Sept. I had a issue last week where my Stenner injector broke and caused some algae but withing 3 days of BBB SLAM it was back to normal. My pool sparkles like never before, no algae, my PH is very stable. I haven't been to the pool store in a year. Just put in liquid chlorine from Wally world and that's it never really adj PH or TA either. With a CYA of 200 or more your FC needs to be maintained at 20 PPM where normally with CYA of 40 or 50 its like 5-6. This is why i always had issues i would keep chlorine at the pool store recommended levels of 3-4 PPM(it needed to be 20 for my CYA of 200+) and had algae issues all the time. Then its off to pool store for algecide, shock, Pool perfect etc. Then your other numbers are all over the place. If you actually had to SLAM or shock your pool with a CYA of 200 the shock level is 80 PPM.

Its a bit daunting(and potentially expensive) to drain most of your pool and refill but it was well worth it to not have any more issues.

 

[gwegan]

The increased error rate I believe comes from the level of accuracy of the test itself. CYA testing is without dilution only accurate to within 10 ppm. So in a normal situation if you get a reading of 40 the actual CYA could be within a range of 30 to 50. There are just too many other uncontrollable factors. So if you dilute that range expands.

 

[duraleigh]

+1 what gwegan says. TFP really doesn't like dilution tests. The CYA test is rough anyway (I think it's +- 15) so it is just an approximation even at 2:1 and much worse at 4:1. Still, it can sometimes reveal some information that's helpful.

The OTO chlorine test should NEVER be diluted. It's a bit of a guess at full strength and becomes wildly inaccurate with any dilution.

I read some stories of folks doubling the pool water sample on the FAS/DPD test......penny wise and pound foolish. The entire point of the FAS/DPD is accuracy so why would anyone dilute to save a couple of pennies?

 

[max2k]

...Bottom line you need to drain probably 2/3 of you water or more in my case it was 3/4 of my water to get it to the 50-60 range...

Luckily it's all in the past, as I said 'I was coming down from 300 ppm CYA' . My current level is 40 and all is well. I managed to survive at FC 16-18 throughout the year - may be DE filter helped, may be VS pump running at low 800rpm speed for 12 hrs every night, may be weekly brushing but somehow algae never took over the pool. The worst I've seen were small quarter size patches in the areas with poor circulation and they were easy to clean up. I simply couldn't afford to replace that much water due to local restrictions and pool size.