Accuracy of the block

[LBI salt pool]

Hey guys. I use the block with the droppers to test my hot tub water pretty much every other day and add liquid chlorine accordingly. Today I decided to do a fas/dot test. Significantly different results. The block shows around 1 ppm. Fas/dpt test was 12.5 ppm. Crazy huh?
So for last few weeks I have been using block as my guide and just adding more and more chlorine Considering I am at 12.5 I would think the block would be orange color

 

[PoolStored]

We recommend you ignore the block test and use the FAS-DPD for daily testing. Use 10ml sample, each drop is .5, to save reagent.

 

[dfwnoob]

I know a lot of people here use the chlorine part of the comparator block as a "is there chlorine here?" test for some reason, but I do not use it all. Never have. I don't think I have any color deficiency in my eyes, but I cannot stand color-comparison tests. Too subjective and both the OTO and DPD-only chlorine tests are very limited with the ranges they can test for (5 PPM and 10 PPM respectively, if I recall correctly).

FAS-DPD is the way to go.

 

[Newdude]

+3. I use my block test as long as it should be orange or really really yellow and it is. The day that it doesn't agree to what I already know, I won't blink to ignore it and use the fas/dpd exclusively.

 

[LBI salt pool]

Thanks guys. So here’s a wrinkle….just tested the pool and was 8.5 ppm which is my perfect target. I did a block test to compare and it’s dark. The hot tub was 4 ppm higher and the block test was way way lighter
This is the pool at 8.5 ppm:

 

[dfwnoob]

I assume that's the DPD-only test and not OTO? That is a known phenomenon of the DPD-only testing which is why it's not recommended. When you hit the upper range of the test, the color starts going the other way. Taylor Technologies notes this in their documentation: "Chlorine levels > approx. 10 ppm may bleach out indicator; to prevent, dilute sample with DI water as necessary and retest."

I'd just stop using the comparator block for chlorine; it's simply not very good.