Converting testing methods - questions

sande005

Bronze Supporter
Aug 19, 2018
930
White Bear Lake, MN
Pool Size
23000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-45
For many, many years I used a LaMotte test kit to check my chemistry. Uses a slide color comparator for FC, CC and pH. Tablets dissolved in the appropriate amount of pool water. Was running low on some of the less frequently used tabs, so made the jump to the TFP test kit.
They generally agreed well on most of the tests. And I love the better accuracy for FC/CC in the TFP kit. Plus the stirrer is fun to use. But, two items are an issue:

CYA - the LaMotte disappearing dot test consistently reads about 15 points below the TFP. I like having a CYA of 40 more than I like 55. Might take a sample to the pool store to settle the matter, but thought I would solicit opinions here, first.

PH - the increments in the TFP test of .3 is just bad. I have to guess whether it is closer to 7.2 or 7.5? The LaMotte does it in increments of .2 and is easier to interpolate to in between values. At least they seem to be in general agreement with each other, even if the margin of error is there. While I have a seasons worth of LaMotte tabs left, is it valid to come up with my own number of drops per ml of water brew to continue to use the Lamott comparator? Haven't measured yet, but it appears to need about twice the water, so may need twice the drops for use. Does the test scale properly?
(yeah, I got the meter too. And it is wonky like many others report...)
 
Might take a sample to the pool store to settle the matter, but thought I would solicit opinions here, first.

This would be an enormous waste of time as CYA is the test pool stores are the worst at. Stay out of the store. Most people on here see about a 10ppm difference between the LaMotte and Taylor CYA tests. I think LaMotte test definitely looks easier, but I can't find any evidence of which one is more accurate.

The reason most people here are fine with the Taylor pH comparator is because pH just doesn't really need to be that specific. As long as you're in the 7s, you're good. Does not having pH resolution down to .2 really matter? I agree the color comparison test is kinda junk but I can easily go "yep, that's somewhere around 7.5, good enough for government work!"
 
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is it valid to come up with my own number of drops per ml of water brew to continue to use the Lamott comparator? Haven't measured yet, but it appears to need about twice the water, so may need twice the drops for use. Does the test scale properly?
The pH test doesn't work like that. Each drop turns the same pH-dependent color, so the test doesn't require a specific ratio of reagent to water; more drops just makes the color easier to see.

The Taylor pH reagent is phenol red. If the LaMotte pH test also uses phenol red as an indicator, then you could use the Taylor reagent in the LaMotte comparator. Experiment with the number of drops to get an easy-to-compare color density.
 
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