Shelley N said:
With a CYA of 50, I need to keep my FC at 4-8 with 5.7 being the goal.
There seems to be a LOT of misconception on the accuracy and precision of the testing we are doing in our pools. It's really a good estimate at best for several reasons. First there is a difference between the precision of a test (What is the actual test number?) and the accuracy of a test (Is the test results repeatable within a specific margin?) As a matter of interest, test strips ARE relatively ACCURATE but they are by no means PRECISE! This is the problem with using strips.
Let's look at the CYA test since it is the MOST subjective. It's accuracy is very good for CYA levels between 30 to maybe about 80 ppm but it's precision is really only about 10 ppm at best and most likely higher because of the subjective nature of this test. This means if you repeat the test on the same sample the dot should disappear in about the same place BUT what is the actual reading? You can't really interpolate between numbers since the scale on the view tube is not linear nor is it truly logarithmic. For example, this means that just because the dot is disappearing midpoint between the 30 and 40 mark it does NOT mean the CYA reading is 35 ppm. It's an invalid assumption. It just means that the CYA
as you are testing it is somewhere between 30 and 40 ppm! Someone else testing the same sample might come up with a different reading because of the subjectiveness of this particular test. This does not invalidate the test by any means because it gives you a baseline to see if and how your CYA level is changing. Realize that the $1000 LaMotte Waterlink colorimeter used in many pool stores has a precision of +10/-25 ppm for this particular test (and this is much more accurate than the human eye and a disappearing black dot). That means that you CYA reading might be 35ppm off from the measured value in a worse case with this setup.
Now let's look at the TA and CH titrations. It's precision is +/- 10 ppm but it's accuracy will change with the actual level of TA or CH since the total number of drops used to reach endpoint changes.
Ditto for the FAS-DPD titrations which have a precision of either +/- .2 ppm for a 25 ml sample or +/- .5 ppm for a 10 ml sample. In this last case it is possible for your reading to have a precision that is off by up to 2 ppm, believe it or not! As FC increases the accuracy also changes because of the increase in number of drop of titrant needed.
We also need to remember that there are a lot of factors that can affect the drop size (angle of the bottle, static charges, amount of reagent left in the bottle) so there is no guarantee that the drop size is going to be uniform each time we test. If we would want to increase the
precision of these titrations (get a more precise number) the we could use a graduated glass burette (a piece of laboratory glassware used for titrations) so the actual AMOUNT of reagent used for the test can be read off the burette (usually in ml.). The use of dropper bottles introduces more error into the precision of our testing. However, it is certainly precise enough for our puposes of testing a swimming pool!
We have to keep this in mind when we test a pool and make changes to the chemical levels. Also, unless you metered the water when filling the pool you really DON"T know how many gallons it actually holds, you just have another 'estimate'.
So when you say your CYA 50 so you need to maintain your FC at 5.7 you are really just deluding your self. Saying that you need to keep your FC between 4-8 is much better and then in actual pracitice you might find that 4 or even 5 ppm
as you test it is too low and you really need to keep it at 6ppm, for example.
The main advantage to testing your own water is that your testing procedure is going to be done the same way each time so your results are more likely to be accurate (repeatable) but if you think they are precise to the degree that test results are talked about on here (or most places for that matter) then you are sadly mistaken.
Anyone who has had college level Analytical Chemistry will understand what I am talking about.
For those that don't just remember, we are talking about swimming pools, not laboratory experiments!
Going back to OTO, let's say that you have found out that when the OTO is a deep yellow your pool is fine, then you know when it's a pale yellow you need chlorine and when let's say when you shock it turns a deep orange, then you know when it's not that deep orange and it's yellow again you are no longer at shock level. A pretty handy test if you ask me (and certainly accurate enough for daily use).
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