Variation of test results of CYA (testing issue)

[surfs_up_today]

I am converting to TFP. Got my test kit. My water is fine, I completed my SLAM and now looking to just maintain. So I had the store test my water just to very the TF-100 test kit results and the CYA is significantly different.

Here are the results:

	Store	TF-100
FC	6.18	6.5
CC	0	0
PH	7.6	7.5
CA	223	250
TA	84	100
Adjusted TA	69
CYA	45	30
Copper	0
Iron	0
Maganese	0
Total Dis Sol	800

The CYA is VERY different. I looked at the pool school notes and it says (in the extended test directions section, last bullet):

• The precision of the test, when done correctly, is around plus or minus 15 for levels up to 90 and plus or minus 30 for levels between 100 and 200. Novices often have problems reading the test correctly, and tend to get higher than actual readings.

Unfortunately, the units were not given in this note. Is the 15 variation percent or PPM? I thought initially it was PPM but maybe is it percent. I will assume percent for the following:

If the store system is reading CYA high (+15%) and the TF-100 is reading low (15%) we get adjusted levels at(PPM):

38.25 Store

34.5 TF-100

Should I try other stores and get CYA re-tested? Or is my CYA test kit bad? This is a huge delta on the non-adjusted readings and is confusing me as which way to go. (I will ignore the variations in alkalinity for now).

Please share your thoughts on what I should do.

 

[BasicTek]

I'm new here too, but everyone on this site will tell you to ignore the store (That's what they told me). I have a store test history for CYA that was off by as much as 200. The owner/manager was a great guy but the store simply didn't test CYA accurately, they never diluted when over 100, and they don't make recommendations based on CYA/FC relationship, so all their tests can do is throw you off. Test yourself, eventually you will have full confidence in it and you will see your pool responds properly to your tests.

Now if you test and see the CYA jumping all over the place then you can question your testing, make sure you test in the daylight (sunlight preferred) back to the sun, fill the container to each 10 mark and then look for the dot (rather than staring down as you fill so you don't trick your mind). I saw those recommendations if your not fully confident. Don't let store results throw you off, then when you are confident check the store, check several and you will see and say (wow and everyone blindly trusts these store tests)

-Tim

 

[cfherrman]

Just drop the numbers in the left column, every time you test add another column, and use that to check your accuracy.

 

[pookiesunshine]

order the 50ppm standard from tftestkits. i was reading about 10ppm lower than actual.

 

[JoyfulNoise]

The bottom line is this - trust your own test results and forget about the pool store. You're assuming that pool store testing is accurate but, based on the experience of tens of thousands of pool owners here on TFP, that is not true. Pool Stores by and large have very sloppy testing practices and while they may sometimes get one or two of the tests right, the typically fail to get the most critical one done right - CYA.

Yes, the turbidity test for CYA has an error of +/-15 PPM but that is not an issue if you do the test correctly and consistently each time. With practice, the test becomes easier to perform. Follow the advice given on TFP on how to do the test.

Here are some tips -

- Do the test outdoors in bright, full sun
- Hold the tube waist high and back to the sun so you're casting a shadow on your tube when viewing and only use two fingers to hold the tube at the top, do not block the tube with you hand.
- Dropper in your test solution up to the first mark, 100ppm, and look to see if the dot disappeared. If it is still visible, add solution to the next mark and check. Repeat the above process until you find the point at which the dot disappears.
- Never stare intently as if daring yourself to find the dot; this isn't a "Where's Waldo" contest. If you stare intently for long periods of time your eyes and mind will trick you into thinking you see the dot.
- Repeat the test 2 to 3 times by simply pouring the solution back into the mixing bottle and trying again. Average your results and always round up to the highest tens value (i.e., an average of 34 => 40ppm).

 

[surfs_up_today]

Thank you. Maybe I am being stupid, but if my CYA test reads 30, and it is + or - 15 PPM per the pool school notes on CYA testing, then my actual CYA is 15-45 PPM. Now FC targets are are a function of the CYA, so the FC targets are all over the map.

 

[Richard320]

What you do is pour the water sample back and forth between the view tube and the mixing bottle. Do it a dozen times if you have to. Eventually you will find a certain number or range that comes up most frequently. Probably find that 9 out of 12 times your readings are all within a 5 ppm range. Use the high end as your CYA level and stop overthinking it.

And just for kicks, take a sample to that same pool store in a few days and see if they get the same number. If you haven;t added any CYA via stabilizer or powdered chlorine, nor replaced large amounts of water, it should be the same. See if they get the same number twice in a row.

 

[JoyfulNoise]

Thank you. Maybe I am being stupid, but if my CYA test reads 30, and it is + or - 15 PPM per the pool school notes on CYA testing, then my actual CYA is 15-45 PPM. Now FC targets are are a function of the CYA, so the FC targets are all over the map.

It's rarely that large of an error. If you do the test consistently and the same way every time as I outlined above, you will get very consistent and accurate results. The biggest problem most people have is that they do the test inconsistently (sometimes outdoors, sometimes indoors, etc). Always do the test the same way and make sure you are doing the test outdoors in the brightest sunlight possible and you will get the right results.

 

[surfs_up_today]

Thank you. I have consistency from test to test. I get the same number. But the accuracy as described in the pool school notes applies to test equipment accuracy, NOT user error. What the note from Pool School says is:

• The precision of the test, when done correctly, is around plus or minus 15. (From JoyfulNoise post #5 above, that is assumed to be PPM of CYA, not percent variation)

Just to restate, a reading of CYA 30 ppm means the actual CYA is in the (wide) range of 15-45 PPM per the note in pool school.

Is the note in pool school wrong? Does it need more explanation?

(I love this system...I went from Green to Clean using the SLAM this year...just trying to figure out the details)

 

[JoyfulNoise]

Thank you. I have consistency from test to test. I get the same number. But the accuracy as described in the pool school notes applies to test equipment accuracy, NOT user error. What the note from Pool School says is:

• The precision of the test, when done correctly, is around plus or minus 15. (From JoyfulNoise post #5 above, that is assumed to be PPM of CYA, not percent variation)

Just to restate, a reading of CYA 30 ppm means the actual CYA is in the (wide) range of 15-45 PPM per the note in pool school.

Is the note in pool school wrong? Does it need more explanation?

(I love this system...I went from Green to Clean using the SLAM this year...just trying to figure out the details)

The biggest source of error in the test is human error because the test requires the use of the human eye. The chemistry of the test is quite sound and produces very low error -

1. The concentration of CYA in solution is fixed;
2. The amount of melamine added by the R-0013 is well in excess of the amount needed to react with the CYA;
3. The R-0013 shifts the pH of the mixture to the acidic region so that there is no melamine-cyanurate solubility issues;
4. Chlorine/chloramine interferences are limited due to relatively low levels of FC/CC

Because of those 4 conditions, the intrinsic chemistry produces a very well defined turbidity upon formation of the melamine-cyanurate crystals. So the intrinsic tolerance in the actual level of CYA in solution is quite small and it is easily ignored (i.e., a few ppm at most).

The main problem is the variation in your eye relative to other people's eyes as well as the variation in environmental conditions, i.e., mostly from poor lighting conditions. Because of that, if you take 100 people and each give them the same sample of water, you will statistically find that the reported value of CYA concentration has a deviation from the real value by about +/-15ppm in a large sample population of testers. If one controls carefully for lighting and procedure (i.e., everyone is taught to do it the same way), then the statistics improve markedly and you get a much smaller error.

SO think of it as a population statistic and not really an intrinsic error in the test.

Your best bet is to assume that the CYA is as you measure it and not worry about testing tolerance and trying to dose based on some range. Otherwise you will be chasing your tail. If you do the test as I described it, you should get very consistent and accurate results. The FC/CYA ratios suggested by TFP include more than enough margin in them that, for normal day-to-day chlorination, your water will be perfectly safe even if you were systematically reading the test too low.