My Tests vs. Pool Store Tests (Huge Difference in CYA)

[spurs21stx]

I did a full testing this morning around 6:30 a.m. and also pulled a sample to take to the pool store to test. All the numbers match up close enough except the CYA number (see picture attached, my results are written in pencil). I am getting a reading of 40 and the pool store got 0 . How can it be that different? They were using the Taylor test kit as I saw the cylinder with the black dot on the table where they do the tests (and also after I questioned the difference) the guy mentioned they are using Taylor testing and it is accurate. He seemed surprised when I told him I am using the same kit at home and explained to him how to run the CYA test.

My other question is, I took the sample at 6:30 am but did not take it to the pool store until around 12:30 pm. I left it in my car during this time---could the heat inside the car eat up that much CYA? Am I running the test wrong at home? As a backup in the past I have had my wife run the test and she gets just about the same as me. I will do that again tonight to make sure.

Any thoughts or theories as to what can be causing this huge discrepancy in the numbers?

 

[jblizzle]

What you found is not unique. Pool store testing is not worth what you pay for it ... and CYA is the one they are the worst at ... usually because they do not use the correct lighting.

There are many many thread in the forum that discuss this. Do yourself a favor and trust your own testing.

 

[JasonLion]

FC and PH can sometimes be dramatically affected by keeping a sample in the car for a few hours, but the CYA result would not be affected at all.

These results actually match up better than what we usually see with pool stores. Test results from pool stores can be wildly wrong for no apparent reason. The CYA test is especially problematic, both because it takes longer to do and pool store staff are often in a hurry, and also because it is affected by illumination and designed to be read outdoors in indirect sunlight, which is difficult to duplicate in a pool store.

Trust your own testing, and just ignore what the pool store says.

 

[spurs21stx]

Thanks guys for the vote of confidence.

Now if I had followed their advice I would have spent 30 bucks on stabilizer to add 7.8 lbs to my pool, using the pool math this would have raised my CYA to 90, yikes!!!

 

[chem geek]

I suspect he was impatient and didn't mix the sample for 30 seconds. He might also have been looking at it with a light overhead directly shining into it. Either problem would lead to a reading that was lower than it should be.

 

[Isaac-1]

I would suggest you try reading the CYA test yourself in varying lighting conditions. The proper condition is outdoors in sunlight during the day (not sunrise or sunset when the light is dimmer) with the sun to your back and the sample held at waist level in your shadow. The mixed sample is good for several minutes so you can pour it back and forth until you are confident in your reading, now try reading it again in a covered area with access to sunlight, and then finally try it indoors under artificial lights. You may want to do it the other way around if your eyes are slow to adjust to the darker indoor lighting.

 

[spurs21stx]

Just to close out this issue I ran the CYA tests again yesterday, outside with sun to my back, and got 40. I then had my wife run it without telling her my result and she also go 40. As most above suggested the pool store employee was either in a rush or his lighting was playing tricks on him. Thanks again for your help.