Are pool store water readings acturate?

[flyboy320]

My Taylor test kit ran out of the reagent to test the CYA levels so I brought a sample of my pool water to the pool store and had them test it. I was only interested in the CYA levels, but they test the FC, TC, CC, PH, hardness, alkalinity, and CYA.

Turns out the CYA was good, but their test showed the PH at 8.1. My Taylor test kit shows it around 7.5. The test they do at the store is one where they use a pipet (I think that's what it is..anyways it picks up a measured amount of water with a plunger you activate with your thumb) and they insert the water into a test tube that's sealed with foil at the top. The test tube then is inserted into a device that I guess measures the color/shade of the water electronically and displays the result on a computer. The test tubes are then recycled (not used again for another test).

It seems that something as elaborate as that should be accurate, but as I said, my test kit shows a much lower PH value. The test kit is about 8 months old. Would a reagent that is a bit old give an inaccurate result, do you think the stores results are the ones I should trust, or are the Taylor tests very accurate?

For comparison I used some cheaper test kits I had lying around, and here's what I got.

Test strips PH7.6
cheap drop test kit #1 PH7.6-7.7
cheap drop test kit #2 PH7.5
Taylor PH7.5

 

[JohnT]

I'd bet you are dead on. Test a sample of distilled water and see what you get. For entertainment, take a sample of distilled water to the store.

 

[flyboy320]

I'd bet you are dead on. Test a sample of distilled water and see what you get. For entertainment, take a sample of distilled water to the store.

OK, I'll give that a try. I don't have any distilled water though, but I do have a RO system in the house, will that give the same results as distilled water?

 

[chem geek]

The water from the RO system should be about as good as distilled. However, I don't think that unbuffered water from any source, be it distilled or from an RO system, is a good idea to test for pH. Carbon dioxide in air entering into RO or distilled water will lower it's pH. In theory, absolutely pure water with no carbon dioxide in it will have its pH drop to around 5.7 when exposed to air and reaching equilibrium. Though this may sound like a problem, the TA is so low in such water that it has virtually no effect on pH whatsoever when added to buffered water such as found in pools.