Different test results just minutes apart. Quick question?

[Mesamav]

I was running my normal tests this morning and something weird happened. It's happened to me a couple of times and I have no explanation for it.
I was only testing my chlorine and PH this morning and my chlorine came up as almost nothing.
I'm in a somewhat unusual situation as I'm taking care of another pool down the street as a good friend of mine has an offer on this house but the bank hasn't approved it yet so until the bank says yes or no, I'm keeping the pool up so I'm using my test kit on both pools.
So anyway, when I put the water in the fill pipe for the chlorine, I dumped a little too much out of the tube so I used my finger to add a few drops by dipping my finger in the pool and letting the drops go into the tube. Not a lot, maybe 5 or 6 drops is all. I had just used the bathroom and I had washed my hands just minutes earlier.
I tested, it came up as .5 so I tested again. This time it came up about 4 ppm. Why did the first test not work? Was my finger dirty and did that use up the chlorine that was in the sample in just a few seconds or could the fact that I'm using the test kit on two different pools have something to do with it? I rinse out the test kit before and after every use including the lid and my reagents are about a week old.
There isn't a problem but I'm just curious why that would happen.

 

[chem geek]

Re: Different test results just minutes apart. Quick questio

Yes, your finger probably still had the remnants from sweat (and other things) or even soap. It doesn't take much to use up chlorine in a small water sample. This is one reason why some commercial/public pools recommend or require people to rinse off before entering the pool. It can cut down the chlorine demand in half. For residential pools, the bather load is so low for this difference to not be noticeable, but your finger is pretty "big" compared to the 10 ml (or 25 ml) sample volume.

It might be interesting to see how different methods or time of washing ones hands and thoroughly rinsing affect this test.

 

[Mesamav]

Re: Different test results just minutes apart. Quick questio

Yes, your finger probably still had the remnants from sweat (and other things) or even soap. It doesn't take much to use up chlorine in a small water sample. This is one reason why some commercial/public pools recommend or require people to rinse off before entering the pool. It can cut down the chlorine demand in half. For residential pools, the bather load is so low for this difference to not be noticeable, but your finger is pretty "big" compared to the 10 ml (or 25 ml) sample volume.

It might be interesting to see how different methods or time of washing ones hands and thoroughly rinsing affect this test.

That would be an interesting topic for the chemical geeks out there.

 

[UnderWaterVanya]

Re: Different test results just minutes apart. Quick questio

Do you dip your cylinders in the pool? I use a cup and have extra ready to go on the counter when testing.

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