Test variances?

[stevodevo]

Hi all,

Just after opinions about the factors that could cause a CH variance between 2 test kits (both Taylor) using the same water.

I've just started testing my CH with my new K-2006 kit and coincidentally found a pool shop near where I live that also uses the exact same Taylor drop tests to do their free testing. I took a sample to them to compare my results with theirs. I was honest about what I was doing and the girl in the shop was really helpful and let me watch exactly how she was doing the tests.

Now I know I'm splitting hairs here, but I got 230ppm and the shop got 210 (even after a second test to check). All other test results (FC, TA etc) were identical to mine. I re-tested with my kit when I got home and still got 230. Can anybody think of any reason that this would happen? I'm seriously not concerned about the 20ppm difference for my pool's sake, but I'd like to know for curiosity.

The only thought I've come up with so far is slightly different drop size? Maybe my bottle has a minutely smaller hole...


Thanks,
Steve.

 

[Bama Rambler]

210 to 230 is only two drops and it's well within the precision of the test. I suspect that the droppers are producing slightly different sized drops or that the operator is causing the difference. Some people have a tendency to release the pressure on the bottle just as the drop is about to release and it causes the drop to be a tiny bit smaller than if you don't release. Also if one of you is using a stirrer and the other isn't, that'll make a little difference.

 

[stevodevo]

Thanks BR. I'll settle on drop size and technique as the answer. Incidentally, use a SpeedStir and the shop used a stirring stick/paddle.

I've been having so much fun with variances between my results and pools shops with photometers, it's really nice to have finally found somewhere that is getting results I can trust. I had a little debate the other day with a pool shop after they got 40ppm (photometer) for TA and I got 90 from 2 different kits of my own. They ended up blaming my reagents and insisted their results and reagents were spot on. Now I have the Taylor kit as my "gold standard" I did a test yesterday using all 3 kits. The result was all 3 kits of my kits tested the same.

It's so liberating to know my results will be accurate from now on. Unfortunately I don't think any of us can completely do away with pool shops (salt, calcium, etc) so it's nice to know I have also found a shop I can trust to buy supplies from... when I need them! Plus, every now and then I can swing by with a water sample to keep me honest and check my reagents are still doing the right job.


Steve.

 

[chem geek]

Taylor's specification for their dropper tips is 24 drops per milliliter, +/- 1 drop. So there is a 4% error possible there and that doesn't account for static electricity affecting drop size (which you can remove by wiping the dropper tip with a damp cloth). There are other errors in titrant concentration so that overall you can expect accuracy to within around 10% (or one drop, whichever is greater) so in your case you are seeing around a 10% difference which is perfectly reasonable.

 

[stevodevo]

Enter Chem Geek to put it beyond doubt with some hard facts... nice explanation. That makes perfect sense.