Bad Technique = Bad Results

[hp223]

I had my pool re-plastered about 2 months ago. I have been using the BBB method and the TF-100 test kit since it was re-plastered.

I suspected the results I was getting for my CH levels was too high using the TF-100 test kit.

I took water samples to 4 pool stores. The results were 220, 220, 285 and 320 for the CH level. My testing showed an range of 430 – 460.

I went back to two of the pool stores using Taylor chemicals and asked them to retest while I was watching. Both again got 220 CH levels.

Thinking my chemicals might be bad I purchased the Taylor chemicals from them. I retested at home using the new chemicals. I got 410 instead of 440. I retested and got 440.

Since I was getting inconsistent results, I decided it was probably something I was doing.

I then remembered one of the instructions I was not following in the Extended Test Kit Directions extended-test-kit-directions-t25081.html

“Hold the dropper bottles vertically and squeeze gently, so that drops come out slowly and seem to hang on the tip of the dropper bottle for a moment before falling."

I was holding the bottle at a 20-30 degree angle and squeezing out the drops. I think I started out holding the bottle vertically but over the last 2 months changed to holding it at 20-30 degrees.

I reran the test holding bottles vertically. The CH level dropped to 310 with both the set of chemicals from the pool store and the chemicals from the TF-100 test kit.

As to why the pool store got 220 CH levels using the Taylor chemicals I think the bottle they were using has a bigger hole for the drops (thus bigger drops) than the “Taylor R-0012 bottle and the TF100 R-0012 bottle. They were not using the Taylor bottle to dispense the drops.

I measured the amount of difference for 30 drops of R-0012 by holding the bottle vertically (1.2 ml) versus holding it a 20-30 degree angle (.9 ml). I used a 2.5 ml syringe I got from the drug store to measure the amount. Holding the bottle at a 20-30 degree angle results in about 25% less volume dispensed which explains why my results were so high. Using the syringe is probably not exact but it’s close enough to show the problem.

 

[duraleigh]

The size of the hole in the dropper tip does not dictate the size of the drop. Done correctly, the fluid moves through the hole and clings to the dropper tip until it is large enough that gravity overcomes the surface tension and the drop falls off.

The TF-100 dropper tips and K-2006 dropper tips are virtually identical (and interchangeable)

 

[hp223]

Then I don't know why both pool stores got the same results of 220 CH. I saw they chemicals they were using and they were Taylor chemicals in a 16 ounce bottles. I watched them drop 5 drops of the R-0011L and 22 drops of the R-0012. The only difference was they used a eyedropper for the R-0010 with a fixed amount marked rather than count 20 drops. When I used both the Taylor chemicals from the TF-100 and the Taylor chemicals I purchased I got exactly the same results of 310. I guess I don't really care that much. I trust the TF-100 test results are correct.

 

[linen]

I wouldn't trust pool store readings, many factors cause their results to be very inconsistent.

 

[duraleigh]

I trust the TF-100 test results are correct.

There is an overwhelming amount of anecdotal evidence on the forum that supports that statement.

Going to a pool store for verification of your testing most always results in confusion.

 

[hp223]

I understand that pool stores give inconsistent results but in this case it allowed me to diagnose what I was doing wrong. Having 4 pool stores testing my CH at ½ to ¾ of my testing results pointed to a possible problem with something associated with me.

I had another post http://www.troublefreepool.com/rising-calcium-re-plastered-pool-t68844.html asking about why my CH was high and no one suggested my testing technique could be the problem.

The purpose of the post was to alert other new users to make sure they follow the direction -
"Hold the dropper bottles vertically and squeeze gently, so that drops come out slowly and seem to hang on the tip of the dropper bottle for a moment before falling."

Holding the dropper bottles at an angle resulted in reading of a CH range of 400-470. Holding the dropper bottles vertically resulted in a repeatable 310 CH level.

If you would have asked me before I found out what I was doing wrong, I would have sworn that I was following directions exactly.

I am an engineer and like repeatable test results and if not an explanation of why they are not repeatable.

-Glenn

 

[linen]

Good point!

 

[langerking]

Yeah, I agree that bad technique can give you bad results. I was doing the CYA test indoors on top of a white towel, completely ignoring the fact that it should be done outdoors with your back to the sun and holding the fill tube at waste level . It skewed my results by about 15 ppm. (Done correctly - 35, done incorrectly - 52)