Comparing test procedures/kits - some observations

[jleonard]

Background

Over the past several weeks, I have been testing my pool water by three different approaches: 1) the TFT kit, 2) the ColorQ 7 system from LaMotte, and 3) the Bioguard Alex system at my neighborhood pool store.

My pool: 35,000 gallon, pebble-tech surface, DE filter, chlorine generator.

Data collection

Below are the results of my two most recent tests (sorry for the formatting, I couldn't figure out how to do tables.) The first column of results is from the TFT kit (ordered from http://www.tftestkits.com/). The second column of numbers are results from the ColorQ system, and the third column is from the computerized, Alex system at the Bioguard store.


4/11/2008
TFT ColorQ Alex/Bio
FC 0.5 0.44 0.1
CC - - -
TC 0.5 0.45 0.1
PH 8.2 8.30 7.3
TA 140 80 188
CH 190 191 200

Temp 77
Salt 3400

4/6/2008
TFT ColorQ Alex/Bio
FC 1 1.36 1.3
CC 0 0.3 0.2
TC 1 1.66 1.5
PH 7.6 8.0 7.3
TA 130 88 147
CH 200 174 203

Temp 65
Salt 3600


Protocols

I follow the prescribed procedures for the TFT test, rinsing the vials with pool water before filling to the appropriate levels. For the ColorQ, I rinse the test tubes and fill to the appropriate levels, add drops, shake and measure in the order prescribed the the instrument (I don't wait long between shaking and measuring). For the Alex, I stand around and wait while the measurements are taken (pretty boring, not nearly as fun as doing it myself.)

Results

I'm pretty comfortable with the FC, CC and TC values - they seem to be consistent across all three tests.

I'm uncomfortable with the other values (pH, TA, and CH). There seems to be too much disagreement between tests. In any case, results of these tests recommend different chemical applications, often in opposite direction.

Questions for the group

1) Would the high salt levels account for some of the test differences?

2) Does the temperature of the water affect the results?

3) Any suggestions for improving this situation?

4) Which test should I "believe."

Inquiring minds (at least mine) want to know! Thanks in advance for your insights.

 

[Guest]

The Alex system is designed to sell chemicals and maximize dealer profit. I would believe that one the least!
That leaves LaMotte and Taylor. If you notice, most of the discrepancies are with the TA and CH tests. The LaMotte reading lower than the Taylor on the TA is common and is because of the difference between a colormetric test vs a titration test. As to which is correct, it really depends on your testing technique. IF you are REALLY doing the tests right I would believe the titrations.
The variations in the pH test I would have to attribute to test procedure. Both tests should be fairly close. You only provided two sets of tests which is not really enough data to see any trends. If you had, say 100 sets of tests then we could see much better how they compare.

 

[JasonLion]

Salt wouldn't have anything to do with it.

Temperature only usually makes a difference with the CYA test, which you were not doing anyway.

The ColorQ tends to have quite a bit of variation on the TA and CH tests so I am inclined to believe the other two tests for those numbers.

The PH difference on the second set of tests looks like a testing error, since the TF Test Kit and the ColorQ are normally very close to each other when FC and CH levels are reasonably low.