Whose test data do you trust?

[sberggren]

Hi I am a new pool owner and have been really grateful for the wealth of knowledge on this forum; I've learned a lot in a short period of time and the information here has already saved me a lot of time and money. Thanks!!!

Here's my issue: I am surprised at the delta between my TFP test kit data and that of the local pool supply company. They are using some kind of machine analyzer which is supposedly more accurate than the test kits (their view). I'll post mainly the Hardness and the Alkalinity data chronology because that is where I'm seeing the difference.
5/19 pool company analysis (last data from prior owner): Hardness 169 ppm Alkalinity 103 ppm. Adjustments by them are unknown.
6/26 TFP test kit (first reading after getting my kit): Hardness 250ppm Alkalinity 160

I added 10 lbs of calcium which brought my harness measurement to 400ppm on 7/3 and it has stayed in that range since.
I have added muriatic acid in half gallon increments followed by aeration to get the PH back up; total acid added has been 2.5 gallons. TFP results were showing slow progress on alkalinity: 7/3 140 ppm; 7/16 120ppm; 7/28 110 ppm. The last acid was added 8/1 using about 3/4 gallon.

7/28 TFP test: Hardness of 375 ppm.
8/4 My wife had a sample tested for correlation expecting to see corresponding data. Pool company tests: Hardness of 175 ppm and Alkalinity at 74 ppm.
In between this time and the below TFP tests I made no changes or additions, just ran the pump and salt water generator as normal. I was on a trip so wasn't able to test on the same timeframe:
8/21 TFP tests: PH 7.5; Alkalinity 130 ppm.

It's a big difference between the two results. Their hardness data doesn't make sense to me, given the calcium I added. But I'm even more puzzled by the alkalinity measurements. I can't seem to bring this down very effectively if I go by the TFP results; in this case the pool company result make more sense given the 2.5 gallons of acid I've added.

Thoughts? Any chemists out there?

Thanks

 

[pooldv]

We trust your test results 100% of the time over pool store test results. We have seen wrong test results from pool stores across the country 1000s of times.

Here are more detailed description test instructions if you want to verify your testing, Pool School - Extended Test Kit Directions

 

[Isaac-1]

TA can be difficult to adjust downward and often takes considerable time as well diligent avoidance of using any chemicals that will raise it. As to those computerized testers used by the pool stores, it is hard to comment without the knowledge of the exact model being used, but all of them are susceptible to a long list of potential problems most notably lack of proper maintenance and calibration along with cross contamination. Keep in mind these systems are used as sales tools by the pool stores, and there is little incentive on there part to get the testing right. In fact they can likely sell the average consumer more chemicals if the testing is inaccurate.

You have already started to prove this to yourself with the tracking of your CH addition, remember CH does no go down on its own, and is only lowered through water replacement / overflow not through evaporation. I suspect if you track the testing over time from that pool store you will see a ping pong effect if you graph out their levels from week to week. On some things like FC this may make sense, but on things like CH and CYA that should be steady or slowly rise depending on your chemical additions it is a clear sign of testing error.

Ike