Numbers don't match up

[JCJR]

EDIT 5/22/07 TMQ I moved this to its own thread so you can get better answers

I'm a newbie and I have been testing my own water. I usually take my water to the pool store to double check my results. I'm not here to bash and I'm glad that I have a pool store close to my house but I have to say that this past week their results were way off from mine.
My numbers were as follows before I added chemicals:
CL 1.5
FC 0
TA 90
CH 260
PH 7.8
CYA 50
Their numbers before I added chemicals:
CL 0
FC 0
TA 70
CH 210
PH 7.2
CYA 40

The PS guy asked what color was my water before giving me the test results and I responded that it was crystal clear but I knew I was low on CL. After seeing that their test showed CL 0 I knew why he asked the question. My first thought was their numbers must be wrong but on the drive back home I started second guessing myself, maybe I was doing something wrong. I retested CL using the FAS-DPD method ( I used the OTO the first time) and got CL 2.0 ( using the 10ml sample the 3rd drop was light pink and 4th drop was clear). Since the first step in that test is to add the powder and water should turn red if FC is present and that is exactly what happened I new my tests were right. Imagine if I did not test my own water.

I am glad to say that I did add Chlorine and my CL is at 4. But this proved to me that the Pool Store do make mistakes with their test, different employees have different results and their test might be setup to sell chemicals.

 

[chem geek]

Depending on the container that is used to carry the pool water sample, it is possible for the chlorine level to drop over time. If there is any contamination in the container or if it is open to the air or if there is a lot of air in the container and shaking occurs, chlorine levels can drop. I've seen this happen when making my own measurements so can imagine it happening in the much longer time it takes to get a sample to a pool store. Now that said, the drop I have seen isn't very large and being careful will usually prevent such problems so in your case it's much more likely that the pool store simply got it wrong for whatever reason.