FC test results varies

kal2002 · Jul 16, 2011

I am having trouble with the FC test lately. I was shocking the pool so I was doing a lot of FC testing. Sometimes after I added the powder, the water did not turn red. It was pinkish and I expected it to be red since chlorine should be around 20. I dumbed it out, rinse the vial with pool water and re-did the process, then the water turned red. Wonder what I did wrong?

Another problem or related problem is the test result. I would do the FC test in the morning and FC was 2 or 3ppm less than it was the night before. So I thought I had to continue shocking. Before I added more chlorine, I re-did the test from the same batch of pool water and the test results would be different. One morning I actually went to the pool and got a different batch of water from the other side of the pool and the FC test results were different too. Now I am beginning to wonder if there is a water circulation problem in the pool. What do you think?

JasonLion · Jul 16, 2011

There are a number of things that could be going on. Make sure the pump was running for at least half an hour before taking the water sample. Take the water sample from 12 to 18 inches down. Make sure you add enough powder, adding to little can cause the results to be falsely low. Hold the dropper bottle vertically, squeeze slowly so that the drop seems to hang on the tip for a moment before dropping off. Do the FC test reasonably quickly, slowing down only at the end to be sure the sample turns completely clear.

kal2002 · Jul 16, 2011

Thanks. The pump was not on that early in the morning unless I am shocking. Here is an example of the FC test results:

7-15-2011: 11:00pm FC was tested twice. FC-15 and 16.5, CC-.5 - pump was off
7-16-2011: 7:15am FC was tested twice. FC-14 and 15.5, CC-.5 - pump was off

I did the overnight test just to comfirm that I was done shocking. When I saw 16.5 from last night and 14 this morning, that means more shocking. AFter doing another FC test and it said 15.5, that means I was done. Now I am not sure if I am really done or not. Would those test results be different if the pump was on?

Butterfly · Jul 16, 2011

kal2002 said:
I am having trouble with the FC test lately. I was shocking the pool so I was doing a lot of FC testing. Sometimes after I added the powder, the water did not turn red. It was pinkish and I expected it to be red since chlorine should be around 20. I dumbed it out, rinse the vial with pool water and re-did the process, then the water turned red. Wonder what I did wrong?
Sometimes for higher FC testing two scoops of powder are needed.
Extended directions: extended-test-kit-directions-t25081.html

Another problem or related problem is the test result. I would do the FC test in the morning and FC was 2 or 3ppm less than it was the night before. So I thought I had to continue shocking. Before I added more chlorine, I re-did the test from the same batch of pool water and the test results would be different. One morning I actually went to the pool and got a different batch of water from the other side of the pool and the FC test results were different too. Now I am beginning to wonder if there is a water circulation problem in the pool. What do you think?
Well, you can check to see if it is a circulation problem.
You can try using a ping pong ball to see how the water is flowing/check out the circulation.
Is the filter clean & does it have good pressure coming from the returns?

You should always have the pump running for at least an hour before testing.

I suggest another OCLT.

kal2002 · Jul 18, 2011

I guess the filter is ok. It was at 14 psi and now it is at 16. Because the pressure has not gone up enough, I have not had it cleaned assuming it it ok. I did not do OCLT last night but I did check FC tonight before 9:00 pm. It was at 10 and 10.5. That means between yesterday morning and tonight, I lost 4 to 5 ppm and I did not add any chlorine last night. That is pretty normal since it was sunny yesterday and today and the water temp. was 80 degrees. I will try and get up early to turn on the pump at least half an hour before I test FC. Frankly, I am getting frustrated and pretty tired of catering to this pool.

crs16 · Jul 18, 2011

Isn't there a percentage error in the test? I thought I read somewhere, extended directions posted above maybe, that the error was +- 10%. If your chlorine levels should be around 20, doesn't that give you 20 +-2? Or am I not understanding it properly.

It is certainly late enough that my mind may be wandering...

JasonLion · Jul 18, 2011

The test is about +-10% from reality, but it is a much much closer match than that to it's self on repeated trials. Another way to say that, is that most of the errors, if there are any, tend to repeat and be the same on repeated trials. Thus, comparing two tests to each other gives you reliable differences between the two results, even though you don't know the absolute match with reality.

crs16 · Jul 19, 2011

JasonLion said:
The test is about +-10% from reality, but it is a much much closer match than that to it's self on repeated trials. Another way to say that, is that most of the errors, if there are any, tend to repeat and be the same on repeated trials. Thus, comparing two tests to each other gives you reliable differences between the two results, even though you don't know the absolute match with reality.

Thanks! I knew there was a different answer to the way I was thinking about it. You're right of course. :-D