Increasing FC Test Result

[Bevostein]

I have had two instances where my FC has risen between two FAS-DPD tests with no bleach or other chlorine being added to the pool. The tests have usually been spaced apart by 8 - 12 hours. It happened last night and I am trying to figure out why. I know it has to be operator error of some sort. Last night I tested at 10:30pm and got a reading of FC 4.5 / CC 1.0. I tested again this morning and got FC 6.0 / CC 0.5. Both tests where it increased were OTCL tests and both evening tests showed CC of 1.0. It also has been early in the week after a weekend of swimming. The other time it happened my first test was FC 14.0 / CC 1.0 and the second was FC 16.0 / CC 0.5.

I test with a Speedstir and use the TF100 kit. My kit is less than two months old and stored inside.

I don't understand how there can be an increase overnight so I know I must be doing something wrong. I know one possibility is that in performing the FC test, maybe I did not get my FC test water totally clear and I needed to add one more drop of R-871. As the water clears, I am usually very conservative about adding that extra drop. If I am "on the fence" as to whether think its totally clear or not I don't add it. I would rather have a slightly low reading of FC than a high reading. That would would possibly account for 0.5 of the difference between evening and morning.

The other possibility I am contemplating is the R-870 powder. The instructions call for one heaping scoop. Everyone interprets heaping differently and I can honestly saying that I have difficulty in making my "heaping" exactly the same from one test to the other. I am wondering if slight differences in the R-870 powder could be driving this as well. Is the test very sensitive to the amount of R-870 powder? Post #2 in this thread seems to indicate that not enough R-870 powder could impact the test.

http://www.troublefreepool.com/help-constistenty-different-readings-tf-100-t38295.html

Thoughts?

 

[Bama Rambler]

Do you have any tablets in that tablet feeder?
How long after the last addition of chlorine did you do the evening test?

 

[duraleigh]

The correct amount of powder is really not very critical. If the solution turns pink/red, you have chlorine in your pool and the powder is adequate for the test. Some rare, rare exceptions but not at your FC levels.

The first thing that comes to my mind is the importance of carrying the test to completion. Your last drop of R-0871 should produce ABSOLUTELY no further color change to the solution. You than subtract that drop that had no effect and the result is your FC.

 

[linen]

As duraleigh said, make sure you carry the test out to completion. Make it a habit of adding the extra drop even thought the solution may appear clear to be certain there is no more change and make sure you have good lighting when doing the test. One reason for this is if you skimp on the R-0870, the end point is still there but can be hard to discern.

 

[jblizzle]

Are you using the light on the Speedstir? I found that it was harder to see if the water was truly clear with that LED on ... so I never turn it on.

 

[Bevostein]

No pucks. Used them when we filled to pool at the end of June to build up CYA but stopped several weeks ago when I got to a CYA of 40.

In both cases, no bleach had been added at least 20-24 hours before either of the evening tests.

Thanks for the advice on the last drop

Not using the light on the Speedstir. I agree on its effect in evaluating the water

 

[scott.MI]

Your not adding the stir bar until you have the proper amount of water measured every time correct?

 

[Bevostein]

That is correct. Put the water in and then add the stir bar. I am baffled by this honestly. I will try adding the extra drop as suggested but even then it doesn't make sense that I would have such a large variance.

 

[DorsalSpine]

That is correct. Put the water in and then add the stir bar. I am baffled by this honestly. I will try adding the extra drop as suggested but even then it doesn't make sense that I would have such a large variance.

How are you holding the bottle? I read in another thread that holding the bottle perpendicular to the test vial instead of vertical affects the size of the drops. That would throw your tests off if you don't hold the bottle of reagent the same way each time.

 

[Bevostein]

I saw that post as well while doing some searches to try and figure it out. I do hold it straight vertical. I do apply a slight amount of pressure to get the drops to drip out. I have seen comments that the correct procedure is to squeeze to get the reagent on the tip of the bottle and then let gravity pull it out. I think I have been consistent in how I do it but maybe the squeeze pressure has been different between the evening and morning.