I've been having problems keeping my FC where it should be for the last two weeks. It should be 4.5 - 5.0 but has been 2.0 - 2.5. I have a Stenner pump that has been is service for about 2 months and until the past two weeks it has done a wonderful job. When the FC dropped I put the outlet tube without the check valve in a bottle for an hour and measured the discharge rate. It was right where it should be. Then I removed the check valve attached the discharge line and measured again and again the flow was right where it should be. From those measurements it appears the pump, inlet strainer, and check valve are working like they should. The pool water is still clear as clean glass (I add chlorine manually to keep the levels up) with no hint of algae and there is no combined chlorine in the samples.
The only other thing I can think of that would cause the low levels of FC is the chlorine being under strength. I buy liquid chlorine that is labeled as 10.5% in 2.5 gallon carboys. I've used the same stuff for several years without any noticeable problems, but now I think the pool store has bought some under strength chlorine, or is diluting the material, or maybe it is just old. Whatever the reason I want to test the strength.
The only testing I can do is with the TFP test kit. So is there a procedure I can use to tell me if the liquid is badly under strength. There is a commercial analytical lab it town I could use, but would like to do some preliminary testing before taking samples to them.