Chemicals Again!

[Simon]

Two questions really:

1) What is the Javel concentration to get a 650MVa reading on the Chlorine sensor I have (it's a Procopi but not sure if this is relevant). The reason is the doser is now acting very temperamentally and I need to be sure it's calibrated correctly). Also, as an aside, do the sensors need maintenance or replacement. Ours is currently 4 years old.

2) Can CYA disappear/breakdown. Last year I had too high CYA, but this year there is no CYA measured on the Scuba+ tester. There has been rain etc and backwashing but not enough to replace all the pool water.

Many thanks in advance....

[Théière]

Concentration varies but the usual 1-1.5ppm should be enough. (lots of things throw the MVa out including CYA) Yes the probes usually need replacing, new probes usually come with a buffer (calibrating solution)

It does seem that a bacteria can breakdown CYA, Chemgeek has done a fair bit of study on this but it does seem to happen on occasions. I am wondering if this is also causing your problems with the doser.

Can you get a free/total chlorine reading with the scuba? 

[Simon]

Many thanks Théière.

I get a Total Chlorine reading on the Scuba+ (around 1.5ppm), and, if I add Chlorine (Javel directly) this will go up (eg I had 4ppm after adding Javel last week) but have not been able to get a Free Chlorine reading at all (it reads "Lo").

You mention the probes need replacing - how often would I need to do this?

Thanks

[Théière]

Hmm, sounds like the scuba+ may have developed a fault. Electrodes need replacing when they won't calibrate any more, they do require gentle cleaning from time to time, best to use the O/H tooth brush whilst they are out [6]

[chem geek]

If the FC is truly low and the TC higher, then that implies Combined Chlorine (CC) higher and could very well be bacterial conversion of CYA into ammonia.  You can do a bucket test to see how much chlorine it will take before you get an FC reading.  1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC.  Or for 10% Javel this is 1 ml in 10 liters for 10 ppm FC.