DPD1 Chlorine test results delayed?

[rwilson86]

Looking for soem advice on the use of DPD1 test tablets for residual chlorine testing. I am a new Pool owner (< 1 month) and have just started using the salt chlorinator after letting the water and residal salts all balance out. Before turnign on the salt water chlorinator I was using granulated chlorine and by addign half a cup a day I was able to keep the chlorign levels at the required levels and were able to confrm this by using the DPD1 testign tablets.

About two week ago the salt water chlorinator was turned on and seems to be workign correctly however when I now test for Chlorine with the DPD1 tablet I dont get an immediate colour change like I was previously, however if I leave the water sample for an hour or two, the colour does eventually change to indicate an almost perfect chlorine level??.

Can anyone suggest why this may be the case, given the fact that the colour change was instant when I testing previously with granualr chlorine I expected the change to be the same with the salt water chlorinator.